inereases with depth. Above this critical depth, the temperature change 
has 2 greater effect than pressure change on sound velocity. The pressure 
change has a greater effect below this depth; As a consequence, a sound 
originating at a depth of about 4000 feet is bent downward by the layers 
of water above it and upward by the layers.of water below. 
8, Sound is therefore channeled horizontally at a depth which actually 
varies in different parts of the ocean, but is sufficiently constant to 
z make the SOFAR system possible, It spreads horizontally in all directions, 
: but many of the sound rays which tend at first to go up or down are bent 
_-—s back into the horizontal channel. 
; 9. When an explosion at a distance of several hundred miles or more is 
i. ‘received at a SOFAR monitoring station, the signal sounds like the roll 
Ab of a kettle drum building up to a crescendo and ceasing abruptly. This 
peculiar quality of the received signal is explained on the hypothesis 
that the sound travelling by the direct route from exploding bomb to 
hydrophone is also travelling by the slowest route, The sound waves which 
are refracted from above or below cross and re-cross the axis of the sound 
channel, actually reaching the hydrophone before the direct sound because 
___ they travel through layers of water in which the velocity is greater, The 
time interval between the first sound reaching the hydrophone and the sub~ 
seguent sharp cut-off is about one second for every thousand miles, 
. DESCRIPTION OF THE EQUTEMENT 
10, The receiving, recording, and timing equipment for one POO station 
consists of: 
(a) A 5-foot cabinet-type panel rack {itsck 1), containing dual 
amplifiers, power supplies, attenuators, and a monitor power  , 
amplifier. 
(b) A 4-foot cabinet-type panel rack {liack 2), containing the auto~ 
ie matic switching device, the "slow-speed" PL Recorder, and a Magnetic 
ee", _ Tape Recorder (number 2). 
(ce) A 4-foot cabinet type panel rack (Reck 3), containing the break 
circuit chronometer, the "high-speed" PL Recorder, and a Magnetic 
1 Tape Recorder (numberl). 
(d) A monitor speaker mounted in a bass reflex cabinet. 
(e) A General Electric voltage stabilizer. 
(f) Connecting cables, spare parts, and a supply of waxed-—paper 
tape for the PL graphic level recorders, 
