II. THE LAMBETH OBSERVATORY.

By ROBERT JAMES MANN, M.D., F.R.A.S.

In the outer courtyard of the Government India Store, situated in the Belvedere Road, Lambeth, and on the bank of the Thames, on the direct line of river-side thoroughfare between the Waterloo and Westminster Bridges, there stands a very complete and pretty little observatory devoted to severe philosophic work, which would certainly not be looked for by the uninitiated in this locality and in these surroundings. The establishment is of modern date, and but little known. Its history, and the reason for its occupying this site, are, however, simple and plain. It rose to its present position when the Indian Store disappeared from the old and traditional ground of Leadenhall Street, and when the Indian administration. took westward wing to find its new and more convenient home in Downing Street. At that time it was thought meet to provide a lodgment for the store department nearer to head-quarters, and a very commodious building was accordingly erected in the Belvedere Road, and opened in the month of February, 1864. The general purpose of this building is to furnish a temporary warehouse in which all articles destined for the military, medical, and educational branches of the Indian service may be received, examined, and packed for shipment.

A considerable number of scientific instruments of various classes have for some time been annually sent out for use in India under the auspices of the Indian Government. In the old days of the Indian administration, the custom in regard to such instruments was that a sealed pattern of each kind of instrument in occasional demand was kept in the store department attached to the establishment in Leadenhall Street, and whenever a supply was required for service in India, the instrument makers of England were invited to send tenders of the prices at which they would undertake to furnish the required articles, constructed in exact imitation of the sealed patterns. In general practice, the maker who offered to supply at the lowest price received the commission; and when it was executed the instruments were sent in to the store, superficially compared with the pattern to see that they were of the kind that had been ordered, and were then shipped off to India, and the transaction was held to be complete.

In this proceeding the real excellence of the instrument, its real fitness for the satisfactory performance of the work it was designed to accomplish, was altogether left to the accident of the way in which the maker might be able or willing to perform his

share of the bargain. It was assumed that if the implements looked like the pattern, it would be all that could be desired, and if by any accident it was discovered that it was not all that was desired, the responsibility was conceived to rest upon the shoulders of the tradesman who had thus badly executed his order. This system was obviously not likely to prove satisfactory where the delicate and exact work of observational science was concerned. In the first place, the sealed pattern assumed a perfection in instrumental construction which will not be obtained after many more centuries have been consumed in unceasing improvement; and in a sense also ordained that Indian science should operate with obsolete instruments, instead of with the best that could be produced in the existing state of cosntructive art. And in the next place, the best makers who had a reputation for excellence to sustain were more or less excluded from the Indian orders because they were expected to compete with men who looked to profit out of low prices, rather than to furnish excellence of work.

It happened while matters were in this position that Colonel Strange, who was the originator, and is now the life and manager of the Indian Store Observatory, was requested to examine a quantity of instruments that had been sent to England for repair, before they were again shipped off for India. In performing this duty, he was constrained to urge that the greater part of these instruments, about which a considerable expenditure had been incurred, were really of an obsolete form and useless for practical work; and to recommend that the whole should be sold for anything that could be made of them, rather than that the serious injury should be inflicted on officers in India of giving them tools that could only furnish erroneous and misleading observations.

The instruments accordingly were withdrawn from the service, and the incident had also a further and more practical result. It opened the eyes of the authorities to the need of a different mode of procedure, and arrangements were forthwith entered upon to secure at least a standard measure of excellence in all instruments sent out to India. At first Colonel Strange was commissioned to examine batches of instruments of various kinds fitfully, as they were prepared for shipment; but after he had pointed out that this could only be serviceable when means of efficient examination were available, he was requested to submit a recommendation as to the appliances which were requisite for the scrutiny. The recommendation was that a fixed observatory should be established for the work, and a plan of the character and instrumental furniture of this testing observatory was suggested. The plan was formally and officially examined by the Astronomer Royal, the President of the Royal Society, and the Director of the Ordnance

Survey; and these practical authorities pronounced that the proposal was sound in principle, and likely to be efficient in operation. The order was then issued that the observatory should be constructed; and the construction, and subsequent management of the establishment, were placed in the hands of the designer, Colonel Strange, who in addition to a natural taste for, and capabilities of, dealing with delicate and fine mechanism, had enjoyed the field experience of thirteen years of service upon the great trigonometrical survey of India.

The first matter that had to be decided in connection with this design was the place where the fixed observatory could be planted. Up to this time all instrumental examinations had been made in the old building that had been used as the Indian store, and that stood opposite to the ancient Company's "House" in Leadenhall Street. It therefore was deemed only natural that the instruments should follow the store, and that the observatory should be planted on the ground acquired by the department in Lambeth; the site was obviously not the best in the world for the purpose, but as no other available position could on the instant be fixed upon, it was ultimately resolved to do the best that could be done in connection with the Lambeth store, and the existing observatory was commenced in the year 1865.

The earliest proceeding was necessarily to neutralize the natural unfitness of the ground and position for exact instrumental work, so far as this could be done. This has been most admirably and most efficiently accomplished, and as follows:-First, twelve iron screw-shod piles with the widest part of the spiral thread 2 feet in diameter were wormed into the gravel, which here lies 24 feet below the surface of the ground. Broad heads of cast iron were next keyed upon the top of the piles. Thick slabs of flag-stone were then laid upon the pile-heads, and the space beneath filled in with concrete cement to the depth of about three feet. A circular platform of brickwork was finally constructed upon the flag-stones, and upon this, two semicircular segments of wall were reared round the circumference, and a solid pedestal of masonry was erected in the centre. This circle of wall, cut by two gaps at opposite points of the circle, and this enclosed central pedestal are the objects which now form the visible base of the observatory, and which carry the chief instruments employed in the scrutiny. A wooden platform, or floor, between the circular wall and the central pillar, to which access is obtained through the gaps, is carried upon beams that are supported from beyond the system of pile-work, and is carefully isolated from all mechanical connection with the piled foundation, so that the observer moves about upon this floor without effecting any disturbance in the position of the instruments carried upon the walls and the pedestal. This method of securing

a firm foundation upon so unpromising a base as a river-bank shaken by the incessant rolling of heavy carts, has been found to be very efficient, if not practically perfect. Since the first setting of the masonry of the platform not the slighest permanent change has occurred in the bearings of the instruments. Passing carts communicate a slight transient vibration for the moment, which, however, has no other inconvenience than causing an occasional brief suspension of observation.

Upon entering this little model testing observatory of the Indian store, then, the general aspect is that of a square room with a raised circular platform in its centre, reached by a small flight of stairs. A broad solid wall-like rim, breast high, and included within an outer suspended pathway for the observer, runs round the platform, and affords a convenient resting-place for instrumental appliances of various kinds. In the midst of this circle a flat-topped pillar forms the bed upon which the instrument to be tried is placed by the examiner.

Overhead the broad glass roof, with the uninterrupted space of clear sky, indicates that the star-rulers of the night can be appealed to whenever such higher and more refined arbitration is deemed desirable.

One of the most important objects that is attempted in this observatory is the examination of the exactness of graduated circles that are designed for horizontal measurement. Unless these circles do measure equal and true degrees in all their parts, it is obvious that the angular intervals recorded from them are not worth the paper upon which the records are inscribed. To accomplish this end four horizontal tubes, called collimators, have been placed on different parts of the circular wall, so that an observer can contemplate each of them from the telescope attached to the instrument under trial. These collimators are in reality only so many fixed and immovable points, occupying for the time known, or more properly ascertainable, positions on the great wall-circle, and therefore including also known, or ascertainable, angular intervals between them. The exact reference-points, or virtual centres in these collimator tubes-"the marks to be collimated or aimed at " -are of a varied character. In one there is a system of diagonally crossing spider-threads, and in another of horizontally and vertically crossing threads, forming the reference-points by their intersection. In yet another tube an artificially fixed star is formed by throwing gaslight through an exquisitely minute aperture, which has its own image reproduced in the focus of a convex lens, so contrived as to fall only one-fifth of an inch from the curved surface of the glass, a proceeding which practically reduces the image of the simulated star to the very smallest dimension to which it is possible to compress it. The four reference-points, or collimators, are,

for convenience sake, so placed upon the wall that the angular intervals included between them on the circular arc are respectively 30, 60, 120, and 150 degrees. This arrangement, by varying the pairs of collimators used, allows twelve different angles to be tested. The test, in plain language, is simply reading these angles off as they are given by the circles of the instruments, and then shifting round the circles again and again, so that exactly the same work is performed in succession by different portions of the same graduated rim. If the graduations of the circle are correct and trustworthy, the angles read off between any given pair of collimators will obviously always have the same value whatever portion of the circle be employed in the reading. This test is a very accurate and a very severe one. The slightest inexactness and failure in the mechanical work at once becomes glaringly obvious, and can be estimated as a question of amount as well as of fact. In practice the process is repeated with each instrument on successive occasions, to test permanence as well as exactness of construction and performance; to see that there are no weak and yielding points, or shifting screws, or attachments, involved in the structure. If the reference-points were simply so many fixed spots, established upon the circular wall, it would be necessary that every graduated circle placed under examination should be truly centred upon one point with the most refined exactness. This necessity is practically avoided by placing the reference-points in tubes behind, or beyond, curved lenses of glass, which have the useful property of always sending parallel rays to the telescope of the tried instrument, and therefore always securing the invariability of each angular instrument however the centre of the reading circle may lie. Colonel Strange has satisfied himself by actual trial that the centre of a graduated circle, or arc, may be shifted a fifth of an inch without making any appreciable alteration in the value of the angles read. It is this especial virtue which converts a mere visible fixed point of reference into what is technically known as the "collimator."

The testing of vertical circles is a far more difficult piece of business than the trial of horizontal ones, for this reason-in vertical circles the telescope which is employed in making each observation is rigidly connected with some unalterable radius of the circle, and any given angle included between reference-points cannot therefore be applied over and over again to different portions of the circle. This however is not of very much moment, because the vertical and horizontal circles of any given maker are turned out by the same graduating instrument, and if the horizontal circle is found to be graduated correctly, the vertical circle may with some confidence be assumed to be of the same excellence. In the case of the larger and more important instruments the examination is rigidly carried out by observing standard Greenwich stars as they