520 
- 2- 
In further illustration of the effect of air cavities it may be of interest to 
compare the record of an ordinary 300 1b. Amato) depth charge (Figure 8 (11)) with that 
of a typical H.1! mine 320 lbs, Amato) (Figure 8 (12)). The H.!! mine in every case 
exhibits a second subsidiary peak of pressure which does not occur in the depth-charge 
record. The secondary peak has on the average a Prax about 12% of that of the primary 
peak and occurs about 1.4 thousandths of a second afterwards. The momentum in the 
secondary is about 10% of that in the primary pulse. It seems probable therefore that 
about 90% of the momentum is communicated directly to the water through the ends of the 
charge case whilst the remaining 19% follows subsequently through the spherical mine 
shell via the air-space. The time of passage of secondary wave through the air space 
(about 1 foot) would probably be of the order of a thousandth of a second as the records 
indicate). 
The record shown on Sheet 5 "h" is interesting. In this case the charge (18% Ibs. 
G.C.) was lashed to the end of a spar of soft wood (about 3 inches diameter). It is 
observed that the maximum pressure recorded is abnormally low, the customary peak of the 
curve having been cut off. It is difficult to reconcile this result, however, with the 
fact that the maximum pressure developed by a 320 1b. H.I! mine or an 184 1b. G.C. charge 
is practically normal in spite of the presence of a large surrounding volume of air. 
In these cases it should be observed that the irregularities produced by the air cavities 
frequently occur subsequently to the maximum pressure being reached. 
Reflection of Pressure Pulse from Surface and Bottom of the Sea. Reflection 
rom the Ilull of "Gorgon". 
It is a matter of importance in considering the effect of the explosion of a charge under the 
water to determine not only the magnitude and form of the direct impulse but also the impulses reflected 
from the surface and bottom of the sea. In the case of surface reflection we are dealing with the 
transmission of a pressure pulse from a denser to a lighter medium in which @se the reflected wave is 
similar to the primary wave but reversed in phase — i.e. the reflected pulse appears as a wave of tension. 
The actual process of reflection of a pressure pulse of large amplitude is complicated by the facts that 
water is incapable of transmitting a large tension and that the surface breaks up into spray at the onset 
of the primary pressure wave. Reflection from the bottom is comparatively simple but presents certain 
difficulties with which we shall deal later. In this case the pulse is travelling froma lighter toa 
denser medium so that the reflected wave has the same phase as the incident wave, i.e. the bottom reflection 
is a wave of positive pressure similar to that of the primary pulse. 
(a) Bottom Reflection 
When the cathode stream is traversed sufficiently slowly across the plate it is possible to record 
not only the primary preSsure pulse which reaches the p.e. gauge directly but also the reflected pulses 
from the surface and the bottom of the sea. From geometrical considerations it is a simple matter to 
determine the most suitable rate of traverse to record these reflected pulses. Typical records showing 
bottom reflections are reproduced on Sheet 8 a to d. 
The following table includes the results of measurements of records of this character. In the 
table hy and ho are the distances of the charge and p.e. gauge respectively from the bottom, the charge 
being 15 feet from the gauge in each case, 
Table wesc 
