of the Cambridge Observatory. 167 



The next step was, to determine the error of each of the 

 clocks upon sidereal time at the place to which it belonged. 

 Mr. Pond (to whose kindness on similar occasions I have often 

 been obliged) favored me with an extract from the books of the 

 Royal Observatory, containing the observed transits on Oct. 21, 

 22, and 23, with the clock errors deduced from them. The fol- 

 lowing is an abstract : 



Oct. 21, by a mean of 12 stars, when the clock was at 21''. 30, it was ll',21 fast, 



22 2 19 .32 10,64, 



23 7 2 .54 10,12. 



From these it is easily found that 



Oct. 21, when the Greenwich clock was at 5". 47, it was 10',98 fast, 

 Oct. 22 5 .21 10,44. 



Consequently at these times the Cambridge clock was fast on Greenwich 

 sidereal time by T 



2- oK^aci respectively. 



The Greenwich observations are reduced by means of the 

 corrections in the catalogue contained in the Nautical Almanac. 

 These corrections are not commonly used in the Cambridge Ob- 

 servatory : but in comparative observations of this description it 

 is evidently right to use the same corrections at both places. 

 With these corrections (omitting the observations of Procyon, as 

 its A.R., as given in the Nautical Almanac, is sensibly erroneous), 

 the errors of the Cambridge clock were as follows : 



Oct. 21, by Castor, when the clock was at 7^ 26, it was 2". l',93 fast. 



Pollux 7 .37 2 . 1,86 



Oct. 22 Arcturus 14 .10 2 .1,79, 



(same civil day) a Aquilae 19 . 44 2 . 2,03, 



aCygni 20 .38 2 . 2,05, 



Mean 13 . 55 2 . 1,93. 



