| Date: | 1891 |
| Author: | Edwin Dunkin, Former Chief Assistant at the Royal Observatory |
| Title: | The Royal Observatory, Greenwich |
| About: | Back in 1862, Dunkin had published a two accounts in The Leisure Hour, Vol XI (London). A day at the Royal Observatory was published in two parts, the first in edition No.524, 9 Jan 1862, pp. 22–26 and the second in edition No.525, 16 Jan 1862, pp.39–43. A night at the Royal Observatory was published in edition No. 526, 23 Jan 1862, pp.55–60. Later in 1868 and 1869, he had two further articles published, the Midnight Sky in London and the Midnight Sky in the Southern Hemisphere. Various aspects of these four works were incorporated into a book – The Midnight Sky, which was first published in 1869 by the same publisher, The Religious Tract Society. Several other editions followed. Originally, the book did not contain a specific section about the Royal Observatory at Greenwich (pp.151–176). This changed when a revised and enlarged and what turned out to be final edition was published in 1891. It is on pp.151–176 of this volume that the account can be found. The six accompanying illustrations are those which Dunkin used in his earlier 1862 account. |
| Images: | Six |
[Note: Pages 151 & 152 are purely historical, and are not reproduced below. The illustrations will be added as soon as time allows]
We will now endeavour to describe, in as plain a manner as possible, what may be seen by a visitor, the peculiarities and use of each instrument, and any other mystery which may be attached to the work of the astronomers.
The principal entrance to the Observatory from Greenwich Park is on the east side, where several interesting objects are placed for the information of the general public. First, there is a galvano-magnetic clock, giving true Greenwich mean time, the hands so arranged as to begin the day at midnight, counting the hours 0 to 24; then an engraved plate, showing that tile, height is 154 feet above the mean level of the Thames; standard Measures of 3 inches, 6 inches, 1 foot, 2 feet, and a yard; a Skeleton barometer, with an indicator set at the present reading and another set at the lowest reading for the preceding twenty-four hours; and a card showing the highest and lowest temperature in the shade, with the amount of rainfall for the same period.
On entering tile courtyard, the first objects presented to the eye are the Astronomer Royal's private house and the more ancient part of the Observatory, consisting of the octagon room, the turrets on the roof, and the dome covering the Ramsden equatorial, while to the left is a range of comparatively low buildings, consisting of the transit-clock room, the computing rooms, and a small fire-proof room. The oldest of these was added by Dr. Halley about the year 1726; the others were more recently built, as the increase of new instruments required. In these unpretending rooms most of the meridian astronomical observations are made, reduced, and prepared for the press.
All the old instruments that are now out of date are preserved; and very interesting they are, as showing the gradual development of the instrumental wants of the staff from the days of Dr. Halley to the present time. Most of these old instruments, however, much as we may now look at them as mere curiosities in the history of the Observatory, have done good work in various astronomical researches. We may see in them the history of the improvement in the construction of astronomical telescope most strikingly illustrated. For example, confining our attention only to the different transit-instruments, no person can view them without being impressed with this visible improvement, which has been gradually developed since the early days of the Observatory. To be convinced of this, we have only to look at the curiously constructed transit of Halley, with its telescope fixed, not in the middle, but on one side of the axis, and having a series of bracing rods applied in a most injudicious manner; then at that the great stride made by the construction of Troughton's excellent transit-instrument, unsurpassed at the time by any other of its class; and, finally, at the massive telescope of the transit-circle, which came into use in the year 1851.
Before describing the separate instruments, we will ask the reader to accompany us to the roof of the octagon room, the highest part of the Observatory, in order to obtain a bird's-eye view of the extent of the establishment. From this elevated Spot the locality of each instrument can be pointed out; in fact, the ground-plan and the relative positions of the different buildings can be seen to great advantage. But what would probably interest a visitor at this moment far more than gazing down upon the roofs of certain comparatively old-fashioned buildings, is the magnificent prospect visible on all sides. If we glance towards tile north, with Osler’s anemometer on the left and the Greenwich time-ball on the right, we cannot avoid being impressed with the visible magnitude of the great metropolis, extending from east of Blackwall to Westminster Abbey, while immediately in front stand the stately buildings of the Royal Naval College and Schools, and the countless objects crowded on the river and its banks. If we turn to the south and west, we look over the well-wooded and undulating surface of parts of Kent and Surrey, forming a pleasing variety to the scene; whilst towards the east, or south-east, Shooter's Hill, with its mimic fortress of Severndroog, is a conspicuous object; and last, but not the least attractive, is Greenwich Park, -altogether a combination of picturesque effect with scientific interest and commercial grandeur which can scarcely be equalled. On summer mornings, soon after sunrise, the distant view over London is exceedingly interesting, for almost every church tower may be distinguished, St. Paul's Cathedral and Westminster Abbey standing out in bold relief, while the objects on the high ground of Highgate, Hampstead, and even so far as Harrow, have been clearly visible.
Entering the transit-circle room by the north door near the entrance gate, it may be noticed that the Astronomer Royal's official room is on the right, and that nearly the whole of the transit-circle room is occupied with the principal observing instrument and its accessories. The transit-circle is placed exactly in the meridian of Greenwich, and it is of no consequence in what way the telescope is directed to the heavens, its central wire represents the position of the meridian line. The telescope is thus capable of being only moved in the direction north and south. All longitudes inserted on British maps are reckoned so much east or west from this central wire. Practically, therefore, the observer, when seated at the instrument, looking south, has his right arm in the western, and his left arm in the eastern hemisphere.
The transit-circle is the embodiment of two well-known astronomical instruments, as its name implies, viz., the transit-instrument and the mural-circle. These two instruments were in use daily till the end of the year 1850. The first observation with the transit-circle was made in January 1851. The observations made are therefore of two kinds, -transits of stars, and measures of angular distance in declination. The first consists in noting or observing the exact time by the clock when the celestial object under view passes the system of nine vertical wires, which are simply lines of spider's web stretched at stated distances across the field in the eye-piece. Supposing that a star is under observation for the determination of the error of the standard clock, the observer, by recording the exact instant when the star passed each of the nine wires, is able, after a slight arithmetical deduction, to find the true error of the clock, and thus he is in a position to communicate, by means of the elaborate machinery of the electrical department of the Post Office, the exact Greenwich mean time to all parts of the country. But this is only one of the practical results derived from these transits. They also form the original groundwork for determining directly the positions of the Sun, Moon, planets, and stars, in right ascension. But secondly, the transit-circle has also to perform the office of the former mural-circle, thus enabling the observer to determine also the position of the object observed in declination. For this purpose, graduated circle attached to the instrument is provided with a narrow band of silver let into the bevelled edge Of the iron circle, on which are engraved very fine lines separated from each other by five minutes of arc. The intermediate space between two lines is read by means of six micrometers, attached to powerful microscopes, placed at intervals of sixty degrees, so that the true mean readings of the circle can be obtained with great accuracy;.
The heavy metal work of the transit-circle was constructed by Messrs. Ransomes & May, of Ipswich, and the optical parts by Mr. Simms, of London. The engineers’ work is massive in all its parts, and has been Constructed with great solidity. The telescope, which is twelve feet in length, and its axis six feet, is made of iron, cast in about four pieces, the separate parts being afterwards firmly bolted together. The object-glass is eight inches in diameter, being sufficiently powerful for the observation of faint stars to the eleventh magnitude.
As it is a strict rule of the Observatory that no visitor, of whatever rank, can be permitted to interfere with the observers during he evening observations, no visiting stranger is admitted inside the entrance gate after about 1 p.m., and before that hour on]}- after special permission has been obtained from the Astronomer Royal or the Board of Admiralty. For this reason we hope that a few popular remarks on the methods of observation will be acceptable to the general reader.
On each Monday morning a list is prepared defining the observing duties of each of the assistants during the week, and this arrangement is strictly adhered to. The assistant charged with the observations with the transit-circle on any day has usually a prepared list of objects which he is called upon to observe if possible, on which the times of meridian passage, with the approximate position in the heavens of each object, perhaps beginning with the Moon at 6 a.m., and ending with Jupiter or some other large planet at nearly 3 a.m. on the following day. His observing watch on such occasions, therefore, occupies his time during twenty-one consecutive hours. This is an extreme case, but it is always possible when the Moon passes the meridian between 6 a.m. and 8 a.m. If we enter the transit-circle room at any time in the evening on a clear night, we shall probably find the observer examining his prescribed list of objects, which may contain the Moon, and certain stars near the Moon, which always receive special attention. Let us now follow him through one of his observations, which will be sufficient to explain the general method of observation with the transit-circle. The object to be observed is within a few minutes of passing the meridian. The shutters of the roof are already open. The observer looks at his list, which, as we have said, contains the time of meridian passage, and also the approximate position of the object in the heavens. He turns the instrument, by means of small projecting handles, to the proper setting, when, if the object is within forty degrees of the zenith, he seats himself in a very comfortable chair with a reclining back capable of being adjusted at pleasure to any inclination. The observing chair rolls backward or forward at the will of the observer, whilst he applies his eye to the telescope, his head being supported by the adjustable back of the chair. When the object is high, the observer is low; if it be a star near the zenith, the observer lies almost on his back. In the illustration on the opposite page, the assistant is supposed to be observing the transit of a star south of the equator, when he usually sits on a movable stage, fitted with steps leading down to the pit where the observing chair is placed. After the object has entered the field of view, it approaches rapidly towards the vertical wires of the telescope; in a few seconds it is behind the first wire; an ivory key is then pressed down, and a signal is instantaneously transmitted to, and a puncture is made on, the prepared sheet of the chronographic apparatus. He repeats the tapping at each of the nine wires; nine punctures will therefore be recorded on the chronograph. With the other hand he turns a delicately constructed micrometer-screw, which carries a horizontal wire, until the wire is exactly on the star, which then appears bisected, the instrument at the time being fixed. The observer then leaves the telescope after having read the figures indicated by the screw-head, technically called a micrometer-head, and proceeds to bisect the graduated divisions on the circle by certain cross-wires in the six large microscopes, the eye-pieces of which are on the back of the western stone pier, as shown in the illustration of this instrument. These microscopes are simply inclined perforations through the pier, the object-glasses being on the eastern side, close to the graduated circle. The observer has now only to read the micrometer-heads of the microscopes, the barometer, and the thermometer, when the observation is complete. It is the duty of the computer, at a future time, to analyze these readings, and to deduce from them the proper results of right ascension and declination, or north polar distance.
The practical object of these astronomical observations is to determine, at a given time, the accurate positions in the heavens of the Sun, Moon, planets, and principal stars, in order that their observed places may be compared with corresponding places computed from existing authorities, to discover the amount of the error of the planetary tables employed in the calculation of the Nautical Almanac. These errors give the mathematician the fundamental means of correcting the old tables, which were originally calculated from data found from inferior instruments, and they enable him to form more perfect theories of the motions of the Sun, Moon, and planets in their orbits, and to construct tables of greater accuracy than those previously in use. The astronomers of all nations have had to rely on the Greenwich observations for most of the raw material necessary for the construction of nearly all astronomical and nautical tables in existence.
The altazimuth was erected in 1847. It is mounted in mounted in a tower or dome, built on the walls of what was formerly known as the 'advanced building,' where Flamsteed’s mural arc and equatorial sector were mounted. The instrument commands the sky to the horizon in all directions, excepting in the north and north-west, where the view is intercepted by the new photographic equatorial and the octagon room. The staircase leading to the dome is partly carried round a three-rayed pier, which serves as a solid foundation for the instrument, the rays being carried up nearly to the floor of the observing room. From the centre of the three rays a massive cylindrical pier, three feet in diameter, is carried through the floor. On this pier the altazimuth is mounted, as clearly shown in the illustration. This instrument was specially designed by Sir George Airy for the daily observation of the Moon, with certain stars for the determination of the instrumental errors, so that its accurate position should be ascertained daily for comparison with corresponding tabular places calculated from the lunar tables. These observations are especially valuable when made during the early and late periods of the lunation, as at those times the Moon is seldom observed with the transit-circle, for though the sky may be unclouded, it is too faint to be seen through the meridian telescope at least four days before and four days after new Moon, in consequence of the proximity of our satellite to the Sun. It frequently happens, also, that at the moment of transit over the meridian in other parts of the lunation, the Moon is temporarily obscured by clouds, though shortly before or after it may be shining brilliantly. Sir George Airy, seeing the great importance of obtaining as many observations of the Moon as possible in every part of its orbit, constructed the altazimuth as the only form of instrument that could be satisfactorily adapted to extra-meridian observations, the results of which could bear comparison with those made under more favourable circumstances with the transit-circle.
Without entering into too much detail, it may be stated that it was the great object of Sir George Airy to design an instrument which should be composed of as few pieces as possible, fixed together in so solid a manner that the movable part of the instrument should be as firmly welded together by bolts as if it were one mass of metal. The engineer has carried out the designs of Sir George Airy so well, that although the altazimuth is about a ton in weight, it can be moved to any position with the greatest ease without any appreciable strain. The movable portion consists of four castings of iron, -the two vertical cheeks, and the upper and lower connecting plates-all, as we have said, firmly fixed together by bolts. The sockets which carry the microscopes for reading the divisions on the engraved horizontal and vertical circles are all cast in the same flow of metal as the cheeks of the instrument. It can thus be easily perceived that these well-devised arrangements must tend to make an instrument of great solidity, as it has been sufficiently proved by the success of the altazimuth in obtaining observations comparable meridian.
The observations consist of transits over a series of vertical wires for the determination of the azimuth, and over a series of horizontal wires for the determination of the altitude of an object, the clock-times of transit at each wire being recorded by a galvanic signal on the sheet of the chronograph. The exact time of the observation is thus obtained for the purpose of computing the tabular places for comparison, both in azimuth and in altitude, with the corresponding azimuth and altitude obtained from the respective readings of the two circles, etc. The transits in both of these elements are observed over six wires, and therefore six clock-times in each instance are recorded, the mean of which, when corrected for the error of the sidereal standard clock, will give the true sidereal time of the observation. It will be interesting, if we follow an assistant while making one of these observations. How intently he appears to be watching the Moon while its limb, or edge, is passing the different wires; at each transit he touches an ivory key, which completes the galvanic circuit and instantaneously a puncture is recorded on the chronograph between the regular two second punctures of the sidereal standard clock Watch now how carefully he is creeping up and down the instrument, reading the four microscopes of the vertical circle, and then the levels which may be noticed attached to various parts, two azimuthal levels being at the extreme top of the altazimuth. We cannot help being struck with the skill of the observer; the massive instrument seems movable at his will, and obeying him without much apparent exertion on his part. Watch again how rapidly he reverses the whole instrument, redirects the telescope to the Moon and before one can realize that the Moon is again in view, tap, tap, once more reaches your ear -another observation is being made and recorded on the chronograph. The most fatiguing part of the observer's duties occurs in cloudy weather, for he is then expected to be on a continuous watch, possibly throughout the night, for the appearance of the Moon. These harassing long, watches are less frequent since 1882, as after that year the altazimuth has been principally confined to observations of the Moon during the first and last quarters of the lunation.
All the transits observed, either with the transit-circle, altazimuth, or great equatorial, are referred to the sidereal standard clock, which is placed in the underground room in the magnetical observatory, in order that it should not be subject to great change of temperature. It was constructed by Dent & Co., and is fitted with a contrivance for sending galvanic signals for registration on the chronograph at every second beat. The clock used in ‘eye and ear’ transits is placed in a recess in the south collimator pier, and is well in view of the observer when making the observation. This clock, Hardy, has been the ordinary transit-clock for a long period, but a new dead-beat escapement was substituted by Dent in 1829, in place of the original escapement. Many other improvements have been made in it since that year. A natural instinct of an ordinary visitor, when his attention is directed to this clock, is to pull out his watch and note how much it is fast or slow of true Greenwich time, which he assumes that this observatory clock ought most assuredly to show. But mark in a moment the hesitation plainly visible on his countenance. The clock and the watch differ by several hours! A word of explanation soon accounts for the apparent anomaly, and our visitor is himself again. This clock, being regulated by the observation of the stars, is made to keep star time, known as sidereal time. The sidereal day, as explained in a former chapter, being nearly four minutes shorter than a mean solar day, the clock which indicates sidereal time will gain nearly four minutes over ordinary mean solar clocks in one day: or in the course of a year twenty-four hours will be thus gained, so that there are 366 days in a sidereal year. No wonder, then, that our visitor found a difference between his watch and this clock. A very slight computation, however, is sufficient to convert sidereal time into mean solar, or ordinary clock time.
The sidereal standard clock and the chronograph are now necessary adjuncts to the transit-circle and altazimuth, as the clock-times of all the transits made with both these instruments are represented in certain punctures recorded on the chronograph. Before, however, describing this apparatus, we may state that the original method of observing transits consisted in counting the successive beats of a clock, and in estimating the exact fraction of the interval between two beats of the seconds pendulum when the star passed the different wires in the field of view in the telescope, an operation requiring considerable skill. The estimated clock-times were entered in the observing book at the moment of each observation, and transcribed afterwards into the usual form for the conversion of the original figures into true sidereal time, or apparent right ascension. Though the chronographic method is now generally adopted, it is occasionally found necessary to revert to the ‘eye and ear’ method, especially when the chronograph is under repair.
The chronographic recording apparatus is placed in the ground floor of the north dome. The clock, which is of peculiar construction, the motion being governed by the conical rotation of a pendulum, gives a sensibly uniform motion to a revolving brass barrel, which is in connection with it. The barrel is covered with woollen cloth, upon which a sheet of smooth white paper is folded, the ends of the paper being gummed together. A spindle which is attached to the clock turns two long screws, causing a travelling frame to traverse the whole length of the barrel. This frame carries two levers, each armed at one end with a pricking point, mounted in such a way that, when the opposite end of the lever is pulled away from the barrel, the pricking end is pushed against it, and makes a permanent puncture on the paper. Two galvanic magnets are fixed on the travelling frame, so as to attract the lever ends opposite to the pricking points. All that is required, therefore, to cause these points to make punctures upon the paper, is to send galvanic currents through the magnets.
One of the prickers is devoted to the registration of the seconds of the sidereal standard clock in the basement of the magnetical observatory, the communication being made by insulated wires connecting the clock with the travelling frame carrying the prickers. The second pricker is used for the registration of the clock-times of observation when a star passes behind the different wires in the eyepiece of the telescope, a similar communication with insulated wires being made. For the proper generation of the galvanic force, a voltaic battery is included in the circuit of each course of wires.
The great equatorial is mounted in a large dome in close proximity to the south-east corner of the record room, and occupies part of the isthmus which connects the Observatory hill with the upper part of Greenwich Park. It is in the form of an octagon, with an attached projection on the north side, containing the staircase leading to the observing room. The object glass, which is 12¾ inches in diameter, was purchased of Messrs. Merz, of Munich, for about £1,200, including expenses. The iron castings for the supports of the telescope, and the engineer’s work generally, were made by Messrs. Ransomes, of Ipswich, and the general optician's work by Mr. Simms, of London.
The appearance of this beautiful instrument never fails to astonish the visitor who, is favoured with the privilege of a personal inspection of its peculiarities and improvements over other telescopes with similar mounting, though it is difficult to explain in what manner these improvements differ from the general construction of other equatorials, without entering into a minute mechanical explanation of all its principal parts, which would be out of place in a popular work. The reader must therefore take for granted, on the evidence of some of our ablest astronomers and mechanical engineers, that, as a specimen of astronomical engineering, it was considered, when first erected in the year 1860, to be a triumph of mechanical constructive art. Even though several much more powerful equatorials have been mounted since 1860, as at Washington, Pulkowa, Vienna, and a few other observatories, yet, taking into account the great stability of the mounting of the Greenwich equatorial, it has played a very important part in the progress of astronomy during the last thirty years.
The great equatorial is provided with all the necessary adjuncts for making extra-meridian observations in every branch of the science. It is furnished with the usual microscopes for reading the graduated circles, both in right ascension and declination; eye-pieces of various powers and construction; spectroscopic apparatus of all kinds used in the comparison of solar and stellar spectra with the spectra of the incandescent vapours of terrestrial substances and gases; and many other appliances for the convenience of the observer too numerous to mention. If the accurate position in the heavens of a planet, comet, or star be required; if the equatorial and vertical diameters of Jupiter, Saturn, or any other planet having a measurable disc be scrutinized with the object of determining the amount of ellipticity; or if the rapid changes in the angular distances of the cusps of the Sun be measured during a solar eclipse: or, indeed, the observation of any other astronomical phenomenon, this equatorial, in the hands of a skilful observer, is fully able to give results of the highest character.
But there is one important contrivance connected with the great equatorial which is very striking to those who see it for the first time. Probably most of our readers know that it is necessary, when a celestial object is under telescopic scrutiny, that it remain visible for a considerable time in the same position in the field of view, in order that the observer may be able to give all his attention to the object he is examining. Now, everybody who, for amusement or otherwise, looks at a celestial object in a telescope fitted with a moderately high magnifying power, cannot fail to perceive, in an inverting telescope, that it is continually travelling towards the left of the field of view, and eventually it passes out of sight. The Earth is, in fact, performing its daily rotation on its axis, thus giving an apparent motion to the heavens from east to west. The astronomer, consequently, has found it necessary to adapt to all fixed equatorials of moderate size a clock movement, which is ordinarily regulated by two balls, suspended to the ends of a horizontal arm which is carried by a vertical spindle, the motive power being a weight. But to carry such an enormous amount of metal as that contained in the great equatorial, a special system of clock-movement had to be devised. Instead of the motive power being a weight, the necessary force required for driving the instrument has been obtained by a direct supply of water from the Kent waterworks. The force of water has been found to be sufficient for working a reaction machine, sometimes called a Barker's mill, revolving four times in a second. A long spindle reaching from the ground floor attached to the lower part of the apparatus acts through two worm-screws, and turns the massive instrument with the utmost precision. The pendulum, which is mercurial, has a uniform conical motion, regulated by a beautiful contrivance known as Siemens' chronometric governor. The success of this interesting clock-movement will be clearly understood when we state that, on one occasion, the equatorial was left by the observer, who was engaged for a time with other observations, with Jupiter visible near one of the wires of a position-micrometer attached to the telescope, and on his return, after an interval of more than an hour, the planet was still near the same wire-a proof of the wonderfully accurate adjustment of this beautiful system of clock-movement.
But suppose we enter the observing room at night, when the observer is actively employed, probably with measuring the amount of the delicate displacements of the lines in the spectra of certain stars, for the determination of the direction and velocity of their motion in the line of sight, we should find our attention first attracted by the working of some kind of machinery. This is the clock-work which gives motion to the instrument, allowing the observer the power of scrutinizing his difficult observations without being disturbed by any shifting of the telescope. Since the year 1875, observations with the spectroscope have been carried on regularly with this instrument with great success. As an illustration, we have only to give the number of measures of displacement of either the F or b lines made in 1884, to show the great activity of the observers in this department of the Observatory. The total number was 731, consisting of 674 measures of the F line in the spectra of forty-eight stars, forty-nine measures of the b line in the spectra of twelve stars, and eight measures of the F line in the spectrum of the great nebula in Orion. In addition to these a large number of measures of the hydrogen or magnesium lines, with the corresponding lines in the spectrum of the Moon, were made, as a check on the general accuracy of the concluded results of the absolute motions of the stars in the line of sight.
It is in contemplation to remove the 12¾-inch object-glass, and to substitute in its place a new object-glass with an aperture of twenty-eight inches, which will enormously increase the penetrating power of this instrument. The tube for this great refractor has been made by Sir Howard Grubb, who has also for some time been employed on the figuring of the object-glass, and it is expected that the new refractor will be ready for mounting before the end of I891.
There are several other equatorials at the Royal Observatory, the principal of which is that formerly belonging to Mr. William Lassell, of Maidenhead. This instrument is a reflector, the aperture of the speculum being twenty-four inches, and it has a good record of successful work when in the hands of its late possessor. Observations are occasionally made with it in its present position, and it is considered to be an appropriate and valuable addition to the instruments of the Observatory.
The oldest mounted instrument in the Observatory at the present time is the small equatorial constructed by the celebrated optician Ramsden. This instrument, which occupies the small dome near the octagon room, and above the chronograph room, is still valuable for certain classes of extra-meridian observations, on account of the purity of the object-glass, which gives a most excellent definition of celestial objects. It was presented to the Observatory in the year 1811, by the representative of Sir George Shuckburgh. It was originally intended to be mounted as an altazimuth in the dome now occupied by the Sheepshanks equatorial; but some doubts were entertained of the firmness of the pier, and the idea was abandoned. This equatorial was subsequently erected in the position which it now occupies, and is used principally for the observations of occultations of stars by the Moon, eclipses of Jupiter's satellites, and a few other astronomical phenomena.
The object-glass of the Sheepshanks equatorial, mounted in a dome over the library, has an aperture of 6.7 inches, and the focal length of the telescope is rather over eight feet. The mounting of this instrument is that usually known by the name of the Fraunhofer, or the German mounting, the telescope being on one side of the axis, and counterpoised by a heavy weight on the other. This equatorial is furnished with a large number of eye-pieces of various powers, a double-image micrometer, wire micrometer, and other convenient appliances for carrying on almost every class of extra-meridian observations, such as the measurement of the angles of position and distances of double stars, occultations of stars by the Moon, the diameters of the planets, tile, observation of comets, both of their positions in the heavens and of the physical appearance of their nuclei. Clock-work is attached to the instrument, which enables the object under examination to be kept always in view, as the motion of the instrument also corresponds exactly with that produced by the diurnal rotation of the Earth on its axis. 1h~ diameters of Mars, Jupiter, and Saturn have been very accurately ascertained by the use of the double-image micrometer, the peculiar construction of which was devised by Sir George Airy.
The photoheliograph, with which pictures of the face of the Sun are taken daily, is an instrument of peculiar construction. The original instrument was made under the direction of the late Mr. Warren dc la Rue, for use at the Kew Observatory, and for some time was transferred to the Royal Observatory. That now employed at Greenwich is one of the instruments made specially for the transit of Venus expeditions. It has an object-glass of four inches aperture and five feet focal length, forming an image of the Sun half an inch in diameter. This image is enlarged by a secondary magnifier to four inches in the camera screen, where the sensitive plate is inserted, the whole length of the instrument being about eight feet. The exposure to the Sun is practically instantaneous, amounting to only a few thousandths of a second in ordinary cases. The definition of the solar spots and faculæ on the photographic plate is generally very clear. The areas and positions of all the spots are subsequently measured, and the results published in the annual volume.
The astrophotographic telescope, for the photographic delineation of a portion of the heavens, has only recently been mounted. It has been constructed in conformity with the scheme agreed to at the Astrophotographic Congress held at Paris in 1887, that all the instruments employed in this important survey of the heavens should be identical in all their essential particulars. This instrument is mounted in a new dome near the computing room, fitted with every convenience for the proper development of the plates.
The reflex zenith telescope is a small instrument designed for the observation of Gamma Draconis, which passes the meridian of Greenwich within a few minutes of arc from the zenith. The importance of the observations consists in the absence of the effects of refraction, and of the errors of the graduated divisions of a circle, on the angular measurement of the distance between the zenith and the star; an important consideration in the determination of some astronomical constants.
Most of the old instruments formerly used in the Observatory may be noticed suspended on the walls of the transit-circle room, or some other portion of the building. Among these we may specially draw attention to the well-known Troughton's transit instrument and mural-circle; Bradley's brass quadrant, on the western wall of the lower computing room; Graham's iron quadrant, which for many years was suspended on the opposite side of the same wall, in a room formerly devoted to the keeping of manuscripts, etc., but recently it has been removed to the west wall of the transit-circle room; Halley's transit-instrument; Bradley's transit-instrument ; and Bradley's celebrated 12½ feet zenith-sector. The last instrument had become famous long before Bradley brought it to the Observatory, for it was by means of this zenith-sector that this great astronomer in 1729 made his discovery of the aberration of light and the nutation of the Earth's axis. This sector was also employed so lately as 1838 in the operations for the verification and extension of La Caille's arc of meridian at the Cape of Good Hope, under the direction of Sir Thomas Maclear.
Most of the equatorials purchased for the transit of Venus expeditions, all of which are valuable instruments of six inches aperture, are preserved at the Observatory; and one, constructed by Cooke has been mounted for occasional use.
We have now noticed the principal instruments which have sustained the astronomical character of our National Observatory in the eyes of the scientific world; but there is also much in the daily occupations of the staff of the highest practical importance which is not only interesting to the savant, but also valuable in connection with the active business of life. Perhaps this intimacy between the inner and outer worlds can have no better illustration than that relating to the business of chronometers and the dissemination of Greenwich time to every part of the country, by means (if galvanic signals, transmitted at stated times to the chronopher at the principal telegraph office in St. Martin's-le-Grand, London.
The room in which all the chronometers are placed for the purpose of rating is situated on the first floor in the great equatorial building, immediately under the observing room. On entering this apartment, a visitor's attention is at once attracted by a universal buzz, sounding almost like the hum of a beehive, for sometimes more than two hundred chronometers, all delicate specimens of ingenious workmanship, are stored here at one time. Everybody has heard the ticking of his watch when placed on the table at night; fancy, then, the indescribable sensation of listening to so many as two hundred chronometers, all registering the time independently of each other, the ticking being considerably louder than ordinary watches. In this room are kept in store, and rated, all the chronometers belonging to the Admiralty which are not required for the immediate use of her Majesty's navy. During about six months, from July to January, a large number are placed here by the makers, on a competitive trial for purchase, the chronometers keeping the best time, with the least change of rate, being bought by the Admiralty after they have undergone a severe trial in different temperatures. A large closet, heated by gas, is in the corner of the room, for the reception of the chronometers when under trial for high temperatures.
Let us follow an assistant, while engaged in the winding and comparison of all these chronometers. He gently opens the lid of each box, winds the chronometer with great care, then as gently proceeds to the next in order, and so on till the whole two hundred are wound up. It is then usually the duty of another assistant to examine each chronometer, to prevent any accidental omission. When the winding is completed one assistant takes his seat at a table, writes down in a book the clock-times of comparison called out by the other, who is comparing each chronometer with one of the sympathetic galvanic clocks, which is always kept to correct time, and thus the error of each chronometer is ascertained, and the difference between this error and that recorded on the preceding day is its daily rate. The rapidity with which this is done is perfectly astonishing, especially when it is considered that the comparison is made to tenth parts of a second. No one, without considerable practice, can hope to rival the skill of the observers in this department. Every one can see the practical importance of the accurate determinations of the daily rates, and it is pleasing to know that the commanders of our noble ships can thus obtain an instrument brought by modern improvements to such a high state of excellence, and then afterwards preserved and rated so accurately for their use, that, with the knowledge of true Greenwich time when at sea, they may be able to guide their ships across tile ocean from shore to shore in safety.
The business of the chronometric department is very extensive. In addition to the duties already mentioned, the management both of the purchase and repairs of chronometer`, which includes the decision of their merits as affecting their price of purchase, the execution of repairs, etc., and the issuing and receiving them to and from the Government ships in commission, is under the sole superintendence of the Astronomer Royal. The Observatory, therefore, performs a national duty of the highest importance, which can well be appreciated by all who take an interest in the welfare of the navy.
Every visitor to Greenwich Park, or, indeed, all who have sailed up or down the Thames, cannot have failed to notice a black ball on the east turret of the principal building of the Observatory. At five minutes to 1 p.m., this ball is raised half-mast, and at about two and a half minutes to one, to the top. Precisely at one the ball is dropped. This public exhibition of correct time has been of the utmost importance to the mariner, who is able to compare the going of his chronometer with Observatory time, without the trouble and expense of coming to Greenwich. How necessary it is to the sailor, before leaving the docks, to know the error of his time-piece, on which he must so constantly depend while steering his course far away from land! The captain of a vessel is always anxious to start with the power of obtaining Greenwich time at any hour of the day or night; he therefore, while stationed in the docks, ought to compare his chronometer at the same hour on several days in succession, by which means he can determine how much it gains or loses in a day. The regular dropping of the Greenwich time-ball gives him an opportunity of making this necessary comparison. A good chronometer will generally have the same rate for a long period. On sailing, when the officer knows the error and rate of his chronometer, by adding daily this rate he is able to fix his longitude from day to day by his sextant observations giving him local time, the difference of which from the true chronometric time determines the longitude of the position of his ship at sea from Greenwich.
This principle of exhibiting the correct time by the dropping of a ball at a stated hour has been carried out in various places throughout the country. At Deal, however, there is a ball dropped at 1 p.m., by a direct galvanic current from the Observatory, the ball being erected for the benefit of the shipping lying in the Downs. After the ball is dropped, a return current is transmitted from Deal a few seconds after one, to inform the assistant at Greenwich that the time of drop was correct. The value of this ball is much appreciated, as it gives the master of each ship the opportunity of checking is chronometer at the last moment before sailing, by which he obtains the accurate error of the chronometer from Greenwich time on leaving England.
Several other time-balls are dropped indirectly by the Greenwich galvanic current; but the Observatory is responsible at present only for that of Deal, which is placed by the Admiralty under the control of the Astronomer Royal; all other time signals are obtained from the Post Office authorities.
The mean solar standard clock, which is accurately set to correct time early every morning, sends a signal to the Central Telegraphic Office every hour, which is distributed throughout the metropolitan district. To the country post offices the correct signal is usually sent twice a day, at 10 a.m. and at 1 p.m. For this purpose a most elaborate contrivance exists at the Central Telegraph Office, called the chronopher, by means of which the Greenwich signal is transferred instantaneously to the various connecting wire., and transmitted to all the principal post offices in the kingdom, or to any private establishment in return for a fixed yearly consideration. The greatest punctuality is exercised in all matters relating to the transmission of galvanic signals to the Central Telegraph Office. For the convenience of the Observatory itself, galvanic sympathetic clocks, in connection with the mean solar standard clock, are placed in several of the apartments, and an additional one of larger size, already mentioned, near the entrance gate. The latter is made much use of by the public, for the regulation of their watches. The majority, however, consider it is some mysterious clock placed there to puzzle the visitors, many of whom have some difficulty in understanding the meaning of ten minutes past fourteen, or half-past fifteen, the dial being divided into twenty-four hours, beginning at midnight. There is generally some one sharper than the rest, who is able to explain this ‘curious’ clock.
The octagon room occupies the whole of the upper floor of the principal building. In the time of Flamsteed it was often used as an observing room. Here the Board of Visitors hold their annual meeting on the afternoon of the first Saturday in June, when a voluminous report of the proceedings of the Observatory during the past year is presented and read by the Astronomer Royal. The octagon room is an elegant apartment, embellished with busts and portraits of several distinguished scientific men, most of whom were noted astronomers or members of the Board of Visitors. The monogram of Charles II., the founder of the Observatory, may still be noticed on the cornice over the middle of each of the six windows. Occasionally, the octagon room is used as a supplementary computing room, and at the present time it is used as such by several lady computers.
The magnetical and meteorological observatory is in the south ground, and is reached by a narrow path. It is a building in the form of a cross, built of wood, and contains three principal instruments on the ground floor, disposed in such a manner as to influence as little as possible the peculiar motions of each other. One is a suspended magnet, intended to measure the magnetic declination, or variation of the needle: another, twisted, by means of the lines which suspend it, away from the magnetic meridian, so that it hangs nearly east and west, measuring the variation in the horizontal force of magnetism; while a third, which is similar to a beam of nicely poised scales without the scales attached, measures the variation in the vertical force. The deviations of these magnets were formerly observed every two hours, night but for many years, thanks to the improvements in photography, the changes in the positions of the magnets have been automatically registered in a more effective manners, superseding entirely the system of day and night watches, practised for so many years. The automatic registration of the magnets is made by instruments placed in the basement of the magnetic observatory, so that they- may not be affected by sudden changes of temperature. This basement contains the three registering magnetometers; a registering barometer; a clock for recording the times on the sheets by causing, at intervals, an interruption of the photographic traces; a mean time clock; and the sidereal standard clock of the Observatory. The magnets arc so arranged that the declination and horizontal force traces are registered on different parts of the same horizontal cylinder, and the traces of the vertical force and of the registering barometer are recorded on one vertical cylinder. The daily photographic sheets, including photographs of the variation in the readings of the dry and wet thermometers, are at once properly marked and timed, and preserved for reference. The principal deviations of the three magnetometers are afterwards extracted and published in the annual volume of the proceedings of the Observatory.
Besides the three principal magnetical instruments, observations are frequently made with a unifilar magnetometer. This instrument consists of an apparatus for deflection of a magnet, and another for vibration, corresponding to the two parts of the process by which the absolute horizontal force of magnetism is determined; the experiments of deflection consist in observing the angular deflection of a suspended magnet, produced by the influence of a second magnet, which is placed on a support at one or more distances from the suspended magnet; the experiments of vibration consist in suspending the magnet which was used as the deflecting magnet in the former experiment, and then observing its time of vibration. Observations for the determination of the magnetic dip, or inclination of the needle at Greenwich, are also frequently made.
In addition to the magnetical observations, the observers in this department note daily, at stated hours, the height of the barometer, the readings of thermometers of various kinds placed in different positions, and numerous other meteorological observations. For example, the temperatures of the air in the shade, as indicated by dry and wet thermometers; the extremes of temperature as shown by the radiation thermometers, as the highest reading with the thermometer exposed to the Sun, and the lowest reading at night with the thermometer placed on the grass; the temperatures of the ground at depths varying from one inch to twenty-four feet, are all regularly recorded. The amount of rain collected in several rain-gauges at different altitudes is ascertained; the amount of cloud is estimated; and many other weather facts are recorded, independently of the photographic registration of the variations of the pressure of the atmosphere and of the changes of temperature. The direction of the wind is registered by Osler's anemometer, placed in the western turret over the octagon room. A large vane, from which a vertical shaft proceeds down to the registering table within the turret, gives motion, by a pinion at its lower end, to a rack-work carrying a pencil. The pencil marks a paper fixed to a board moved horizontally and uniformly by a clock, in a direction transversely to that of the motion of the pencil, and it oscillates according to the Movements of the vane. The sheet of paper is divided into lines corresponding N., S., E., and W. of the vane, with transversal hour lines; the direction of the wind may therefore be easily found for every hour of the day and night. The instrument also registers the force per square foot of the wind, and the amount of rainfall. A second anemometer was devised by tile Rev. Dr. Robinson, of Armagh, and is chiefly intended to register the velocity of the wind. A record of the amount of sunshine is kept, as registered by an ingenious instrument contrived by Mr. J. F. Campbell and afterwards modified and improved by Sir George G. Stokes, consisting of a sphere of glass nearly four inches in diameter supported concentrically within a well-turned bowl, in such a manner that the image of the Sun, formed when the Sun shines, falls always on the concave surface of the bowl. The registration is made according to a plan suggested Sir G. G. Stokes. The electrometer now in use was contrived by Sir William Thompson, and consists of an aluminium needle hanging from an insulated Leyden jar, and taking up, within insulated quadrants, a fixed position under a constant electrical charge. The atmospheric electricity is collected by a Thomson's water-dropping apparatus. The needle carries a small Mirror which reflects the light from a slit illuminated by a gas-jet through a semi-cylindrical lens to a drum covered with sensitized paper and moved by clock-work, and on this the variations in the amount of electricity in the air is automatically registered.
Abstracts of the meteorological observations are sent weekly to the Registrar General, and published in the weekly report of births and deaths.
Most of the calculations in the astronomical department are made in the two principal computing rooms, which occupy the central part of the old buildings facing the courtyard, the lower room communicating with the upper room by means of a spiral staircase. The Astronomer Royal's official room is between the lower computing room and the transit-circle room. A glance at the numerous documents and letters on all kinds of scientific subjects, almost covering his table, is sufficient to give an idea of his extensive correspondence, not only on astronomical subjects, but also on a great deal of miscellaneous business; while maps, models, and drawings vouch for the varied nature of the vast scientific employment of his never-resting mind. But let us walk quietly into the next rooms, the lower and upper computing rooms, and admire the order which is evident at a glance. If we look over the shoulders of some of the assistants, we shall probably find that one computer, generally a junior, is entering the observations made on the preceding night, while another is employed on an advanced stage of the computations, referring, minute after minute, to a rather ponderous-looking book, full of logarithmic tables; and a third is arranging in order for binding some of the manuscripts and letters of the preceding year. The principal assistants are generally occupied in the examination or revision of what is performed by the computers. These duties are never ending: as soon as one set of computations is finished, another is ready to be commenced; indeed, the quantity of work involving a considerable amount of monotonous labour performed in these computing rooms is almost marvellous.
But computing is not the only day-duty of the assistant. Observations are also made during the computing hours, when practicable. The Sun, Venus, Mercury, and most of the principal stars can be seen and observed at all hours of the day with the transit-circle. It is therefore the duty of the assistant in charge of the observations for the day to be always on the look-out in clear weather for any of these objects that happen to pass the meridian in the daytime. The arrangement of the observing duties of the assistants for the week is drawn up, as we have already stated, every Monday morning, generally by the Chief Assistant, and afterwards submitted to the Astronomer Royal for his approval. It is then placed in a prominent position for the daily inspection of the assistants, who can then see at once all their observing engagements during the ensuing week. Each observer's watch usually extends, for the transit-circle observations, from six o'clock in the morning till three o'clock on the following morning. There are, however, occasional modifications of this rule, and it is only on rare occasions that the watch extends to the extreme limits. The subjects of observation are various. The assistant is expected to observe the positions of the Sun, Moon, all the major planets, the brightest of the minor planets, stars specially required for the determination of the error of the sidereal standard clock, and stars of all classes that are required to be observed for some particular purpose. With the altazimuth, when the Moon was regularly observed throughout the lunation, the observer was required to watch from moonrise to sunrise, or from sunset to moonset, unless he was able to secure a successful series of observations in the interval. The duty of watching for the Moon to appear is exceedingly harassing in cloudy weather, as it frequently necessitates the disturbance of the night's rest of the observer. Occultations of stars by the Moon; eclipses, transits, and occultations of the satellites of Jupiter; or of any other occasional phenomenon usually observed with the equatorials, are also provided with observers, who are expected to attend to the respective observations. Visitors to the computing room, though it is too sacred a place for many to have this privilege, are often interested to notice how quietly an assistant leaves his calculations to perform some observing duty. He may be seen looking, intently at the face of a chronometer set to sidereal time, while he is noting the number of minutes that will elapse before the Sun, or some other important object which it is his duty to observe, passes the meridian. At the proper moment he may be seen to leave the room, book in hand, when, having completed his observation, he returns to his seat to resume his calculations. In fact, what strikes a visitor most, on a casual inspection of these computing rooms during the busy hours of the morning, is the quiet, orderly manner with which everything is done. Every one knows precisely what his duty is, and how to perform it to the satisfaction of the Astronomer Royal. Ten to twelve assistants and computers are generally present to perform the reduction of the astronomical observations, and to carry on the business correspondence of the office. The personal staff of the Observatory consists of the Astronomer Royal, who is appointed by the Prime Minister, and holds his warrant of office under the Royal Sign Manual; the Chief Assistant, who is of a rank superior to the other assistants, and competent to represent the Astronomer Royal in his absence four first-class assistants; and four second-class assistants. The assistants are appointed after a severe competitive examination, under the control of the Civil Service Commissioners. The number of computers is variable according exigencies of the service; but usually about fifteen to twenty are employed.
Considering the enormous amount of correspondence on questions affecting different sciences-for we have already said that the Astronomer Royal is frequently occupied on important business unconnected with astronomy-and the yearly increase of folio volumes of computations, it is not to be wondered at that considerable space is required for the reception of the accumulation of manuscripts from year to year. A new fireproof record room was erected in 1855, which contains the largest portion. Many of these manuscripts are of great interest and value. In this room are stored those of Flamsteed, Halley, Bradley, Maskelyne, Pond, Airy, and some collected by the present Astronomer Royal.
All the correspondence of the Observatory, of both receiver and sender, is preserved on the shelves of the record room, or in fire-proof closets in the Astronomer Royal's official room; and also all the calculation books, from the date of the appointment of Sir George Airy, are ranged in order, so that they are capable of being referred to at any moment. In fact, we can realize with perfect confidence the remark of the great French astronomer, Delambre, that if the whole edifice of astronomy were by any series of casualties to be destroyed, it might be rebuilt with the rich store of materials gathered together within the walls of the record room of the Royal Observatory. The yearly accumulation of papers is very great, and, owing to want of space, some of the least important manuscript volumes have been placed in one of the libraries.
It is not difficult to perceive, from the preceding notes, that the assistant-astronomers of the Royal Observatory are not occupied on the most interesting and romantic branches of astronomy, for their attention is not often directed to the mere gazing at planets, stars, or nebula:, to the watching of the changing appearances of the spots on the Sun, or to the varying illumination of the mountains in the Moon, the observation of which affords so great a charm to the intelligent amateur astronomer. But it is to the regular observation of the right ascension and declination of the Sun, Moon, planets, and stars, as they pass the meridian at all hours of the day and night, and to observe the position of the Moon in the heavens from day to day with the altazimuth, that the observer's attention is particularly directed -a class of observations that requires not only every attention in the use of the instruments, but also brings in its train such an enormous mass of intricate computations as none but professional astronomers could for a moment undertake. Add to these the difficult and delicate observations with the spectroscope, the scrutiny of the face of tile Sun by photography, and the astrophotographic survey of a limited zone of the heavens, and we shall have some slight conception of the valuable astronomical researches carried on in our National Observatory.
In addition to the internal work of the Royal Observatory, Sir George Airy the late Astronomer Royal, undertook several personal scientific investigations of great importance, especially one of great magnitude, by which the progress of physical astronomy has been much accelerated in the most marked manner. The reduction of the observations of the Sun, Moon, and planets, made at the Royal Observatory from 1750 to 1835, was an enormous undertaking; and, though a considerable staff of computers were employed, this work required more than ten years to complete. Sir George Airy was also foremost in observing the total eclipses of the Sun of 1842, 1851 and 1860; in researches on compasses in relation to iron-built ships; in the determination of the longitudes of Paris, Brussels, Washington, Alexandria, the island of Valencia, Glasgow, Edinburgh, Cambridge, and other places; in determining the density of the Earth by an elaborate series of pendulum experiments made in the Harton Colliery, near South Shields; in the arrangements of the observers and instruments in the expedition for the observation of the Transit of Venus in 1874; and in a vast number of other researches, too numerous to mention-so many are they that a total enumeration of Sir George Airy's contributions to astronomical science since he took the highest academical degree at Cambridge in 1823, would be giving a complete abstract of the progress of astronomy in the nineteenth century.
Before closing this brief account of our National Observatory, there is one class of correspondence which, during the author's long connection with it, he has never known to fail, and which should be alluded to here, to show that, even in this the nineteenth century, there are paradoxes of all kinds, both scientific and social, who call upon the astronomer for advice under difficulties. For it must be acknowledged that the Greenwich astronomer, in addition to his stated public duties, is also very generally supposed to devote some attention to astrology, if we consider how often applications are made to him for some anticipatory information of the events of the life of the applicant. This belief in the powers of the astronomer has evidently descended from the days when he rose very little higher than the astrologer in the estimation of all classes of the public. It must be confessed that in the early days of astronomy, before the invention of the telescope, the astronomical student had little hesitation in occasionally practising the black art, either on his own account or for the benefit of some anxious inquirer after the secrets of futurity; and therefore it was a natural consequence that the astronomer and astrologer were frequently found in the same individual. Even the celebrated Flamsteed amused himself; on his accession to Greenwich, by drawing the horoscope of the Observatory, which is still preserved among the archives in the record room. The belief in nativity casting is considered in this modern educational age to be well-nigh exploded; but still there are many persons - and those not always the most humble in station - who believe that the Greenwich astronomer is not continually contemplating the starry heavens in vain. This is very evident from the fact of individuals calling frequently at the Observatory gate, requesting information about their future destiny; letters even have been received enclosing Post-Office orders, requesting a nativity cast in return. The following letter, received in 1876, we give verbatim, as an illustration of many such:- ' Female, born August 13, 1844, at twenty minutes past eleven in the morning, wishes to know if a young man who has paid her attention for some time if she is ever likely to be his wife, and when will he be prosperous in life.' Stamps to the amount of one shilling were enclosed. On one occasion, a well-dressed young woman, apparently in great distress, called at the author’s private residence, to know whether an uncle who had gone somewhere in the Pacific, and who had not been heard of for several years, was dead or alive. She left in tears, because she was informed that the stars were unable to satisfy her wishes. But, as a final example of the march of intellect in the nineteenth century, we cannot give a more appropriate conclusion to our astrological experiences than by inserting a copy of a letter which was received more than thirty years ago:- 'I have been informed that there are persons at this Observatory who will, by my inclosing a remittance and the time of my birth, give me to understand who is to be my wife. An early answer, stating all relative particulars, will greatly oblige.'
It has always been the object of successive Astronomers Royal to make the fundamental work of the Observatory coincide with the terms expressed in the original warrant of 1675. How faithfully this ruling idea has been carried out is shown in the sublime works of the modern physical astronomer, who never could have devised the accurate theories of the motions of the Earth, Moon, and planets, without relying on the Greenwich observations of the last 140 years. Lieut. Winterhalter, U.S.N. who was the delegate of the United States Naval Observatory at the astrophotographic congress held in Paris in 1887, has truly remarked, that 'although the instruments and buildings at Greenwich have been repeatedly changed, and the methods of using, them as well, when required by the advance of astronomical science, the objects had in view from the foundation of the Observatory have been steadily pursued. The utilitarian spirit in which it was founded and maintained, and the well-defined purposes of its existence unswervingly carried out - to these must be ascribed in large measure the pre-eminence in its particular sphere which it holds in scientific and public estimation. The distinguished astronomers who for long periods have directed its labours, while impressing upon their administration their individuality, have consistently subordinated their personal work and inclination to the fundamental idea, for its faithful and systematic execution. However great has been the extension of the work during two centuries, that idea is still the governing one. It has never been attempted at Greenwich to develop all branches of astronomy, but those undertaken may in their present state well be taken as models. We are moved to respect the order, regularity, and efficiency of the Observatory in the manifold branches in which it engages. It may well be doubted whether these can be found in a greater degree elsewhere.’
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