APPLICATION OF THE ELECTRIC TELEGRAPH. 849 



EXPERIMENTS MADE IN EUROPE FOR THE DETERMINA- 

 TION OF GEOGRAPHICAL LONGITUDE BY THE 

 ELECTRIC TELEGRAPH. 



During the summer of 1852, the Royal observatory at 

 Greenwich was put in galvanic communication with the 

 principal telegraph offices in London, for the purpose of 

 determining differences of longitude with other observa- 

 tories, British and continental. The first comparison 

 was made between the observatories of Greenwich and 

 Cambridge. The electric current was made to pass 

 through the coils of a telegraph needle at Greenwich, 

 and through the coils of another telegraph needle at 

 Cambridge, so that the completion of the circuit at Green- 

 wich produced a movement both in the needle at Green- 

 wich and in that at Cambridge. The order of operations 

 was as follows : 



At 11 P.M., Greenwich mean solar time, Greenwich 

 commenced by giving five signals at intervals of about 2 

 seconds each. The turn-plates were changed, and Cam- 

 bridge responded by five similar signals. These were 

 merely to say "All is right." Greenwich then gave 

 groups of signals at intervals of 10s. to 15s., in numbers 

 of from three to nine signals in a group (some of them 

 being transits of stars) to llh. 15m. Notice was pre- 

 viously given of the number of signals to be expected in 

 each group, by giving the same number of warning signals 

 at intervals of about two seconds. Then Cambridge gave 

 similar groups of signals to llh. 30m. Then Greenwich 



