543 



(c) The mounting of gaviges on a steel ring (Figure ik) resulted 

 in greater damage than any type of wooden frame mo\mting. This in- 

 crease varied from 6 to 10^ over the type shown in Figure 13 to 30 to 100^ 

 over the regular type of Figure 11. The change in type of gauge backing 

 from spruce to oak increased damsige 2 to i*-^. Typical results with the 

 steel ring are shown in Table VII. 



An additional experiment in which a chain was stretched between 

 gauges moimted on a regular wooden frame (Figure 11) to prevent their 

 swinging away from the explosion more than about 1 in. resulted in no 

 increase in damage over that usually produced. 



It was concluded that, in general, the lighter the wooden frame 

 the more the damage, and the greater the charge-to-frame distance, 

 the more the damage. 



A suggested explanation for the remarkable increase in damage 

 (25 to 100^) when gauges were mounted on a steel ring rather than at 

 the comers of a square wooden frame was that the wood, being a medixim 

 less dense than water, reflected a rarefaction wave which partially 

 destroyed the shock-wave. Another proposed explanation was that the 

 damage was increased by the more rigid backing of the steel ring. To 

 test the reflection theory independently of the rigidity with which 

 the gauges were mounted, shots under three sets of frame conditions 

 were fired. One was a frame consisting only of the ring; another 

 consisted of the ring and an "outside" wooden frame (Figure 13); and 

 the last consisted of the ring and an "inside" wooden frame (Figure 11) . 

 For this and the following frame experiments, except where otherwise 

 noted. Lot k steel diaphragms were used, the charge-to-gauge distance 

 was k8 in., the charge employed for each shot was 2.31 lbs. THT, and 

 two shots were fired for each arrangement. 



It was found that the average ratio of the damage obtained with 

 the outside frame and ring, to the damage obtained with the ring alone 

 was O.9O; the average ratio of the damage obtained with the ring and 

 inside frame to the damage obtained with the ring silone was 0.59« 



To msike sure that the decreased damage found in these cases was 

 caused by reflection of the shock-wave from the wood and not by some 

 phenomenon involving actual contact of the wood and the gauges, the 

 following experiment was next performed. Shots were fired in which 



(1) boeirds were lashed to the ring outside the circle of the gauges; 



(2) boards were lashed to the ring within the circle of the gauges 

 but below the plane of the gauges so as not to be directly between 

 the charge and the gauges; (3) boards half as wide as in (2) were 

 lashed to the ring as in (2) . The average ratios of the damages 

 obtained compared with the damage obtained with the ring alone were 

 found to be 1.00, 0.82, and O.9O, respectively. 



To investigate the possible effect of movement of the gauge as 

 a whole on the damage of the diaphragm, shots were fired using the 

 ring alone in which one, two, and three gauges were used. In the 



