Page 375 equipment and instruments 4534 



pricker centerpiece may be tested in a similar manner by pricking several points in the 

 paper with it in various positions. The prick points should coincide in one hole. 



Each arm of a protractor should be tested for straightness against a straightedge. 

 It may also be tested by drawing a fine line along the fiducial edge of the arm with a 

 chisel-edged pencil, and then reversing the arm to the opposite side of the line; if the 

 fiducial edge still coincides with the pencil line, the arm may be assumed to be straight. 

 If the extensions to the arms are to be used, this test should be made for the entire 

 length of the arm with the extension locked in place. 



After the arms and centerpieces have been tested, the protractor should be placed 

 on the aluminum plate and centered precisely at the intersection of the two lines at 

 right angles. The fixed arm should be moved to coincide with the central line, and the 

 right and left arms moved to coincide with the lines at 90° to the central line. The 

 two movable arms should be clamped in these positions with their arms coinciding 

 precisely with the lines. If the angles read exactly 90°, the verniers are correctly set 

 with reference to the graduations at these points. If they do not read exactly 90°, the 

 verniers should be adjusted by loosening the screws which hold them and moving them 

 so that they will read precisely 90°. The verniers are then locked firmly in position by 

 tightenmg the screws. The adjustment should be checked. 



When the two movable arms are set 90° to the right and left respectively of the 

 fixed arm, a line drawn along the fiducial edge of one of these arms should be a straight- 

 line extension of a line drawn along the fiducial edge of the other. Likewise, each arm 

 when set at 180° should form a straight-line extension of the fixed arm. 



Metal protractors should be tested at the beginning of a field season, before plotting 

 a smooth sheet, and at least once a month during their use. 



4534. Use of Three- Arm Protractor 



The three-arm protractor is used almost constantly in liych'ographic surveying. 

 With it three-point fixes may be readily plotted, as well as angles from a known position. 



To plot a three-point fix (see 333) , the left movable arm is turned away from the 

 fixed arm by the amount of the left angle, the arm is clamped in place and the final 

 setting made with the tangent screw and vernier. The procedure is repeated for the 

 right angle. After the angles have been set the instrument should be grasped by the 

 metal cu'cle and the fixed arm, and centered at the estimated position of the fix. With 

 the fingers of one hand used as a guide to keep the fixed arm bisecting the center station, 

 the protractor is moved toward or away from the stations until each arm*bisects its 

 respective station. During this operation speed may be gained by keeping the side 

 arms at approximately equal distances from the respective stations. A prick mark or 

 pencil mark is then made at the center of the protractor, the position thus marked 

 representing the location of the observers at the time the angles were taken. 



In using a metal protractor it will occasionally be found that one or more of the 

 plotted control stations are obscured by the metal circle. To overcome this difficulty 

 a protractor of transparent material may be used. 



To plot an angle from a known position, the instrument is centered accurately over 

 the position by means of a transparent centerpiece, the fixed arm is rotated until it 

 passes through the plotted position of the initial object, and the right or left movable 

 arm is set at the desired angle. The cuts may then be drawn along the fiducial edge 

 of the arm with a chisel-edged pencil. Before each cut is drawn, the position of the pro- 



