W e have deferred our account of the Astronomical Observatory until the principal Instruments which form its fundturo were described. We cannot attempt such a minute delineation as would suffice to guide any one who wialled to erect such a building. It will be enough to state some of the properties which a well-contrived observatory should possess, and this may help an intelligent person to form a judgment after examining several of those which exist.
An observatory, as was well remarked by Romer, is nothing more than a covering for the instruments and a protection for the observer from the inclemencies of the weather. This should be steadily borne in mind by an architect who is called upon to furnish a plan for such a building, especially if he be limited in cost. The best situation is a gentle eminence which commands an uninterrupted view of the horizon, but which should not be abrupt or very high. The observatory at Cambridge is nearly perfect in this respect. The best foundation is undoubtedly rock ; the foundation must be solid enough not to change rapidly. Absolute immoveability is not to be obtained. The supports of all instruments which are carefully watched show slow movements depending either on temperature or moisture, or some causes which are more obscure. No rule can be given as to the depth at which the foundations of the piers supporting the instruments should be laid, The deeper, broader, and more solid the better. The outer earth should not touch the base below the surface, end the outer walls should freely from it. The floor should be quite clear of a pier or its foundations. With these precautions the change of position in the instruments will be very slow, and when it is slow and uniform the effect is easily taken into account. In some of the earlier observa tories, the instruments are placed high above the ground, and in several of the Italian and some Continental observatories they are at. the summits of lofty towers. We need not say that any elevation beyond that which is required to command the horizon and keep the building dry and well ventilated is injurious. If distant meridian marks can be erected to the N. and S. it is of advantage, but this con dition is not essential.
The instruments which are required for an observatory depend of course upon the class of observation's which are to be pursued there. It has been too much the custom to build observatories nearly alike and to pursue exactly the same objects. We shall mention those instruments in order which may be considered important enough to give the name of observatory to their enveloping buildings.
The transit and its clock. These, on some scale or other, are re quired by almost every observer, as the time enters nearly into every observation, and a transit is the best instrument for getting it, and a good well-fixed clock is wanted for keeping the time when got. In principal observatories the transit is generally from five to ten feet focal length. We think the latter size unnecessarily large for the objects usually observed, and faint objects which require light, and consequently a telescope of large aperture, might be turned over to the large equatorial and micrometer. A 5-foot transit with an object glue of the best quality will show everything that is usually observed, as well as the largest instrument, and is much more manageable. It can be levelled by one person with a hanging level end reversed by hand. The time can be got to the tenth of a second by a 45-inch or by a 30-inch transit, if they are of the beat kind and well handled, so that these are sufficient for the meet delicate determination of time. The larger transits are necessarily supported between two atone pillars, and we strongly recommend that the smaller transits should also be so mounted, when practicable, and on a sound and detached founda tion. The clock is firmly fixed to its own insulated pier, or against a solid wall where there Is not room or convenience for a separate pier. The beat situation is with its face looking towards the observer when he is looking south, and so that the observing chair is not likely to strike it. The windows should be on the north side of the transit and circle rooms.
The meridian circle, which, In England at least, is always a circle, and, so far as we know, there has been hitherto no material improvement in the plan first followed by its inventor, Troughton. Perhaps if the circle were cast in one piece, the whole would be firmer. The standard size of the mural circle is six feet, but it has been made of eight, of five, and of four feet. The transit and circle require two observers, and consequently separate apartments, as the transit observer while counting the clock would be disturbed by any noise. Where two observers cannot be afforded, a transit circle may be made to answer both purposes, and such are used, in preference it would seem, in the German, Russian, and Italian observatories.