Page 703 



THE SMOOTH SHEET 



7631 



the path of the transmitted sound wave, other data obtained during the progress of 

 such a survey must be appraised with the acoustic distances, and the most probable 

 positions adopted. Such supplemental data are sextant angles, bearings, and dead 

 reckoning. 



To avoid confusion, too many positions should not be plotted at any one time before 

 they have been analyzed and accepted (see 7637) . In areas of systematic sounding lines 

 not complicated by crosslines, or development, three or four lines may usually be 

 plotted at one time, but in areas where previous lines have been plotted only a compara- 

 tively few positions should be plotted before analysis. 



Nimierous pencil notes are necessary to keep track of position numbers and other 

 data. These must be placed directly on the smooth sheet, since this type of plotting 

 is done without a sheet cover (see 761). A soft pencil should be used so that the marks 

 may be easily and completely erased. 



7631. Plotting the Distance Arcs 



Before plotting is begun a small circular piece of heavy celluloid should be fastened 

 over each R.A.R. station to protect the center from wear due to repeated use of the beam 

 compass. The best adhesive is non vulcanizing rubber cement, thinned by an approxi- 

 mately equal amount of benzol or, as a substitute, benzine. The underside of the 

 celluloid and the respective area on the smooth sheet are covered with a thin coat of 

 cement and allowed to dry until 

 sticky, after which the celluloid is 

 pressed into place. Care should be 

 taken to prick the point in the 

 celluloid directly over the station 

 hole. These protective celluloid 

 pieces should not be confused with 

 those described in 7341 for plotting 

 the distance circles, which are 

 fastened temporarily to the sheet. 



All R. A. R. positions must be 

 plotted by means of their distance 

 arcs so that a study can be made of 

 all the factors influencing the final 

 positions. The distance arcs are 

 plotted directly from the data in 

 the Bomb Record (8311), in which 

 the measured time intervals have 

 been reduced to the plotting velocity 

 of 1,460 meters per second (see 7341). The radii of the arcs are found on the smooth 

 sheet by applying short increments or decrements of time to the plotted distance circles. 

 Thus, the plotting radius for a 16.83-second time interval would be obtained by plotting 

 the increment 1 .83 seconds from the 15-second distance circle. To plot these increments 

 or decrements on the sheet, three methods are available: 



a. With Odessey R.A.R. protractor. — This device, the construction of which is described in 4537, 

 consists of closely spaced concentric circles on a piece of transparent celluloid, the circles being spaced 

 according to the plotting velocity, 1,460 meters per second, at the scale of the survey. When the device 

 is placed on the sheet so that the circle corresponding to the time increment or decrement is tangent to 



p 4? 



Figure 160.— Plotting R.A.R. positions with Odessey protractor. 



(Position illustrated is 33 seconds distant from station A and 27.5 seconds 



distant from station B.) 



