4535 HYDROGRAPHIC MANUAL PaGE 376 



tractor should be checked to see that it is properly centered over the known position 

 and that the fixed arm passes through the initial point. 



In plotting hydrography accuracy should be the objective of the beginner. Speed 

 will be gained with experience. 



4535. Care of Three-Arm Protractor 



The metal three-arm protractor is an accurate and expensive instrument and 

 requires as much care to preserve its initial accuracy as does any delicate survey 

 instrument. Each protractor should be stored in its own wooden box when the day's 

 work has been completed, and a visual check should be made to ensure that each acces- 

 sory is in its proper compartment in the box. 



During use, especially during rough weather, provision must be made to eliminate 

 any possibility of the protractor's slipping off the plotting table. Ingenuity will suggest 

 several methods of securing the instrument on the table when not in actual use. The 

 arms and the extension arms of metal protractors are straightedges which may be 

 easily bent if the protractor is dropped or carelessly handled. After any accident to 

 the instrument the straightness of the arms should be carefully verified before subse- 

 quent use. 



If a metal protractor is exposed to salt spray or happens to be immersed in salt 



water, it should be cleaned and oiled immediately in accordance with the instructions 



given in 4313. 



4.536. Celluloid Protractors 



The transparent protractor (fig. 78) in most general use is constructed of celluloid, 

 with a solid disk about 12 inches in diameter containing a circle graduated in degrees, 

 and one fixed and two movable arms also of celluloid extending about 13}^ inches 

 beyond the edge of the disk, each containing an etched line radial with the center of 

 the protractor and which serves as the plotting edge. Each of the movable arms 

 contains a vernier graduated in 2-minute intervals. Angles can be set or read to the 

 nearest minute. A setscrew on each movable arm clamps the arm to the solid circle 

 near its outer rim, and each arm may be set and clamped in place independently of 

 the other. 



Although limited in accuracy the celluloid protractor has several advantages over 

 the metal protractor. Its transparency makes it possible to see the plotting sheet and 

 the stations thereon. Positions may be plotted quite close to the plotted positions of 

 the control stations. The protractor is lighter in weight, more easily read, and con- 

 sequently can be set more quickly than the metal protractor. It is easily cleaned and 

 contains practically no metal to stain the sheet. The solid celluloid disk serves to 

 protect the sheet from being soiled by the hands of the plotter. 



A celluloid protractor may be used whenever desired for boat-sheet plotting, but 

 its use for smooth-sheet plotting is restricted by the limitations in 7622. 



The celluloid protractor is most useful in launch hydrography, where the positions 

 are usually comparativel}^ near the control stations and rapid plotting is necessary, 

 especially where there are currents. Compared to the metal protractor, it can be more 

 easily held in place when the launch is pitching and can be set more readily regardless 

 of vibration from the launch motor. It is less likely to be damaged if thrown off the 

 table by the violent motion of the launch in a rough sea. 



Test for accuracy. — Celluloid protractors cannot be adjusted. They should, how- 

 ever, be tested on the aluminum plate in order to determine whether the scribed lines 



