984 
2. 
(444) 
(iv) 
charge which was actually a sector of a sphere. (See 2 of Figure 5). The 
fabrication is indicated by Figure 8. The advantage of this form was that 
about one-seventh the amount of explosive ( 7~v 40 gm) was required which 
permitted the use of flash charges in areas where half=-pound charges were 
too large. 
Flash charges for depths down to 600 feet. The construction of a flash charge 
which would be waterproof in deep water and would not collapse under the 
hydrostatic pressure was accomplished by casting the pentolite in a tin can 
and using a thick (1/4 to 1/2 in.) disk of lucite for a window; Figure 9 shows 
the details. Photographs of this type of charge are given in Figure 5 (4a, 
4b). The window is clamped against a rubber gasket and a 1/32 in. hole is 
drilled in the window for an argon filling hole. This hole was later closed 
with a tapered plug of wood and Bostik. A metal tube, closed at one end and 
soldered to the base of the tin can, projected into the explosive to comprise 
the detonator well. The charge was detonated by inserting primacord into the 
open end of a non-electric blasting cap, wrapping the junction carefully with 
rubber tape, inserting the blasting cap into the detonator well, and initiat- 
ing the primacord. 
Other sources. Some single pictures of damage to model structures were taken 
with a #22 photoflash lamp or with a gas-filled Kodatron flash tube. Both of 
these lamps had to be protected from the explosion by a metal case with lucite 
window. The usual explosive flash charge technique was not employed in this 
instance because the final damage to the model would have been affected by the 
explosion of the flash charge. 
Underwater equipment. -- Figure 10 shows a sketch and Figure 11 photographs of a 
typical experimental set-up for underwater photography. The majority of experi- 
ments were carried out in rings of this sort. While the circular shape is not the 
most convenient for the suspension of miscellaneous objects, it is generally less 
subject to destruction from the explosions. Lightweight objects, such as paper 
diffusing screens, were stayed from the ring by light line, while cameras, gauges 
and other heavy gear were fastened directly to the ring or to sturdy auxiliary 
structures bolted or welded to the ring. 
Firing devices. -- The flash charge and the target charge were generally fired a 
short time apart by connecting them with primacord. Since it was sometimes 
undesirable to have primacord extending between the two charges, a method was 
developed for simultaneously firing two charges which were separated in space. 
The method was to discharge a 40 microfarad condenser, charged to 600 - 1000 volts, 
through two No. 8 SSS seismographic caps connected in series. Under these con- 
ditions the caps detonated within a few microseconds of each other. Primacord 
was still necessary to obtain delays, but more control was obtainable over the 
geometrical configuration of the primacord since it was no longer necessary to 
have 
a continuous length between the two charges. Furthermore, this eliminated 
the necessity of having primacord in the field of view rhen photographing explod- 
ing charges. 
Methods of motion picture photography. 
Relatively slow phenomena, such as bubble growth and structural damage, can be photo- 
graphed successfully using motion picture technique. We have used two conventional movie 
cameras and an Eastman High Speed camera, all of which require a continuous light source. 
For pictures in which a total duration of only 100 milliseconds is required, such as studies 
of damage from a 25 em charge at 600 ft. depth, a single #31 photoflash lamp has been used. 
For longer durations it is possible to use several photoflash lamps tripped in series. 
Eastman Super XX film or its equivalent has been used in all underwater movies. 
Cameras for motion picture photography. -- 
(1) 
Eastman high speed camera. Figure 12 shows the Eastman high speed electri- 
cally driven 16 m camera with attached Lord vibration mounts. The mounts 
slide into the tracks shown inside the case, (Figure 13) and shock mounts 
10 15518 
