530 EEPORT— 1894. 



the Rodman gauge gave pressures varying from 2-i-G to 38"9 tons per 

 square inch. 



In the second series, all conditions being the same, except that the 

 charge was fired from the extreme rear, the maximum chronoscope 

 pressure was 14 '3 tons, while the Rodman pressure varied from ol-G 

 tons per square mch to over 50 tons per square inch, that pressure being 

 the highest which the instrument was capable of registering, every obser- 

 vation in this series with the gauge placed at the seat of the shot being 

 over fifty tons. 



Shortly afterwards the Rodman gauges were destroyed, two of them 

 being blown from the gun. 



These discrepancies led the Committee to investigate with certain 

 powders the variation in pressure indicated when a gauge was placed at 

 the surface of the bore and at the exterior of the gun as with the Rodman 

 gauge. 



For this purpose they used the crusher gauge, which admits of being- 

 placed in both positions. 



With pebble powdor the gauge placed at the interior of the bore gave 

 14'5 tons; placed under precisely tJie same conditions at the exterior 

 it gave 27 tons per square inch. With R.L.G. the similar figures were 

 re.spectively 20 and HZ tons, and with L.G. respectively 19'5 and 45"5 tons 

 per square inch. 



The error I have just discussed was due to the position of the gauge ; 

 but R,odman's pressures and the pressures of the Explosive Committee 

 were exaggerated from another cause. It will be readily understood that 

 if a pressure of, say, 20 tons per square inch be suddenly applied to a 

 gauge, and if the resistance to the motion of the knife be initially trifling, 

 a certain amount of energy will be communicated to the piston and knife ; 

 and the copper when measured will indicate not only the gaseous pressure, 

 but in addition a pressure corresponding to the energy impressed upon 

 the piston during its motion. 



This cause of eiTor can, however, be eliminated by producing beforehand 

 by mechanical means a cut indicating a pressure a little less than that to 

 be expected. 



Rodman admits that his chase pressures are erroneous ; their ex- 

 aggeration is no doubt greatly due to the causes I have just pointed 

 out ; but in my opinion, based upon long experience, no gauge of tlli^< 

 description placed in the chase, where the products of explosion arc 

 moving with a very high velocity, can be depended upon to give reliable 

 results. 



If we disregard the energy of the moving products and suppose thi-, 

 gauge to be acted on by pure gaseous pressure, with a projectile moving 

 at the rate of 2, .500 f.s. (and such velocities are now quite within tluf 

 range of practical ballistics), the projectile would pass the entrance to the 

 Rodman gauge in something like the ^7,-J^„„t]i part of a second. It is 

 difficult to imagine that the full indentation could be given to the copper 

 in this small fraction of time, and, if it were not so given, the gauge 

 would indicate the pressure at a point considerably in advance of the 

 gauge. 



On the other iuind, if, as would generally be the case, the products of 

 explosion moving at a high velocity acted on the piston, the energy of 

 these products would be reconverted into pressure, and the gauge would 

 in this case give too high a result. 



