20 



A MANUAL OF TOPOGRAPHIC METHODS. 



automatically at regular intervals. Those in use upon the Geological Sur- 

 vey break the circuit every two seconds, and the end of the minute is indi- 

 cated by breaking at the fifty-ninth as well as the fifty-eighth and sixtieth 

 seconds. When one of these chronometers is connected with a battery and 

 a chronograph is put in the same circuit, the beginning of every even sec- 

 ond is recorded upon the chronograph by a jog on the paper, and the dis- 

 tance between the jogs in each case represents, therefore, two seconds. The 

 observer at the instrument is provided with a telegraph key, which may also 

 be put in the circuit with the clnonometer and chronograph, and as a star 



Fig. 2. — Chronograph. 



near the meridian crosses a thread in the telescope he records that fact by 

 pressing on the key, which makes a record upon the chronograph along 

 with the record of the chronometer. 



An illustration of the form of chronograph in use upon the Geological 

 Survey is shown upon Fig. 2. 



FIELD WORK. 



Since the observations tor latitude and longitude, though different, 

 are made with the same instrument, at the same time, and by the same 

 party, certain parts of the work apply equally to both determinations and 

 may be described once for all. 



