354 HISTORY OF ASTRONOMY. 



second and fourth quarters ; and between the two sets of 

 observations at each place the poles of the battery were 

 to be reversed. 



The transit clocks were corrected by two distinct 

 methods in the same manner as in the operations for de- 

 termining the longitude of Cambridge. 



The final results for the difference of longitude as 

 determined by the two methods A and B are the 

 following : 



Method A. Method B. 



Mean of the first series 17m. 29 -256s. 17m. 29 -340s. 



" second series 28'538s. 28-4763. 



Mean of the two series 17m. 28-897s. 17m. 28'908s. 



These determinations rest upon 1104 signals. The 

 last result 17m. 28'9s. is the best that can be given for 

 the difference of longitude of the two observatories. It 

 is however remarkable that the difference of the results 

 given by the two series is O791s., which is probably due 

 to personal equation of the observers. 



The time employed by the galvanic current in passing 

 between the two observatories, a distance of 270 miles, is 

 0'109s., which indicates a velocity of only 2500 miles per 

 second. It is however to be remarked, that from Green- 

 wich to London and thence to Ostend, the whole of the 

 line is subterraneous or sub-aqueous, and it is considered 

 probable that the observed retardation belongs almost 

 entirely to this portion of the line. 



In May and June, 1854, the difference of longitude 

 between Greenwich and Paris observatories was deter- 



