THEODOLITE. 



55 



focal distance is KU inches. The telescope with the eyepiece commonly 

 used has a power of about 30 diameters. 



The circle is read by means of two microscopes, placed opposite one 

 another. Within the field of the microscope is a comb stretching over the 

 space of 20 minutes. This comb has ten teeth, divided into two parts by 

 a depression, each corresponding to 2 minutes. Tarts of a minute down to 

 2 seconds are read by means of a micrometer screw moving a pair of fine 

 threads in the field of the microscope. 



Fiq. 7 — Eight-inch Theodolite, detail. 

 INSTRUCTIONS FOR THE MEASUREMENT OF HORIZONTAL ANGLES. 



The following general precautions should be observed in the measure- 

 ment of all horizontal angles in the primary triangulation. 



The instrument should have a stable support, which may be a stone 

 pier, a wooden post, or a good tripod. If a portable tripod is used, its legs 

 should be set firmly in the ground. 



The instrument should be protected from the direct rays of the sun by 

 means of an umbrella, or a piece of canvas like a tent fly. It should also 

 be shielded from winds which nun - jar or, twist either it or its support. 



The foot screws of the instrument after it is leveled for work should 



