88 4 
was evolved depending on the use of soft copper plugs to register the momentum 
acquired by steel pistons exposed at their outer ends to the pressure in'the water and 
free to move inwards towards an anvil. The construction, theory, and use of the 
gauges are described in Part Il. of this report, Part J. dealing only with the results 
obtained, and it is sufficient to say here that the measurements from a set of gauges 
enable a stepped diagram to be drawn, representing the average pressure during 
different periods at a given place in the water, as illustrated for example in Fig. 1. 
A curve drawn through the steps in such a way as to take in as much as it leaves out 
is assumed to represent the time-history of the pressure. The maximum pressure 
was determined separately by a somewhat different type of gauge. 
Some preliminary experiments were made at Portsmouth, but as soon as the 
gauges had passed out of the experimental stage approval was obtained for carrying 
out a systematic programme of investigation in deeper water. The locality selected 
was Troon, on the Firth of Clyde, and H.M. Drifter “‘ Malapert ” was allocated for the 
purpose. The experiments at Troon commenced in August 1918 and were concluded 
in the following April. A list of the charges fired is given in Appendix I., they were 
mostly of regular Service types and ranged from 40 lbs. of explosive to 1,900 lbs. 
One hundred and seven shots were fired altogether, amounting to about 21 tons of 
explosive. 
The executive arrangements and the working party and ship were under the 
direction of Lieut.-Commander D. Errington, R.N. 
(2) Scope of the Results. 
Broadly summarised, the experiments have shown that the pressure wave from a 
submerged charge of high explosive is a very regular and symmetrical phenomenon. 
In the region investigated, that is to say, at distances between 25 and 100 feet from a 
5U0-lb, charge and corresponding distances from other charges, the pressure wave 
follows very approximately the simple laws of sound, the velocity of the-wave being 
the same as that of sound and the pressure falling nearly in simple proportion to the 
distance. All the principal features can be expldined from the standpoint of acoustic 
theory. For example, the influence which the surface of the water exercises on the 
pressure at any given point can be completely accounted for by assuming that the 
pressure wave is reflected from the surface as a wave of tension. 
The pressure wave from a big charge is more intense and more sustained than 
that from a small charge, the two pressure waves being connected by a definite 
relationship deduced from the principle of dynamic similarity. 
Other questions examined include a comparison of various high explosives 
(T.N.T., 40/60 amatol, 80/20 amatol, guncotton and ammonium perchlorate) and 
also of some powder charges; the effect of surrounding a charge with a large air 
space, as in a buoyant mine; the effect of exploding a charge on the sea-bottom 
instead of in mid-water; the influence of the shape of the charge as affecting the 
symmetry of the pressure wave ; and the effect of composite charges, made by lashing 
together several charges, of which only one is primed and fired. 
By observing the deformation of standard mine cases suspended at known 
distances from various charges a beginning has been made in the investigation of the 
relative damaging power of different pressure waves, and it has been proved that the 
extent to which a structure is damaged is not entirely determined by the maximum 
intensity of the pressure, but also by the period for which the pressure is sustained. 
(3) Nature and Genesis of the Pressure Wave. 
Before describing the results in detail it is useful to picture what probably 
happens at the moment when a submarine charge is fired. Assume that the charge 
is a sphere of T.N.T., weighing 300 lbs. and therefore 22 inches in diameter, and 
that detonation is initiated at the centre. The velocity of detonation in T.N.T. is 
24,000 feet a second, so that in about four hundred thousandths of a second the whole 
of the explosive is transformed into incandescent gas under enormous pressure. The 
globe of gas expands’ rapidly (but much less rapidly than it would in air, because it 
has the additional inertia of the surrounding water to overcome) and in a thousandth 
of a second the volume of gas has probably expanded 5 or 10 times and the pressure 
fallen to 2 or 3 per cent. of its first intensity. As the pressure falls the gases expand 
more slowly. Jn the first few hundredths of a second levitation has no time to lift them 
