TELEGRAPHIC DETERMINATION OF LONGITUDE. 429 



the same circuit, the mark made on the chronograph-pai^er, instead of 

 being a straight line, will be broken at regular intervals as shown at a. 



0. a a aha aha a 



By means of a little instrument called, a break-circuit key, in the 

 liands of the observer, and included in the same circuit, tlie electric 

 current may be interrupted, causing the pen to make a similar mark 

 as shown at h, on the occurrence of any event, such as the passage of 

 a star across the wires of the telescope. 



With a finely-divided scale the position of this arbitrary mark, with 

 reference to the nearest second mark, may be accurately established, 

 and the exact time accurately ascertained to within -gL of a second. 



By means of these instruments, the error of the chronometer is 

 found at each station with great accurac}', and, the times shown by the 

 faces of the chronometers being compared by telegraph, the difference 

 of time and corresponding difference of longitude are readily deduced. 



The time occujsied by an electric impulse to traverse the wire from 

 one station to another, and act upon the telegraph-instruments, though 

 generally very small, is too great to be neglected, but is easily ascer- 

 tained and allowed for. 



Suppose a to be a station, one degree of longitude east of another 

 station &, and that at each station there is a clock exactly regulated to 

 the time of its own place, in which case the clock at a will be, of 

 course, four minutes faster than the clock at b. Let us also suppose 

 that a signal takes a quarter of a second to pass over the telegraph- 

 wire connecting the two stations. 



Then if the observer at a sends a signal at exactly noon, by his 



clock,to6 12" O- 0' 



It wiU be received at 6 at IPSO 0'.25 



Showing apparently a difference of time of. 3 SQ'.'ZS 



Then if the observer at 6 sends a signal at noon by his clock.. 12* 0"" 0' 

 It will be received at at 12" 4" 0'.25 



Showing an apparent difference of time of 4"" 0'.25 



One-half the sum of these differences is 4 , which is exactly the 

 difference of time and of longitude ; and one-half of their difference is 

 0'.25, which is exactly the time taken by the electric impulse to trav- 

 erse the wire and telegraph instruments. This is technically called 

 the " wave and armature time." 



The error of each chronometer being ascertained by observations 

 of stars at each station, and the difference of the chronometers being 

 in this way shown by the exchange of signals, the difference of the 

 local times, which is the difference of longitude of the two stations, is 

 easily deduced. 



Some English astronomers have objected that, where the line is, as 



