94 10 
to take cinematograph records ; the actual measurements are from photographs, and 
cannot be regarded as very accurate, since it is difficult to expose the camera at exactly 
the right moment. The significance of the dome measurements is discussed in 
Section 22. Some photographs of typical domes and plumes are reproduced at the 
end of this report. 
When a charge is fired at a great depth the dome and plume efiects appear in an 
entirely different form. For example, when a 300-lb. Amatol charge is fired at a depth 
of 200 feet the surface above the charge, over an area about 200 feet in diameter, is 
observed to quiver, and a slight flicker of spray is thrown up, but the surface as 
a whole does not rise at all and there is no sign of the whiteness caused by disinte- 
grated water—on the contrary, if the sea is perfectly smooth the surface appears 
darkened, as still water is darkened by a catspaw of wind. Nothing further is seen 
until about 25 seconds later, when a large volume of creamy green fluid begins to 
pour up at the surface, consisting of an emulsion of bubbles and water, the residue of 
the explosion products. 
The fact that the surface in this case is not broken at the moment of explosion is 
a proof that the energy of the pressure wave is completely reflected (except a minute 
fraction which passes into the air) and since it is acase of reflection in a dense medium 
at the surface of a light medium the pressure wave must be reflected as a wave of 
tension. The pressure in the wave front when it reaches the surface is 0°2 ton 
per square inch, and the reflected wave starts downward with a tension expressed by 
the same figure ; it is clear therefore that the water is able to support a momentary 
tension of this amount without breaking. To discover how much tension sea water 
is capable of supporting, a series of 40-lb. and 300-lb. amatol charges were fired at 
different depths. The point dividing complete reflection on the one hand and 
complete disintegration of the surface on the other is not very distinct, and depends 
moreover on the state of the sea, the surface breaking more readily when there is any 
lop. but approximately the minimum depth for complete reflection at a flat calm 
surface was found to be 60 to 80 feet for a 40-lb. charge and 125 to 150 feet for 
a 300-lb. charge, corresponding in both cases to a pressure of about 0°3 ton per 
square inch, and it may be concluded that this is about the greatest tension that sea- 
water is capable of supporting, even momentarily. 
The idea of the reflected tension wave leads to a simple theory of the effect which 
the surface exercises on the pressure at any given point in the water. Assume, to 
begin with, that the pressure wave is completely reflected, without breaking the 
surface. The effect at a point B, Fig. 2, is found by superimposing the effects of the 
pressure wave X direct from the charge A and the reflected tension wave Y, arriving 
by the path A C B. The tension wave is weaker than the pressure wave in the ratio 
AB 
A‘'B 
and arrives later by an interval 
A’'B — AB 
a > 
a being the velocity of sound in sea-water. The result is (as shown at Z) that the 
first part of the pressure wave arrives at B entirely unaffected by the proximity of the 
surface, but after a certain interval the remaining pressure is obliterated by the arrival 
of the tension wave. 
If the pressure is strong enough to break the surface the matter is not so simple, 
but there is reason for believing that the same rule holds, at all events very nearly. 
To take an imaginary case, suppose that a plane wave of the form shown in Fig. 1 
travels vertically to the surface, and that it disintegrates the water to a depth of 2 feet. 
This means that the first 4 feet of the pressure wave (from t= 0 tot= ‘8 x 10°°, 
Fig. 1) fails to get reflected, its energy being spent in giving an upward momentum to 
the disintegrated water. The remainder of the pressure wave, from t = °8 X 10°, is 
reflected from the new surface, 2 feet below the original surface, as a wave of tension. 
The point to observe in the present connexion is, that while the tension wave has 
been shorn of its first 4 feet it has also 4 feet less distance to travel, and the moment 
of arrival of the front of the reflected wave at any given point in the water is there- 
fore the same as it would have been if complete reflection had occurred at the original 
surface. It may be concluded therefore that the rule stated in the preceding para- 
graph still gives the correct moment for the obliteration of the pressure at any given 
