TEST B: UNDERWATER EXPLOSION 



few thousandths of a second, and failed to penetrate 

 the capillary tube appreciably; but the slow-moving 

 water wave had no trouble in exerting its effect through 

 this tube.* A number of bottom-mounted hydrophones 

 (inductiphones) were used also. These, too, measured 

 wave height in terms of excess pressure at the bottom 

 of the lagoon. 



Some wave-height gages were mounted directly on 

 the target vessels. These gages, called portable record- 

 ing echo sounders, made continual records of distances 

 to the lagoon bottom. Thus as a ship rose and fell on 

 a great wave, its changing elevation was recorded. t 

 Some echo sounders were mounted on buoys, which rose 

 and fell more freely than the cumbersome ships. 



A few of the buoys sent off their wave-height data 

 at once, by radio, to scientists in a PBM plane circling 

 10 or 15 miles away. 



The last important date before B-Day w^as 19 July 

 1946, when the final rehearsal was held. It went 



* These instruments, powered hy batteries, could only operate 

 for limited periods and therefore had to he started almost at the 

 last moment before the explosion. To make final adjustments on the 

 starting mechanisms a number of picked men were kept aboard cer- 

 tain of the target ships until a few hours before Mike Hour. These 

 men breathed a great deal easier when the small boats showed up 

 on schedule to take them out of the lagoon. 



t The echo sounder's main coynponent is a7i imderwaier "trans-' 

 ducer," or combination transmitter and receiver of souyid waves 

 in water. The transducer gives out a sudden pulse of sound, listens 

 for the echo from the bottom, and measures the time interval before 

 the echo is received. An increase in time interval corresponds to 

 an increase in depth. 



49 



