120 36 
this theory to the pressure wave from a 300-lb. amatol charge at a distance of 
55 feet from the charge, where the muximum pressure is about U"7 ton per square 
inch, Under this pressure the density (p) of the sea-water is 0°5 per cent. higher 
than normal, and the modulus of compressional elasticity— 
pilp 
dp 
is about 4°5 per cent. higher (Tait, Challenger Report), so that the velocity a’ of 
sound in the compressed medium is 2 per cent. higher than in the-undisturbed water. 
The velocity of the medium itself is— 
Le = 23 feet per second, 
ap 
or 0°5 per cent. of the velocity of sound. Consequently the velocity of the pressure 
wave, a + w, should be 2°5 per cent. higher than the velocity of sound. Theoretical 
considerations therefore confirm the experimental results in indicating that the 
velocity of the pressure wave does not exceed that of sound by more than a few per 
cent. In the region near the charge the difference is no doubt greater. 
Thirty years ago Threlfall and Adair made a series of experiments on the 
velocity of the pressure wave generated by small charges in sea-water (Proceedings 
of the Royal Society, 1859). Sensitive contact makers were mounted on piles at 
approximately 30 and 200 yards respectively from the charge, and the closure of the 
contacts was recorded on a pendulum chronograph. With a Y-oz. charge of guncotton 
they found a velocity of 5,7U0 feet per second, and with a 4-lb. charge of guncotton 
the velocity was 6,600 feet per second. It is difficult to reconcile these results, which 
among themselves were very consistent, with those obtained in the present experi- 
ments. Sir R.-Threlfall has informed the writer that the possibility of a disturbance 
having been propagated through the sea-bottom and the supporting piles was 
carefully weighed and rejected at the time of the experiments. 
The determination of the velocity of the pressure wave is of more than theoretical 
interest ; it is a factor which has to be known in estimating the effect of the surface 
on the pressure at a point in the water (Section 8); moreover, the demonstration that 
the velocity is practically the same as that of sound strengthens the argument that 
the pressure wave in all respects approximately obeys the simple acoustic laws. 
(24) Measurements of the Echo reflected from the Bottom. 
All that was attempted in tls direction was a measurement of the whole time- 
integral of the pressure in the reflected wave. Tor the purpose of this measurement 
it was necessary to modify one of the gauges described in Section 18 in such a way 
that the main pressure wave would have no effect except to set the gauge in readiness 
to be operated by the reflected wave. The bottom of a GZ gauge (Fig. 35) was fitted 
with a shutter A, Fig. 39, held by a powerful spring B against a detent C in one of 
the piston holes; the pressure of the main pressure wave forces in the detent C 
against a spring and also holds the shutter against the bottom of the gauge; when 
the pressure of the main wave ceases the shutter is pulled round by the spring 
against a stop D, in the next piston hole to the detent C, and exposes two piston holes 
E containing working pistons, which are afterwards operated by the reflected wave. 
The two remaining piston holes were blocked. The time-constant of the shutter was 
measured by the spark chronograph described in Section 27, and was found to be 
about 7 < 107° second. The working pistons were # inch long, with 1? inch free 
travel. Lead crushers were used in place of coppers. 
In one typical experiment with this gauge, the charge (300 lbs. 40/60 amatol) 
was 344 feet deep; the gauge was 40 feet deep and 50 feet away from the charge ; 
the bottom, which was mud, was at 21 fathoms. The time-integral of pressure at a 
distance of 50 feet from this charge is I = -68 (Section 4); the distance travelled by 
tke reflected wave, from charge to bottom and from bottom to gauge, was 154 feet, 
so that if complete reflection occurred the time-integral of pressure of the reflected 
wave would be "185; actually the gauge gave it as only “V68, corresponding to a 
coefficient of reflection of 0°37. In three similar experiments the values found for 
this coefficient were 0°29, 0°47, 0°47. It may be concluded therefore that reflection 
of a pressure wave from a mud bottom reduces the time-integral of pressure to less 
than half its incident value. 
