S86 SECONDARY PRESSURE WAVES 



position of about 800 Ib./in.^ is in reasonably good agreement with the 

 value 600 Ib./in.^ from Eq. 9.12. The measured value of 400 Ib./in.^ 

 at the lower rest position is somewhat less than the computed value of 

 600 lb./in.2, indicating that assumption of a rigid boundary overesti- 

 mates the effect of the bottom even on this small scale. The comparison 

 of impulse with theory shows an unexpected rise at greater depths, but 

 it should be pointed out that no correction of experimental data for the 

 added pressures from bottom reflections was made (surface corrections 

 were applied when possible). 



600 



10 15 



DEPTH (ft) 



Fig. 9.7 Peak pressure and positive impulse 3 feet from 0.055 pound loose 

 tetryl charges fired in 24 feet of water. 



The most surprising feature of the peak pressure and, to a lesser 

 extent, impulse curves is the sharp maximum for a depth slightly less 

 than that of the upper rest position. The observed pulse in this narrow 

 region is of very large amplitude and relatively short duration and the 

 optimum peak pressure is more than double the level at greater depths. 

 Reproductions of experimental pressure-time curves for 280 gm. cast 

 charges in Plate X show the difference between this pulse at 4.3 foot 

 depth with a normal pulse at 8 foot depth. The reality of this pulse 

 has been confirmed for several sizes and weights of small charges and 

 for 300 pound TNT charges, but its occurrence is extremely sensitive 

 to charge depth and is realized experimentally with some diflficulty. 

 The existence of this ''anomalous pulse" is not predicted by any of the 



