RADIO ACOUSTIC RANGING—ADAMS 235 
intensity to actuate it is, of course, the one that is used in R. A. R.-— 
other sound waves arriving later serve only to prolong the received 
signal. Unfortunately, the sound traveling via the direct path is 
almost always canceled by the sound wave reflected from the surface 
of the water. This surface-reflected sound wave is reversed in phase, 
and as the length of its path is nearly equal to the direct path, almost 
complete cancelation of the two takes place. In actual experience the 
sound via the direct path is rarely received at distances greater than 
7 or 8 miles. 
The result of this is that the useful sound wave in R. A. R. is the 
one that is reflected at least once from the ocean bottom and, depending 
on the depth of the water and its physical characteristics, the sound 
may arrive at the receiving station after having been refletced a num- 
ber of times between surface and bottom. 
Another complication is the fact that the surface boundary is hori- 
zontal, but the ocean bottom is not. A sound wave is reflected from a 
boundary in the same way as a ray of light is reflected from a mirror, 
the angle of reflection being always equal to the angle of incidence. If 
the water is deep at the bomb explosion but the receiving station is 
located in comparatively shallow water, as is the usual case in R. A. R., 
it is obvious that the bottom slopes upward along the effective path of 
propagation. In such a case, each time the sound wave is refiected 
from the bottom, its direction of propagation is changed toward the 
vertical, and if enough reflections are involved and the slope of the 
bottom is sufficiently great, the successive angles of reflection may be 
decreased until the sound wave is reflected vertically upward or it may 
actually reverse its horizontal direction of propagation, and never 
reach the receiving station. This condition is aggravated in shoal 
water where more reflections take place in a given horizontal distance 
than do in deep water of the same characteristics. This partly explains 
the difficulty encountered in sound transmission from deep water on 
the Continental Slope to shoal water on the Continental Shelf. It also 
explains the difficulty encountered in R. A. R. where there are inter- 
vening shoal areas between the bomb explosions and the receiving 
stations. 
The path of a sound wave is also affected by refraction. Wherever 
a change in the velocity of sound takes place along the path, the sound 
wave is refracted. If pressure were the only characteristic affecting 
velocity of sound, its constant increase with depth would cause a con- 
stant increase in velocity, and the sound wave would be refracted in 
the arc of a circle concave upward. It is rare, however, that pressure 
is the only variable involved. The temperature of the water varies 
and normally decreases with depth more than enough to overcome the 
increase caused by pressure, until the depths become comparatively 
