OF THE OBSERVATORY OF CAMBRIDGE BY GALVANIC SIGNALS. 503 



It is needless to say that the determination of the instrumental errors of the Transits 

 was carefully attended to at both Observatories. The following reduced results of the 

 Greenwich transit observations, communicated to me by Mr Airy, suffice for converting the 

 signal-times noted at Greenwich into sidereal times. 



o 1 



Method (A). 



h. m. I. 



May 17. 11.0 Mean Time, Transit-Clock slow +8,77 



12.0 +8,79 



May 18. 11.0 + 9,48 



12.0 +9,51 



Method (B). 



h. m. s. 



May 17. 11.0 Mean Time, Transit-Clock slow +8,87 



12.0 +8,89 



May 18. 11.0 +9.54 



12.0 +9,57 



In the reduction of the Cambridge transit observations, the level error was the mean of 

 two levellings, taken on May 17 and May 21 ; the collimation error was the mean of two 

 results obtained by the collimating eye-piece on May 24 ; and the azimuth error was deduced 

 from transits of Polaris SP and Polaris on May 16. The three errors were calculated on the 

 supposition that the pivots of the transit-instrument are cylindrical and equal, (which, however, 

 is not strictly the case), and the errors of the transit-clock were determined accordingly. 

 The longitude will first be calculated from clock-errors founded on the above supposition, and 

 I shall afterwards correct the result by taking account of the true forms of the pivots. 

 The following are the errors of the transit-clock (Hardy), deduced immediately from the 

 observations. 



Method (A). 



h. m. m. t. 



May 17, at 9.49 sidereal time, Hardy slow 1.20,66 

 18, at 14. 9 1.22,55 



Method (B). 



ft. Jit. tit. S> 



May 17, at 11.49 sidereal time, Hardy slow 1.20,84 

 18, at 14.17 1.22,72 



The daily losing rate of Hardy was found to be 1%58 by observations of Mr Todd on 

 May 16 and 17, l s ,58 and l s ,59 by observations of Mr Breen on May 17, 18 and 19, and l',57 

 by observations of Mr Dunkin on May 18 and 19. The adopted losing rate for May 17 and 

 May 18 is 1%58. 



It is now required to state what comparisons were made between the transit-clock and 



the three chronometers, for the purpose of converting the noted signal-times into sidereal 



times of the Cambridge Observatory. The transit-clock will be designated by the letter H, 



the mean-time chronometer by the letter W, and the sidereal-time chronometers by X and U. 



Vol. IX. Part IV. 65 



