the charge was thrown over the side versus the direct wave 
travel time. This will also plot as straight line depending 
on the constancy of the ship's speed and course. Besides 
checking the value of D, this graph is useful in giving 
the velocity of the shooting ship when there is no pit 
log on that ship. Further, the navigation plot in con- 
junction with the ship's log gives the range of the reverse 
point when no shot was fired at this station. 
IX. Ry - D Versus Ry Graphs 
Depending on the thickness of the isothermal surface 
water layer, the direct wave is recorded from six to forty 
miles. On those refraction records for which there is no 
D present, R, is used to determine the range. In the 
2 
a 
range at short distances (less than 40 miles), and curved 
at 
laboratory the R 
versus p° plots are used to find the 
ray path calculations are used for the longer shots. However, 
at sea it is desirable to have a quicker method for the 
rough travel time plots. For this purpose the BS D versus 
Ry curves are included (see Figure 20). Knowing the value 
of Ry and the depth of the water, the expected time interval 
between R, and D can te read off the graph. These curves 
are not accurate for the longer shots because the velocity 
structure in the water becomes important, and the simple 
straight line, average velocity calculations are in errore 
These curves are plotted from the equation, 
