APPLICATION OF ELECTRIC TELEGRAPH. 307 



In the summer of 1847, the experiments were renewed 

 with more complete apparatus. After 10 o'clock, P.M., 

 the three stations at "Washington, Philadelphia, and 

 Jersey City, opposite New York, were converted into 

 temporary telegraph offices, as well as astronomical sta- 

 tions. The astronomical observations for clock correc- 

 tions and personal equations, were arranged by Mr. S. C. 

 Walker, on the model of Struve's celebrated chronometric 

 expedition between Pulkova and Altona. 



The following is the method of comparing the local 

 times at the different stations. A signal is given at the 

 first station by pressing a key, as in the usual mode of 

 telegraphing ; and the observer at each of the other sta- 

 tions hears the click caused by the motion of the armature 

 of his electro-magnet. The most obvious method of com- 

 parison consists in simply striking on the signal key at 

 intervals of ten seconds ; the party at the first station re- 

 cording the time when the signals were given, and the 

 party at the second station recording the time when the 

 signals were received. After about twenty signals have 

 been transmitted from the first station to the second, a 

 similar set of signals is returned from the second station to 

 the first. The objection to this mode of comparison is that 

 it requires the fraction of a second to be estimated by the 

 ear. The party giving the signals strikes his key in coin- 

 cidence with the beats of his clock, so that at his station 

 there is no fraction of a second to be estimated ; but at 

 the other station, the armature click will not probably be 

 heard in coincidence with the beats of the clock, and the 



