644 



To reduce ambiguity in interoretation, the records of all except a few 

 early shots were made with artificial light provided by firing ten photo- 

 flash bulbs simultaneously with the shot. The flash bulbs were fired in 

 two specially«designed parabolic reflectors each holding five bulbs. 

 Number 21 or 22 bults were used for the high velocity shots; number 31 

 bulbs were used for the low velocity shots to take advantage of the longer 

 duration of the flash from the focal plane tyne of flash bulb. Illumination 

 from the flash bulbs was sufficiently intense so that variations of natural 

 lighting could be neglected. The increase in illumination also allowed 

 narrowing the vertical collimating slit in the GenereLL Radio camera from 

 0.1 in. to 0.01 in. This greatly increased the definition of the resulting 

 record although it made focussing and aiming a little more difficult. 



Synchronization of the photoflash bulbs, detonation of the charge, and 

 remote control of the cameras were obtained by the use of the time delay 

 circuit described in the Underwater Photography II report. 6) Both cameras 

 ran appro :d.mately 2.5 sec using 120 volts for the Fastax and 107 volts for 

 the General Radio cameras. These applied voltages caused the General 

 Radio camera to run 50 ft of film through at about 300 in. /sec and the 

 Fastax, 100 ft of film at about 3000 frames/sec. V/hen the cameras were 

 started, a switch in the primary of the timing spark coil was closed so 

 that the timers were operating only during the time the cameras were 

 running. 



Those parts of the recording mechanism which had to be relatively 

 close to the exploding charge, i.e., the two cameras and the timing 

 mechanism, were protected by being enclosed in a specially constructed 

 reinforced concrete box with a safety plate glass window. 



2, Tank Experiments 



In order to determine the dependence of dome velocity upon surface 

 tension it was necessairy to construct an experiment in which a reasonable 

 amount of water would be involved. Accordingly, a welded tank was con- 

 structed of l/8 in. sheet iron to the dimensions of a 3 ft cube with open 

 top. This tank was supported on a heavy base constructed of angle iron 

 such that the upper edge of the tank was five feet above ground level. 

 This placed the upper edge exactly in line with the optical axis of the 

 lens of an Eastman High Speed camera located inside the laboratory and 

 focussed on the center of the tank through a plate glass window as 

 indicated in Fig. 2. All the equipment was thus protected from the spray 

 resulting from the explosion. With the exception of the camera, all the 

 supplementary equipment was the same as that described under the 

 preceding section. As with the Fastax camera, the High Speed camera was 

 placed on its side so that measurements of successive frames would be 

 greatly simplified. Illumination was again obtained with synchronized 

 flash but, because of the decreased, distances involved, only five flash 

 bulbs were used for each shot. 



The explosive used for all of the tank shots was the Engineer's 

 Special Cap manufactured by the Hercules Powder Company of Wilmington, 

 Delaware, It comprises a metal tube 1/4 in. in diameter and 3 in. long 

 with the charge located in the end away from the detonator wires. The 



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