328 



HYDROGRAPHICAL SURVEYING [chap. xiv. 



Galvano- 

 meter. 



Pre- 

 arranged 

 Method of 

 sending 

 Signals. 



Use of 

 Sidereal 

 Chrono- 

 meter. 



about ; but in many instances the local arrangements will not 

 admit of this. 



Telegraphic instruments differ very much ; but it does not 

 much matter which are used, as long as they are similar at 

 both ends. The deflection of an ordinary galvanometer needle 

 of Wheatstone's instrument, or of the Morse recorder, or of 

 the more delicate mirror of long submarine cables, will all 

 serve our purpose. Preference is given to one or the other 

 by different observers. The writer prefers an instrument 

 giving a sound to the silent movement or the suspended 

 mirror. 



Each signal will consist of one deflection, and the key 

 should be kept pressed down for about a second. 



In sending the signals, it must be clearly arranged before- 

 hand what is going to be done. 



A good plan is as follows : 



In commencing, give a warning, say of tliree rapid signals, 

 at ten seconds before an even minute by the sender's watch. 

 The first signal will then go at the even minute, and at every 

 ten seconds another, missing the fifty seconds, to mark the even 

 minute, for tliree minutes, ending with another even minute. 



After an interval of three or four minutes, a similar set will 

 be sent in the reverse direction. 



If at the receiving ends the signals agree, this will be quite 

 sufficient, unless we intend +o use another watch. 



An example of a telegraphic meridian distance is appended. 



It will be seen that the resistance of the wire and instru- 

 mental retardation was less on one day than on the other, 

 amounting at one time to nearly a tenth of a second, and at 

 the other to only twenty-five thousandths. 



If possible, a sidereal chronometer should be used at one 

 end and a solar chronometer at the other. A chfferent fraction 

 of a second having to be estimated at each signal prevents any 

 bias on the part of the observer receiving the signals, which is 

 the weak point when using two solar chronometers. The 

 coincidence of beats occurring every 3 minutes 4 seconds is 

 worth more as an accurate comparison than any number of 

 estimations of fractions of a second, and should be used in 

 preference, provided the signals have been sent automatically 

 by a clock. 



