ON METHODS FOR MEASURING PRESSURES IN THE BORES OF GUNS, ool 



Majoi" Rodman appears to have considered it impossible that any 

 gauge could rightly indicate a pressure liigher than that indicated by 

 another nearer to the seat of the shot. This, however, is not so ; nothing- 

 is more certain than that, with the powders known as 'Poudres brutales,' 

 and, possibly, in a less degree with all explosives, motion is communicated 

 to the shot by a series of waves or impulses ; and it is easy to see that, if 

 the position of a gauge coincided with the ' hollow ' of a wave, while that of 

 a more forward gauge coincided with the ' crest,' the latter might easily 

 show the higher pressure. Later on I shall revert to this point. 



The crusher gauge is a modification of the Rodman gauge, designed 

 to overcome some of the defects of that instrument, and it is now almost 

 universally used for the direct measurement of pressure : it is shown in 

 the diagram exhibited (fig. 2), and its action is easily understood. The powder 

 gases act upon the base of the piston, compressing the copper cylinder ; 

 the amount of crush on the cylinder serves as an index to the maximum 

 tension acting on the piston. It is usual, where possible, to employ in 

 each experiment two or three gauges so as to check the accui'acy of the 

 determination. Properly used, very great confidence may be placed in 

 their results ; but, as may be gathered from my remarks on the Rodman 



Fig. 2. — Crusher Gauo:e. 



gauge, this and all similar gauges will cease to give reliable information 

 as to the energy th.at can be impressed on a projectile, or as to the mean 

 pressure on tlie surface of the bore, if there be any probability of the 

 products of explosion being projected into them at a high velocity. In 

 such a case the pressure indicated would not be the true gaseous pressure, 

 such as, for instance, would exist were the products of ignition retained 

 in a vessel impervious to heat until the waves of pressure generated by 

 the explosion had subsided. But I defer an examination of the results 

 given by the crusher gauge until I compare these results witli those given 

 by the indirect method of deducing the pi'essure from the motion of the 

 projectile within the bore. 



The method I have adopted for this purpose consists in registering the 

 times at which a projectile passes certain fixed points in the bore of a gun. 

 The chronoscope (figs. 3 and 4), which I have designed for this purpose has 

 been so often described that I shall only here briefly allude to it. It consists 

 of a series of thin discs made to rotate at a very high and uniform velocity 

 through a train of geared wheels. The speed with which the circum- 

 ference of the discs travels is between 1,200 and 1,300 inches per second, 

 and, since by means of a vernier we are able to divide the inch into 



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