That an opinion may be formed of the present state of this question, it appears necessary to give some account of the Dublin instrument. The account of the Green wich mural circle, which has furnished most of the obser vations that are opposed to those made by Dr. Brinkley, has been given in a former volume.
The instrument at the observatory of Trinity College, Dublin, is a vertical circle, eight feet in diameter. It was planned, and partly executed, by the celebrated Mr. Rams den, and finished by Mr. Berge. The following particulars may be sufficient to render intelligible the method of making the observations, and the degree of accuracy to be expected. (See Transactions of the Royal Irish Academy, vol. xii.) The circle is supported in a frame, which frame turns on a vertical axis. The upper part of the frame is of cast iron, turns in a collar, and is connected with the lower part of the frame by four hollow brass cylindrical pillars. The lower part of the frame, which is also of cast iron, terminates in a pivot of steel, which turns in a socket of bell-metal. This socket is moveable south and north by one screw, and east and west by another, for the purpose of adjusting the vertical axis.
The axis of the circle, a double cone four feet in length, is supported on Y's, which are themselves supported by strong bars of brass attached to the cylindrical pillars. The pressure.of the weight of the circle and its axis, is relieved by an ingenious application of friction wheels and the lever. There is also an ingenious contrivance for ad justing the axis horizontal.
The limb of the circle is brass, and is divided into inter vals of five minutes, which intervals are subdivided by micrometer microscopes, into seconds and parts of a se cond, as usual. There are three microscopes, one called the bottom microscope, opposite the lowest point of the circle; a second opposite the left extremity of the hori• zontal diameter, and a third opposite the right extremity of the horizontal diameter.
The frame carrying the circle turns on the vertical axis with the greatest steadiness. The circle also turns to gether with its horizontal axis on the V's with equal steadi ness.
The vertical axis of the instrument is adjusted by a 'Er plumb line. The plumb-line by which this adjustment is
performed is about ten feet long, and is suspended from a point about eight inches from the centre of the top of the frame, and passes over a point below eight feet from the point of suspension. By the help of this point, which is moveable by a screw, and by the moveable socket below, the axis of the instrument is made vertical. The adjust ment of this axis, as to the north and south position, is, it is evident, of the most essential consequence to the ex actness of the zenith distance observed. It is likewise evident, that, from the great interval between the upper and lower parts of the instrument, the relative position of the point of suspension, and of the point below, may be altered by change of temperature. To obviate this incon venience, which would be fatal to the accuracy of the ob servations, the point of suspension is on a compound bar, formed of bars of brass and steel, and the point below is also placed on a similar compound bar. By this the distance of the plumb-line from the vertical axis remains always the same. This contrivance appears to answer in a very satisfactory manner.
The axis of the instrument being adjusted vertical, and the plane of the circle in the meridian, and facing the cast, let b, 1, and r be the zenith distances of a star, as shown by the bottom, left add right microscopes respectively. When the plane of the circle is on the meridian, and facing the west, let 1/, r', be the zenith distances of the same star as shown my the respective microscopes. Then the true observed zenith distance = z ;) and 3 3 the correction of the mean of the three microscopes r' —- 3 For objects above seven degrees from the zenith, the ob servations are frequently made both on the east and west positions of the face of the circle, at the same passage over the meridian. One observation is made just before the object comes to the meridian, and then, the circle hav ing been reversed, the object is again observed. The ob servations are reduced to the meridian. This method of observing adds greatly to the exactness of the results.