859 
ABSTRACT 
In the Buresu of Ordnance the damaging efrect of 
underwater weapons is at present measured by piston geuges. 
Since the difference in peak-pressure and impulse between the 
better explosives is only of the order of 30%, it is important 
to have a reliable theory of the gauges which are used to compare 
explosives end weapons. Therefore, the theory of piston gauges 
is considered here. It is now customary to use such a gauge in 
the "side-on" orientation, namely, with its pressure-sensitive 
face perpendicular to the face of the shock. In this nosition 
theoretical interpretation of the measurements is difficult be- 
cause of diffraction and nossible Mech reflection. In the same 
orientation exnerimental disnversion is larger than for the "heed- 
on" position (where the shock is incident normally on the sensitive 
face of the gauge). Hence, in this revort, the theory is given 
for the head-on orientation. The effect of rarefactions emitted 
by the recoiling pistons on themselves and on esch other is cal- 
culated. This effect is small; it alters the pressure by only 
a few vercent. On the other hand, the recoil of the front face 
of the block affects the pistons so much that it makes the hesd- 
on gauge useless if the block 1s not thick. Therefore, the block 
should be thick; the necessary thickness is estimated here. 
Another correction to be considered is due to the Bernoulli effect. 
= 1S 
