34 A MANUAL OF TOPOGRAPHIC METHODS. 



COMPARISON OF TIME. 



After time lias been thus observed the chronometers at the two stations 

 should be compared by telegraph. 



Chronometers are compared in the following manner: The chronometer 

 at one station being in circuit with the chronograph and recording upon it, 

 the chronometer at the other station is switched into the general telegraphic 

 circuit, by which it is brought to the first station and switched into the 

 local circuit there, so that the two chronometers register upon the same 

 chronograph, their beats being marked side by side by the same pen. 



Fig. 3. — Switchboard. 



After this has gone on for a minute or more the operation is reversed, the 

 chronometer at the first station is switched into the telegraphic circuit and 

 made t<> record upon the chronograph with the chronometer at the second 

 station. Of course the observers are informed of the hour and minute at 

 which the joint record upon the several chronographs begins. 



This method constitutes what is known as the automatic exchange of 

 signals. 



The arbitrary exchange of signals is made as follows: 



Each chronometer recording on its own chronograph as usual, and each 



local circuit being connected with the main-line circuit, the observer at one 



station breaks the circuit by means of the main-line talking-key, which 



break is recorded on the chronograph sheets at both stations. The breaks 



