8 INSTRUCTIONS FOR MAKING FOREST SURVEYS, ETC. 



Vernier compasses are provided with a revolving 

 graduated ring dial which may be set according to the 

 magnetic variation, thus reducing the reading to true 

 north instead of magnetic north. 



Clinometer compasses are provided with a small pen- 

 dulum hung from the center pivot, which is used to 

 determine a vertical angle. 



Prismatic compasses are sight compasses with a 

 " floating" dial which may be held in the hand. The 

 sight is taken and the direction is read in the same 

 operation. 



Mirror compasses are provided with a reflecting sur- 

 face on the inside of a hinged cover, and the reflec- 

 tion of the reading is noted at the time the sight is taken. 



Alidade compasses are provided with at least one 

 straight edge parallel to the line of sight. The bottom 

 of the compass is smooth so that the instrument may 

 be laid upon a map and the straight edge used as a 

 ruler. 



Solar compasses are provided with a special attach- 

 ment which can be revolved independent of the com- 

 pass for taking observations on the sun and determin- 

 ing the cardinal direction without using the compass 

 needle. 



Compasses are also used as a part of the equipment 

 of transits, levels, and plane tables, and in such cases 

 these instruments should be constructed of nonmagnetic 

 materials, in order that the needle may not be deflected. 

 Iron, nickel, cobalt, and manganese are the most mag- 

 netic substances. 



The instruments for determining grade or vertical 

 angles are: 



