The transit, meridian circle, and clock are the instruments on which exact astronomy is founded, and they differ from other astronomical instruments in the observatory they require. This should be for each a square or oblong room, from 14 to 20 feet in its smallest direction, namely, north to south, and 10 or 14 feet high, with a slit of from 18 to 30 inches wide, cut in the direction of the meridian through the roof and side-walls, to about six Inches below the height of the centre of the telescope. As this must be open during observation, It must be closed to exclude the weather by shutters easily removed. If the building is not very large, the vertical slits may be secured by a shutter in one or two pieces, and the top by one or more shutters turning on a hinge, pretty much like a box-lid, in a way that any good carpenter will understand. A specimen of what appears to us a very good fundamental observatory is the working part of that at Oxford, consisting of two square rooms like those described, separated by a small waiting-room between them and by au entrance. The slits in larger rooms may be closed by shutters which elide back and forwards by pulling the ropes attached to them. The shutters, where the opening is wide, are in halves, meeting in the middle. This is the Greenwich abutter at Brussels the shutters are in one piece. At Cambridge the shutters are double, as at Greenwich, and the ropes connected with both sides are drawn simultaneously and either way by turning a winch. But whatever be the plan of the shutters, the opening must be wide enough to let the air within and without the building come to the same temperature very nearly. A telescope never performs well optically which peeps through a narrow slit, and the refraction must always be uncertain where the temperature of the air varies rapidly and irregularly.
The next instrument is the equatorial, which, when it is used for exact measurement depending upon the graduated circles, should scarcely have a larger telescope than one of five feet, with circles of from two to three feet diameter. Such an equatorial is intended at times to supply the place of the meridian instruments when the phenomenon observed is not visible on the meridian, and yet where the observations cannot be confined within the limits of the micrometer. The equatorial is necessarily an indifferent instrument, except need differentially ; and in this case, the immoveability of the foundation is of less Importance than the extent of the horizon. It is generally raised high enough to overlook the circle and transit rooms, and has a revolving roof with a slit on one side, which thus gives a command over the entire heavens. If an observer wishes for one instrument which will make good observations in any part of the heavens, be must confine himself to the altitude and azimuth circle under a revolving dome ; but though everything may be dote with this instrument, it must be admitted that it is with considerable expense of thought and calculation.
To be able to pursue some of the most interesting departments of modern astronomy, an observer must possess a telescope of large size, equatorially mounted, and carried by clock-work. This is especially designed for making all micrometrical observations, as those of double stars, diameters of planets, ecc.., and requires a graduated circle only for finding or Identifying objects. Such an instrument is the superb telescope presented by the Duke of Northumberland to the Cambridge Observatory, which was mounted entirely on the plan of the astronomer royal. Instruments of this class aro almost necessarily on the ground, and may be in an isolated building, if a better horizon can bo thus commanded. It is not a simple thing to construct a rotatory roof of 25 or 30 feet diameter, light enough to be easily moved, and yet of sufficient strength, since it must necessarily have a slit on one side from top to bottom. The astronomer royal adopted at Cambridge a
Chinese-looking form, namely, 3 flatter cone on the frustum of a sharper cone. There is a flattish channel on the top of the circular wall, in which half a dozen cannon-balls are laid at equal distances, and the roof rests on them, with another channel In its curb. This appears to be the cheapest, easiest, and most certain mods of getting a rotatory roof, but there is a good deal of rolling. The conical form is of much simpler construction than the spherical, and the shutters can be better applied to it ; but in appearance the spherical form has greatly the advantage of any other form. For measuring objects which do not require a large field, the astronomer royal has applied a divided eye-piece, which answers very well. Both that and the usual micrometer are very limited as to the extent of angle they can measure; and where it is proposed to make most accurate measure ments of angles, which may extend to 1', the proper instrument Is the heliometer, or equatorial with a divided object-glass. It is with this last-mentioned instrument that Beascl succeeded in establishing the existence of parallax, which had baffled astronomers ever since the existence of parallax was proposed as a test of the motion of the earth.
Lastly, if it is wished to cultivate that department of astronomy which wo will call the Herschelian, an observatory must be furnished with gigantic reflectors, as it seems hopeless at present to expect that refractors can be made large enough to transmit sufficient light. These instruments however must almost of necessity be used in the open air.
A private astronomer would probably do most to advance the science by confining himself chiefly to extra-meridian observations, which may be taken up and left off at pleasure. A small transit for time, and as large an equatorial telescope as be can afford to set up, would be sufficient to employ his leisure very profitably.
An observatory is a very dull and uninteresting sight to any one who is not acquainted with the purposes to which it is applied; and we can scarcely conceive how Lalande could say, or others repeat after him, that a person would learn more of astronomy in one night in an observatory than in six months elsewhere. We should say there was no worse school ; and that a person would learn astronomy far bettor from a celestial globe and a fine sky. It is probable, however, that Lalande supposed his learner to possess some elementary knowledge, and to be acquainted with the geometrical part of astronomy ; in which case he would, no doubt, learn that in an observatory which is not to be learned or understood elsewhere.
There is no perfect model of an observatory, as respects the building, to which we can refer. Cambridge is, perhaps, the best, but on a larger scale than is necessary. We have already mentioned Oxford ; Greenwich has nothing to recommend it as a building, but the good ness of the instruments, and the number and methodical arrangement — — - of the observations and computations may be judged of from the printed Observations.
if an architect should have to construct a first-rate observatory, wo should advise him, after learning what instruments are to be accom modated, to study each of these observatories, and arrange the rooms In the most convenient manner. The beat appearance which the case admits of may be given afterwards, but ho shculd not be very rigorous as to outside symmetry. It is scarcely possible to unite convenience as an observatory with a regular exterior, except at a considerable expense.