Longitude hehveen Melbourne and Hobart Toiun. 95 



stars equations of condition were formed, the solution of 

 which by the method of least squares gave the most probable 

 values of the collimation, azimuth, and clock errors, the level 

 error having been previously found by means of the striding 

 level. The positions of the azimuth stars are taken from 

 the Melbourne General Catalogue for 1870, and those of the 

 clock stars from a specially prepai-ed list. The places of these 

 latter stars differ slightly from their places as given in the 

 EnglUh Nautical A Inutnac ; the resulting clock errors are, 

 however, generally within one-hundredth of a second of 

 what the latter places would produce. 



At Melbourne I observed with the transit circle, which has 

 an aperture of 5 inches and a focal length of 6 feet; the 

 eye piece used has a magnifying power of 167 diameters. 

 This instrument does not admit of reversal, but the collima- 

 tion error is found according to Bessel's method, with two 

 collimators. The level error is obtained by means of reflec- 

 tion from a surface of quicksilver, and the azimuth error is 

 found from the transits of circumpolar stars in the ordinary 

 way, one star being generally observed above the pole and 

 another below. 



At both places self-recording chronographs were employed; 

 that of Professor Harkness was a barrel one, regulated by a 

 vibrating spring. The timepiece which marked the seconds 

 on the chronograph sheets, and which transmitted the 

 signals through the telegraph lines to Melbourne, was a box 

 chronometer, No. 1520, by T. S. & J. D. Negus, of New York, 

 the going of which quite justifies the fame enjoyed by those 

 celebrated makers. The Melbourne clock was the famous 

 Frodsham, No 991, which continues to perform as well as 

 it did some years ago, when its going was declared to be the 

 most remarkable for accuracy on record. It is attached to a 

 chronograph by Siemens and Halske, of Berlin, which regis- 

 ters on a fillet of paper, the motion of which is governed 

 by means of a Froude's fly. 



The usual practice was to commence observing a set of 

 stars soon after sunset ; and as soon as the telegraph lines 

 were clear from their ordinary work, the Hobart Town 

 clock was made to transmit its time to the Melbourne 

 chronograph, on which the Frodsham clock marked its 

 seconds at the same time. After this the Frodsham clock 

 sent its time to the Hobart Town chronograph, where it 

 was registered simultaneously with the Negus chronometer. 

 Now, taking the results as recorded on the J^Ielbourne 



