4532 HYDROGRAPHIC MANUAL PaGE 374 



Extension arms. — A removable extension, about 15 inches long, is provided for 

 each of the tlu'ee arms. These extensions are not interchangeable, each extension 

 fitting only its corresponding arm. Wlien in place they form true, straightedge exten- 

 :sions of the arms of the protractor. Setscrews are provided for fixing them in place. 



For plotting angles on stations at great distances, metal protractors have some- 

 times been fitted with a second set of extension arms to be attached to the first set. 

 It is difficult to provide sufficient rigidity for these extensions, and their use is not 

 recommended, especially since extremely small angles, which usually occur in observa- 

 tions on distant signals, require very accurate plotting. 



To overcome the lack of rigidity, braces in the form of metal arcs with central slots 

 have been devised for attachment to the arms at the points where the extension arms 

 are fastened. These braces can be used only for comparatively small angles. After 

 the extension arms have been added, the brace is placed in position and the setscrews 

 are screwed down through the slot in the brace and into both arms. When these are 

 used at least one setscrew in each brace must be loosened each time a new angle is set 

 on the protractor. After the brace has been locked, the setscrew on the vernier should 

 be released and the angle verified before the position is plotted. 



4532. Adaptation for Wire-Drag Plotting 



Wire-drag plotting is facilitated by a simple adaptation of the metal three-arm 

 protractor. In the base of the transparent centerpiece which contains crosslines and 

 a center hole, a small eccentric hole is drilled at a distance from the center corresponding 

 to the length of the towline at the plotting scale. A special pointer is mounted on top 

 of the centerpiece in such a way that the pointer is in line with the two holes, the end 

 of the pointer reaching to the graduated arc of the protractor. 



In a wire-drag survey the three-point fix is plotted in the usual manner, the center 

 of the protractor indicating the position of the guide launch. The buoy angles are 

 then set with the new device; the position of the near buoy is plotted through the 

 eccentric hole, and a short direction is indicated to the distant buoy. Different tow- 

 line lengths or smooth sheets of different scale will require different centerpieces. 



4533. Test and Adjustment of Three-Arm Protractor 



Protractors, when first purchased or when repaired at the Washington Office, are 

 verified for accuracy before being issued for use. But since both metal and trans- 

 parent protractors are subjected to exposure and hard use in the field, they must be 

 frequently verified during use. An endless amount of trouble will result from the use 

 of inaccurate protractors. 



An aluminum plate with accurate angles of 90° on each side of the center may be 

 requisitioned from the Washington Office for use in testing protractors. If the con- 

 struction of such a plate is undertaken in the field, it should be made with extreme 

 accuracy on metal or on metal-mounted paper. 



In testing a protractor the centerpiece with the transparent bottom and crosslines 

 should be used. The bottom of the centerpiece should be ffush or nearly flush with 

 the hub of the protractor to avoid parallax. The centerpiece should be tested for 

 concentricity by placing the intersection of the crosslines above a marked point on a 

 sheet of paper and rotating the centerpiece in the socket. If the intersection of the 

 crosslines coincides throughout the rotation with the marked point, the centerpiece 

 may be assumed to be correct. The other centerpieces should also be checked. The 



