Page 377 equipment and instruments 4537 



on the arms are straight and whether there are any errors greater than 2 minutes in 

 the graduations. 



Method of use. — The celluloid protractor is used in a manner similar to that de- 

 scribed for the metal protractor, except that there are no vernier tangent screws. 



Where one or more stations on a boat sheet are just beyond reach of the arms of a 

 celluloid protractor, a system of lines 3 or 4 inches long can be drawn radiating from each 

 station. By the use of these, an arm can be pointed toward a station beyond its end. 



Care. — Celluloid protractors are constructed from a perishable material and reason- 

 able care must be taken to prevent their deterioration. Continued exposure to the sun 

 gradually renders them opaque and may distort the material so that the graduations 

 are no longer accurate. 



The instrument is furnished in a leatherette-covered cardboard folder, and when 

 not in use should be kept in this folder. Celluloid protractors should never be stored 

 touching or in close proximity to steel or felt. A chemical reaction occurs between these 

 two materials and the material of which the protractor is made. 



The celluloid protractor, being of light material, may be easUy blown overboard if 

 used in an exposed place. This possibility should be guarded against by securing the 

 protractor to the edge of the plotting table with a twine or tape lanyard of sufficient 

 length to permit use of the protractor on the sheet. 



45S7. The Odessey R.A.R. Protractor 



Where distance circles at regular intervals are drawn on R.A.R. hydrographic 

 sheets, a series of concentric circles drawn on celluloid can be used advantageously to 

 plot the residual distances between the distance circles and, under certain conditions, 

 to plot the positions themselves. 



The device must be constructed for the scale of the sheet and the interval between 

 distance circles. It is simply a series of closely spaced concentric circles drawn to repre- 

 sent distances from the center in terms of meters or time intervals, whichever are used. 

 Circles should be drawn so that those of the same unit interval are readily distinguish- 

 able. The following scheme has been found satisfactory for a 1:80,000 scale using 

 time intervals: A circle for each 0.05 second of time; those representing seconds — -in 

 heavy solid lines; those for half-seconds — ^in heavy pecked lines; those for tenth- 

 seconds — ^in fine solid lines; and those for 0.05 seconds — in fine broken lines. 



A similar scheme can be adopted for other scales and for distances in meters. 



The diameter of the outer circle on the device must be at least equal to, and prefer- 

 ably double, the interval between distance circles on the sheet. The system of concentric 

 circles is either drawn or reproduced photographically on clear celluloid of low distor- 

 tion. The lines must not be so heavy as to decrease appreciably the transparency of 

 the celluloid. 



In plotting R.A.R. distances, the device is placed on the sheet so that its center 

 marks the increment or decrement from the nearest distance circle on the sheet, the 

 partial distance being measured by the closely spaced concentric circles. The interpolated 

 concentric circle on the device will be tangent to the distance circle on the sheet. The 

 center of the device then marks the correct distance, assuming there is no distortion 

 in either sheet or device and that the circles on both are accurately constructed. If 

 the diameter of the largest concentric circle on the device is double the interval between 

 distance circles on the sheet, the device may be used to compensate for the distortion 

 of a hydrographic sheet between distance circles. 



465382—44 26 



