- 9 
salinity quoted in various oceanographic reports for the 
particular area and season involved. Sound velocity is 
not critically dependent on the variations observed in 
salinity so that with the bathythermographic observations 
that have been taken the surface sound velocity can usually 
be quoted to t 2 feet per seconde The average vertical 
velocity is obtained from "Tables of the Velocity of Sound 
in Pure Water and Sea Water," published by the Hydrographic 
Department, Admiralty, London (1939). A check on this 
value is obtained from the R® versus D® plots. These two 
values usually agree to ¢ 10 feet per second, but for 
the sake of uniformity the British Admiralty Table value 
is used in all refraction calculations. 
VIII. Check Plots 
In order to insure confidence in the data used in 
the travel time graphs, two check graphs are made. These 
are the R© versus D© and the navigation plots. 
gs7--aplotted against De will produce 
straight lines except for changes in depth or slope of the 
bottome These graphs will then bring out any obvious 
errors that have been made in R or in D. The slope of this 
line is the square of the ratio c,/@,, which gives a check 
on the value oLedned from the Eritish Admiralty Tables for 
Cy» The value for Cy can not be read from this plot to 
better than # 10 feet per second. 
The navigation plot is a graph of time of day that 
