CHAPTER XIV 



MERIDIAN DISTANCES 



Telegraphic — Chrononietric. 



Under this head we shall consider all the methods available 

 for our purposes of obtaining difference of longitude. 



TELEGRAPHIC MERIDIAN DISTANCE. 



Where a telegraph can be used, it is of course the best, and 

 at the same time the simplest, means of obtaining difference 

 of longitude. 



This method consists in sending a current through the wire 

 at a known local time from one place, the local time of arrival 

 at the other place being noted. The difference of these is the 

 difference of longitude. 

 Retarda- In theory, the passage of the current through the wire is 

 instantaneous ; but in practice it takes an appreciable time, 

 when the distance is considerable, and the electrical condition 

 of the wire is not fu-st-rate ; and to eliminate this, and also to 

 decrease errors of sending and receiving, we must send several 

 sets of signals in both directions equally, the mean of which 

 will give the true time. 



A little consideration will show that if a signal is sent from 

 A to B, a place to the westward, and it takes two seconds to 

 traverse the wire, the time at B wiU have had those two 

 seconds in which to catch up the A time, which is so much 

 ahead ; or, in other words, the difference of the two times as 

 shown will be two seconds too little. Whereas, if the signal 

 is sent from B to A, the watch at A, already ahead, will 

 advance another two seconds before the signal arrives, and 



B2G 



tion. 



