TRANSIT CIRCLE

A meridian instrument which combines the properties of the Transit Instrument and the Mural circle, that is, which can be employed for determining at the same transit of a celestial object, both its R. A. and its zenith distance. A remarkable instrument of this cla was erected at Green wich, by the Astronomer Royal, Mr. Airy, in the year 18.50; and, from the beginning of the year 1851, all the meridian observations have been made with it. It consists mainly of a telescope of 12 feet focal length with an oiyect-gla of 8 inches clear aperture, carrying on each side of its centre a circle of 6 feet diameter. The tube of the telescope is of iron cast in four pieces, viz. two immense cones at its eye end and object end, and a large central cube in two pieces, having attached to them in the same casts the pivots on which the telescope revolves. The cones weigh each about If cwt, and the central cube with its pivots weighs nearly 8 cwt. The two parts of the cube are bolted together on flanges with planed surfaces, as also are the cones to the central cube. The surfaces of the pivots were hardened after the casting, by chilling them, to a depth of about a quarter of an inch, and were afterwards ground and polished to a true cylindrical form after a great expenditure of time and labour. The circles are carried on cylindrical rims on the east and west sides of the central cube. The western circle has a dished surface of about 3 inches in breadth with its concavity westward, and carries two bands, one on its eastern and one on its western surface, the latter being divided to 5' of space. The eastern divisions are merely given for the convenience of setting the instrument accurately to given objects by means of a pointer; but the western divisions are for the observations of zeniih distance. For the reading of the circle six long microscopes or lunettes are fixed in the solid stone of the western pier, having their object-gla es arranged in a circle of about 5^ feet diameter en the inner or eastern surface, and their eyepieces in a circle of about 2 feet in diameter at the back of the piers. The centres of these circles are in the same horizontal line with the centre of the telescope, and at the height of 5 feet from the floor. The microscopes, whose axes lie in a conical surface, are each 4.^ inches in length, and their axes are not quite perpendicular to the portions of the surface of the circle opposite to them, but are inclined in such a way that the light thrown through corresponding illuminating holes (cut in the pier) from a central gas lamp shall be reflected truly into them without the aid of specula. The error of collimation of the telescope is found by means of two 5 ft. telescopes, one to the north and the other to the [s. 1431]