RADIO ACOUSTIC RANGING—ADAMS 231 
nected with the chronometer and makes a mark on the tape once a 
second. Another stylus is connected with the chronograph amplifier 
and is actuated by the reception of the bomb signal and later by the 
reception of the radio signals. This record permits the scaling of the 
time intervals to the nearest 0.01 second by interpolation. 
A different instrument, called the Dorsey chronograph, designed 
and built by the Coast and Geodetic Survey, is also used for the same 
purpose. It incorporates electric time measurement, starting with a 
piezoelectric crystal, and provides much more constant and correct 
time than a break-circuit chronometer. The tape in this chronograph 
runs at a speed of about 5 centimeters a second, and a mark is made each 
tenth second and the seconds are numbered. The instrument is also 
automatic in that the electric bell signal signifying that the bomb has 
struck the water starts the tape moving and the timing stylus begins 
marking tenth seconds. When the hydrophone is actuated by the 
bomb explosion, the next tenth second is marked zero and the mark at 
each subsequent second is numbered. Time intervals to the nearest 
0.01 second can be interpolated from this record by eye. 
BOMBS 
For use in R, A. R. a sound of great intensity reaching a peak almost 
instantly and one that will travel through the water in all directions is 
required. The explosion of a trinitrotoluene (TNT) bomb has been 
found best adapted for the purpose, although any type of explosive 
suitable for use under water can be used. Dynamite has been used, but 
it is more unstable and more dangerous to handle than TNT. The 
frequency of a bomb explosion is below 300 cycles, which is lower than 
most interfering noises. 
The TNT itself does not have to be in a watertight container. For 
best results the container should be made of a comparatively strong 
rigid material so that the gases generated are contained until detona- 
tion is complete, whereupon the container bursts. The resulting ex- 
plosion produces a highly compressed sound wave that has a greater 
range than one from an explosion in a comparatively fragile container. 
For long distances and best results, hollow cast-iron spheres with 
half-inch walls, containing from 1 to 4 pounds of TNT, are used. 
These spheres have to be especially cast and are expensive and heavy 
to handle. For ordinary distances and for perhaps 95 percent of the 
cases, ordinary commercial tin cans with a friction top are used. 
Three sizes of the latter are commonly used, 14, 14, and 1 pint, depend- 
ing on the distances involved and the characteristics of the area being 
surveyed. 
Ordinary commercial detonators made of fulminate of mercury are 
used with standard waterproof fuse to detonate the TNT. Tin cans 
619830—45—16 
