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EQUIPMENT AND INSTRUMENTS 



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graduated to half -degrees, with a vernier on each movable arm which permits a setting 

 to the nearest minute. The graduations of the circle should be accurate to within 

 one-half minute, and the arms should be truly radial also within one-half minute. 



The center arm of the protractor is fixed at the zero of the scale. Each movable 

 arm has a vernier, setscrew, and tangent screw. The graduations of the circle are 

 marked with two sets of numbers so that angles may be read either clockwise or counter- 

 clockwise from the zero of the scale. The construction 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 being 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 accommodate a small left angle. 



Figure 78.— Metal three-arm protractor with attachments, and celluloid protrat-Uir. 



At the center of the graduated circle is an open metal socket into which may be 

 fitted any one of several interchangeable centei-pieces. One of these centerpieces has a 

 pricker which is held above the paper by a small spring until the plotter is ready to 

 mark his position. When the pricker is depressed by hand, a small hole is made in the 

 paper, a shoulder preventing the pricking point from being pushed into the paper more 

 than just enough to mark the position. A second centerpiece is provided with a trans- 

 parent bottom on which are etched two perpendicular crosslines which mark the center 

 of the graduated circle. Tliis centerpiece is used in setting the protractor on any pre- 

 viously plotted position. A third centerpiece has a small round opening through which 

 a plotted position may be marked by pencil. 



A magnifying glass of low power is attached to the instrument so that it can be 

 rotated around the center, in order to facilitate reading the graduated circle. 



