80 INSTRUCTIONS FOR MAKING FOREST SURVEYS, ETC. 



roller D. This will happen in every case and the num- 

 ber of this mark on the vernier will determine the 

 second decimal. In the diagram the first mark to the 

 right of the zero falls opposite a mark on roller D and 

 therefore the reading is 0.01. Thus the total reading 

 is 25.71 square inches. Use a magnifying glass if 

 necessary. 



(4) The area in acres is found by multiplying the 

 figure given by the planimeter by coefficient determined 

 by the scale on which the map is drawn. If the scale 

 be 1 inch to the mile, 1 square inch will represent 640 

 acres. If it be one-half inch to the mile, 1 square inch 

 will represent 4 square miles and the acreage will be 

 determined by multiplying the instrument reading by 

 640X4, or by 2,560. If the scale be 2 inches to the 

 mile, 1 square inch will represent 160 acres; and so on 

 for any desired scale. 



(5) Blueprints and other photographic papers are 

 never exactly to scale, but a conventional mile on the 

 print can be planimetered, and the reading thus ob- 

 tained will be known to represent 640 acres. 



(6) On important work the area should be plani- 

 metered several times and the results averaged. 



(7) For practice, a regular figure, such as a square 

 containing a known number of square inches, should 

 be planimetered until the reading on the instrument 

 agrees substantially with the known area. 



(8) Only an expert should attempt to adjust a 

 planimeter. If the instrument does not work properly 

 it should be returned to the property clerk for repairs. 



