118 # 
only about 30 inches per second, and its measurement would be a matter of very 
great difficulty under sea conditions. Un the other hand, to explore the pressure 
for severai thousandths of a second would require pressure-bars of enormous length ; 
even for one-thousandth of a second the time-piece alone would have to be 84 feet 
long. It is feared that the utility of Hopkinson’s method will be confined to tank 
experiments with very small charges. Work on these lines is in hand at the Research 
Department, Woolwich. 
The dome of broken water which is thrown up at the surface when a submerged 
charge is fired (Section 8) is a phenomenon closely parallel with Hopkinson’s 
experiment. The pressure wave of the explosion is reflected from the surface 
as a tension wave, and the excess of tension over pressure causes separation of 
the upper layers of water, which rise with a certain amount of trapped momentum. 
If it could be assumed that the water separates at the least excess of tension, 
the velocity U with which the uppermost layer begins to rise above the surface 
would afford a measure of the maximum intensity P of the pressure (that is to 
say the maximum pressure that would exist ata point in the surface directly above 
the charge if the wave went on instead of undergoing reflection). It is easily shown 
that 
= 2P 
ap 
a being the velocity of the pressure wave and p the density of sea-water; or U = 66P, 
if U is expressed in feet per second and P in tons per square inch. With a 300-lb. 
charge of 40/60 amatol at a depth of 343 feet the gauge measurements show that 
P = about 1:2, so that the uppermost layer of water should begin to rise with a 
velocity of 79 feet per second, and under the action of gravity alone the dome should 
reach a height of nearly 100 feet. Actually the observations recorded in Section 26 
show that it rises only to about 35 feet. A 40-lb. charge at a depth of 18 feet and a 
1,900-lb. charge at a depth of 64 feet should also give an initial velocity of 79 feet per 
second, but in these two cases the actual height of the dome is about 25 feet and 
55 feet. The observed height is always much less than the calculated height, and the 
difference is greater for small than for big charges. The most probable explanation 
is the resistance of the air; this would make all the results low, but would have less 
effect in the case of big charges because they throw up a greater volume of water. 
By means of cinematograph records it may be possible to make direct determinations 
of the initial velocity U, and it will be interesting to see whether results obtained in 
that way agree better with the gauge results. In any case, however, the assumption 
on which the calculation is based is not entirely correct, since it has been shown in 
Section & that sea-water is able to bear considerable momentary tensions without 
breaking. 
Another method for determining the time-pressure curve, due to Sir J. J. Thomson, 
depends on the property possessed by quartz, tourmaline, and some other crystals, of 
liberating an electric charge under the action of pressure. The charge liberated at 
any instant is proportional to the pressure at that instant. A pair of electrodes on 
the crystal are connected to plates in a cathode-ray tube, producing an electrostatic 
field which deflects the cathode ray to an extent proportional to the pressure on the 
crystal. The movement of the ray is recorded on a photographic plate. An 
alternating magnetic field, of about a hundred cycles per second, produces a second 
movement of the ray, at right angles to that imparted by the piezo-electric action of 
the crystal, so as to draw out the deflection of the ray into the form of a loop, from 
the shape of which the whole time-history of the pressure can be deduced. This 
method has been developed at Shandon by Mr. David Keys, and successful experiments 
have already been made with miniature charges. No great difficulty is anticipated in 
applying the method under conditions parallel to those of the present experiments, 
and the results should be a valuable check on the gauge measurements. It may be 
hoped that this method will enable the time-pressure curve to be studied more closely 
and in detail than is possible by any other means at present known. For example, it 
is probable that the time-pressure curve of a charge surrounded by a big air-chamber 
is less simple than that of a naked charge ; it very likely has secondary peaks or 
irregularities, and the same may be true of charges fired on the bottom. The present 
gauges are not well adapted to show these features, and if they exist it is probable 
that they can only be brought to light by methods giving a continuous trace of the 
pressure. 
