ox METHODS FOJJ INIEASLKLNG TllESSURES IN THE BORES OF GUNS, 539 



chamber in the same experiment. The pressurep:, moreover, arc often 

 greatly above those which would exist were the charge absolutely confined 

 in a close vessel. 



A very striking instance may be cited from the early experiments of 

 the Explosive Committee with a M.L. 10-inch gun (fig. 8). The first round 

 was fired with a charge of 87^ lb. Belgian Pebble, the charge Ijeing lighted 

 in two places. The maximum pressure with the chronoscope was 25*2 

 tons. With the crusher gauge the pressure in the chamber varied from 

 22"2 to 24:"8 tons per square inch, while the energy developed by the 

 powder on the shot was 6,240 foot-tons. With the second round, all con- 

 ditions being the same except that the charge was fired at a single point, 

 the chronoscope pressure was as nearly as possible the same ; but the 

 chamber pressure was, at the rear, 79-1 tons ; in the middle 52-0 tons ; at 

 the seat of the shot 39-5 and 48-0 tons per square inch. A similar large 

 excess of pressure was shown at points 1 foot and 2 feet in advance of the 

 .seat of the shot, and the crusher gauges did not show their normal pres- 

 sures until points 5 or 6 feet from the seat of the shot had been reached. 



Yet with the violent difference in pressure shown betweeii the crusher 

 gauges in this round and in the previous round (which I have just cited), 



Fig. S. — Position of Pressure Plugs in 10-inch Gun. 



C 2 4 6 



the difference of energy developed in the shot was exceedingly trifling, 

 being only 6,249 foot-tons, as against 6,240. 



I believe I have expressed pretty clearly my views that crusher 

 gauges placed in the chase are for absolute determination not of much 

 value, and their main use, if used at all, is to give comparative results. 

 But the same remark does not apply to crusher gauges placed in the 

 chamber. 



Gases moving at a high velocity in the chase are, so to speak, perform- 

 ing their proper function ; but the same is not true of those violent waves 

 of pressure in the chamber which appear to accompany the explosion of 

 all brisante powders, and which occur either when the projectile has hardly 

 moved at all or when it is moving with a comparatively slow velocity. 



It is our object, and in this we have had great success, to avoid these 

 waves as much as possible ; and in attaining this end our indebtedness to 

 the crusher gauge is very great, as this instrument has made plain to us 

 not only the extreme violence but the variability of these oscillations. 



I have heard it urged that these waves of pressure are, after all, not of 

 high importance, because their maxima act at the same time only upon a 

 very small section of the bore, and the continuity of the metal is amply 

 sufficient to resist the stress. 



