164 Proceedings of the Royal Society of Victoria. 



and Observing Telescope were in a line with it, in the direction of 

 the meridian. An image of the white disc attached to the 

 pendulum of the clock was formed at the tail piece of the 

 Invariable Pendulum, by the proper collimating lens, and was 

 brought into the field of view of the Observing Telescope by a 

 plain mirror, fixed at the object end, which could rotate in a 

 plane at right angles to the meridian, thus enabling the 

 observer to bring the eye end at any convenient altitude. 

 The distances of the several parts from the tail piece were 

 as follows : — Observing Telescope, o.g. 70-3 inches ; disc on 

 Shelton pendulum, 48 inches; collimating lens, 12-2 inches; arc 

 scale, about 1 inch for No. 11, and 0*5 inches for No. 4 and No. 6. 

 The arc scale was about 18 inches, and the plane of suspension 

 about 68 inches above the floor of the room, which is 84 feet 

 above sea-level. 



The operations consisted in observations of coincidences, 

 temperature, pressure and arc of vibration, and comparison of 

 the Shelton clock, with the sidereal standard clock of the Obser- 

 vatory, the rates of which are always known from transit 

 observations, and finally, the reduction of observations. 



Observation of Coincidences. 



The diaphragm in front of the disc carried by the pendulum of 

 the Shelton clock was so adjusted as to have its imner edges 

 tangential to the disc, when at rest. (The disc is of white caixl 

 two inches in diameter and turned to a true circle in the lathe). 

 The image of the disc formed at the tail piece by the collimating 

 lens was made of the same width as the tail piece, and the ai-c 

 of vibration at the commencement was made a little less 

 than the arc of the clock. The Disappearance Z>, and the 

 Reappearance R, of the apparent right edge of the disc 

 (actually the eastern edge) was invariably observed. The times 

 of the first four swings were observed by the Seth Thomas, a 

 sidereal clock kept in the room for the purposes of the Observa- 

 tory, which, being near and almost in front of the observer, could 

 be seen better than the Shelton, and had the advantage of 

 being directly compared with the transit clock on the tape 

 chronograph ; but this method introduced confusion, and was 



