| Date: | 1861 |
| Author: | Anonymous |
| Title: | The Equatorial in Greenwich Observatory |
| About: | Published in The Illustrated London News on 24 August 1861, p.205, this is probabally the earliest published article about the Great Equatorial following its erection in 1860. The the ‘official account’ of the instrument was published in the volume of Greenwich Observations for 1868. |
| Images: | One |
[Note: The illustration will be added as soon as time allows]
THE EQUATORIAL IN GREENWICH OBSERVATORY.
This splendid instrument, which forms the latest addition to the instrumental equipment of the Royal Observatory, was completed in the spring of last year, its construction having occupied several years. It was designed by the Astronomer Royal, G. B. Airy, Esq.; and his plans were carried out by Messrs Ransomes and Sims, of Ipswich, to whom the whole of the engineers' work was intrusted; Mr. Simms, of Fleet-street, supplying the optical parts, with the exception of the object-glass, which was furnished by Messrs. Merz and Sons, of Munich.
The instrument is erected on the third or highest floor of an edifice specially built for its reception, and is surmounted by a movable dome. The weight of the large masses of iron forming its component parts are supported and stability obtained by massive piers of solid brickwork and masonry, sinking several feet below the general foundation of the building, and rising to within a few inches of the floor of the room represented in the Engraving, Upon these piers are placed the supports for the upper and lower pivots of the framework generally known as the polar axis, which in this instrument is composed of a pair of skeleton prisms, each formed of three bars of wrought iron, tied together by diagonal and transverse bracing and thrusting-rods, and connected with each other at each end by oval frames having pivots at their centres turning on friction-wheels carried by the supports above noticed.
Midway between the extremities of the polar frame, and firmly fixed I' to it, are the supports and Ys of the declination axis. To this axis is firmly attached the telescope- tube and the declination-circle, a circle of cast iron 5 ft. in diameter, carrying on its circumference a band of silver divided into degrees and parts of degrees, and read by two microscopes at one end of the axis, reached by a swinging ladder hung from the roof of the revolving dome. The other circle seen in the drawing in nearly the same position is the clamping-circle: it is fixed to one of the prismatic cheeks of the polar frame.
The telescope-tube is of mahogany. It holds at one end the object-glass 13 in. in diameter and 18 ft, focus; at the other the flanges and tubes necessary for carrying the peculiar descriptions of eyepieces requisite for the various observations this instrument is designed to make.
Upon the lower pivot of the polar axis turns the hour-circle, 6 ft. in diameter, graduated on a silver band round its circumference into hours, minutes, and seconds, and read by microscopes fixed to the lower oval frame. When the instrument is in use this circle is geared with the driving- clock, the motive power being water acting by means of a reaction-machine or Barker's mill, and the regulating power a conical pendulum controlling the speed of the mill by an apparatus known as Sieman's [Siemens’] chronometric governor.
In making astronomical observations it is absolutely necessary that the position of the observer be perfectly easy and comfortable. For this purpose the instrument is provided with an adjustable chair or couch carried by a framework so constructed as to command easy access to the eyepiece in any position of the telescope, and furnished with adjustments and contrivances so complete that the observer can, without taking his eye from the telescope, move either himself, the instrument, or the dome into any position he pleases. The theatre-steps and inclined circle seen on the right of the drawing are for the purpose of supporting the observer when from the position of the object under examination the before-mentioned chair cannot be conveniently employed.
We may here remark that the sanctity of the observing room is seldom disturbed by the presence of such visitors as the artist has inserted in the drawing: they are introduced merely to relieve the monotony of the picture, and to give an idea of the general proportions of the instrument.
On the merits of this equatorial it is almost needless to comment. Suffice it to say that every want that modern astronomy could suggest, and every appliance that modern science could devise, have been supplied, and the result is that the ideas of the practical astronomer and the workings of skilful mechanics have produced an instrument that will long remain a monument of its designer's skill.
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