350 HISTORY OF ASTRONOMY. 



gave signals to llh. 45m., and Cambridge to 12h. Om. 

 This closed the night's signals. From 135 to 150 efficient 

 signals were given. The evenings selected for the deter- 

 mination of the longitude of Cambridge were those of 

 May 17 and 18, 1853. On May 17, Mr. Dunkin observed 

 transits and galvanic signals at Greenwich, and Mr. Todd 

 observed at Cambridge. On the morning of the 18th, 

 the observers were interchanged, and Mr. Todd observed 

 transits and signals at Greenwich, while Mr. Dunkin ob- 

 served at Cambridge. The errors of the transit clock 

 were determined by two methods : method (A) in which 

 the Nautical Almanac stars were employed, but no par- 

 ticular care was taken for the identity of the stars at the 

 two stations; and method (B) in which the same stars 

 were observed at both stations, but no attention was 

 given to the accuracy of the assumed right ascensions. 



For comparison of the Cambridge transit clock, with 

 the chronometers used at the railway stations, Professor 

 Challis employed three chronometers. It was found 

 necessary to reject the comparisons of one of them. 



The results of these operations were as follows : 



Method (A). Method (B). 



May 17, by 146 signals, East longitude of Cambridge 22-660s. 22-601s. 

 May 18, by 135 signals, " 22-713S. 22'782s. 



Mean 22'687s. 22'691s. 

 Mean of the whole 22 -689s. 



The result arrived at in 1829 by transmission of chro- 

 nometers was 23*54:3. 



