236 | ANNUAL REPORT SMITHSONIAN INSTITUTION, 1944 
great. This decrease in temperature causes a decrease in velocity 
which refracts the sound wave downward. 
Thus it is seen that for any given case in R. A. R. the path along 
which the received sound has traveled may be very complicated. It 
may have been reflected a number of times from the bottom and the 
surface, and between these reflections it may have been refracted, either 
upward or downward, or perhaps in both directions at different depths. 
The excellent results obtained in R. A. R. are due to the fact that 
water is a relatively good medium for the propagation of sound, even 
though its physical characteristics, and consequently the velocity of 
sound, vary with time, place, and depth. It is due also to the good 
reflecting qualities of the water surface and the ocean bottom. The 
sound is confined vertically and is reflected and amplified, somewhat 
as it is in a speaking tube. Little of the energy of the sound wave is 
actually lost:in reflections, although when the sea is rough or the ocean 
bottom irregular, some of the sound energy may be dissipated. 
R. A. R. IN PRACTICE 
Subaqueous sounds have been detected with instruments of only 
ordinary sensitivity at a distance of 400 nautical miles (740 km.). A 
sound propagated vertically downward by an electromagnetic oscilla- 
tor in a depth of about 200 fathoms (365 m.) has been heard after 
having been reflected 23 times alternately from the bottom and the 
surface. In R. A. R. the longest distance that has been measured is 
184 nautical miles (340 km.). This was in connection with a test 
which was concluded at that distance, but there was no observable 
diminution in the intensity of the received sound as compared with 
that received at somewhat lesser distances. In actual hydrographic 
surveying, distances of 100 miles (185 km.) or more have often been 
measured. Shore stations are much more efficient in this respect than 
sono-radio buoys, although returns have been received from sono-radio 
buoys at distances of 100 miles (185 km.). The type of area in which 
sono-radio buoys are preferred to shore stations ordinarily limits their 
range to about 30 or 35 miles (55 or 65 km.). 
The operation of R. A. R. to control hydrographic surveys is now a 
routine procedure. The position of the survey ship is fixed regularly 
by R. A. R. at intervals of 10 minutes or less with as much casualness 
as if three-point sextant fixes were being used. 
The positions of the receiving stations are plotted on a projection, 
just as the positions of triangulation stations are. Because of the long 
distances ordinarily involved, the distortion which occurs in a plotting 
sheet made of even the best drawing paper has considerable effect. 
For this reason, a number of uniformly spaced concentric circles are 
