50 



HYDROGRAPHIC MANUAL 



Figure 9. — Three-arm metal protractor. 



struction of the protractor permits setting a 

 small angle approaching 0° on one arm only, 

 the minimum angle which can be set on the 

 other arm is about 12°. In order to plot all 

 three-point fixes, two metal protractors are 

 required, one constructed to accommodate a 

 small right angle and the other to accom- 

 modate a small left angle. A magnifying 

 glass of low power is attached to the instru- 

 ment so that it can be rotated around the 

 center in order to facilitate reading the grad- 

 uated circle. At the center of the protractor 

 is an open metal socket into which can be 

 fitted any one of three centerpieces. 



3-18 Test and adjustment of three-arm 

 metal protractor. — Metal protractors should 

 be tested, and adjusted if necessary, before 

 the beginning of a field season, before plot- 



ting a smooth sheet, and at least once a 

 month during their use (see 6-48). An alu- 

 minum plate with test lines accurately drawn 

 will be furnished by the Washington Office 

 on request. 



In testing a protractor the centerpiece 

 with the transparent bottom and crosslines 

 should be used. Place the center of the pro- 

 tractor exactly over the intersection of the 

 test lines and rotate the centerpiece. The 

 intersection of the crosslines should coincide 

 with the intersection of the test lines at all 

 times. The pricker centerpiece may be tested 

 in a similar manner by pricking several 

 points in paper as the pricker is rotated. The 

 prick points should coincide in one hole. 



Each arm of the protractor should be 

 tested for straightness against a straight 



