1074 
Details of charges fired. 
16-1b._ charges. 
Three charges were fired; one each at depths (measured to the ctarge centre) of 8, 16 
and 32 inches. 
4Y-oz. charges. 
Three charges were fired; one each at depths (measured to the charge centre) of 2, 4 and 
8 inches. 
Expe rimental procedure. 
The tests were carricd out in a disused gravel pit, in about 15 ft. of water. 
16-1b. charges. 
Figures 1(a) and 1(b) show in plan and elevation the layout used for the larger charges. 
A row cf photoflood bulbs fixed to a vertical pole and spaced as shown In figure 1(b) was 
viewed using a double mirror periscope through the horizontal slit of a drum camera. The lowest 
lamp and the centre of the lower periscope mirror were each set at approximately 3 in. from the 
water surface. The charge was arranged to Vie in the plane containing the lamp standard and the 
lower periscope mirror. The method by which it was Supported, and its helght adjusted, will be 
apparent from figure 1(b). The length of the bamboo pole used in the charge mounting was 
approximately 10 ft. and the sealed cans uscd to provide the necessary buoyancy were about 4 ft, 
below the charge. Each charge was fired through a switch operated by the shutter of the drum 
cameras 
The complete mechanism used for supporting the charge was destroyed by each shot, 
4Y-oz, charges. 
Figure 1(c) shows the arrangement used for the 4-oz. charges. 
A lens cover plate provided with 4 ins slots, together with a rotor having a single 
4 in. slot was used on the Marley camera. With this combination a sequence of 60 pictures is 
recorded during one complete revolution of the rotor, with a lens aperture of about f/12. 
To record the plume with this aperture, artificial illumination was necessary. This 
was provided by the two flash bombs shown in figure 1(c), each consisting of a 20z, pellet of 
C.E. surrounded by 2 oz. of aluminium powder, Ouring the tests the camera Was shielded from the 
direct flash of these charges by cardboard screens. 
The detonators used to fire the underwater charge and the flash bombs were wired in 
parallel. To ensure adequate illumination in the early stages of the explosion very short delay 
detonators were used in the flash bombs. 
Records obtained. 
16—1b. charges. 
Figures 2, 3 and 4 show the records taken on the 16-1b. charges at depths of 8, 16 and 
32 in. respectively. 
The lamp images focussed on the moving film give rise to luminous traces, As a shock 
front rising between the camera and the lamp standard intercepts the light pencils subtended at 
the camera by each lamp, the rays are refracted momentarily out of the camera slit causing small 
breaks in the lamp traces. The traces are terminated as the lamps become obscured by the rising 
water surface. 
Breaks marking the progress of the shock wave can be seen in figure 4, but they are 
absent in figure 2. This indicates that a shock wave travelling faster than the water surface 
was not detected with the charge centre 8 in. below the surface. With the charge 16 in. deep, 
figure 3 ses. 
