ON METHODS FOR MEASURING PRESSURES IN THE BORES OF GUNS. 525 



iiicli, with the lightest projectile, to 9-6 with the heaviest ; and with 

 this powder it will be observed that the energy developed increased 

 steadily and considerably with each increment in the weight of the shot, 

 while the low chamber pressure shows that, even with the heaviest shot, 

 the projectile must have moved a considerable distance from its seat 

 before the charge can be considered to have been entirely consumed. 



I have mentioned the discrepancy between Rumford's two series of 

 experiments. This discrepancy was very great, the one series giving, for 

 a density of unity, a tension of about 190 tons per square inch, or 29,000 

 atmospiieres, the other series giving a tension of over 101,000 atmospheres. 

 It is remarkable that Rumford makes no attempt to explain this discre- 

 pancy, but, as he deliberately adopts the higher tension, it is not impro- 

 bable that he was led to this conclusion by an erroneous estimate of the 

 elastic force of the aqueous vapour contained in the powder or formed by 

 its explosion. He considered, relying on M. de Betancourt's experiments, 

 that the elasticity of steam is doubled by every addition of temperature 

 equal to 30° F., and his only difficulty appears to have been — he expresslj' 

 leaves to posterity the solution of the problem — why the tension of fired 

 gunpowder is not much higher than even the enormous pressure which 

 his experiments appeared to indicate. 



It will be remembered that Rumford's apparatus consisted of a small 

 but strong wrought-iron barrel, terminated at one end by a small closed 

 vent, so arranged that the charge could be tired by the application of a 

 red-hot ball. At the other end it was closed by a hemisphere upon which 

 any required weight could be placed. His method was as follows : — -A 

 given charge being placed in the bore, a weight judged to be equivalent 

 to the expected gaseous pressure was applied. If the weight were lifted, 

 it was increased until it was just sufficient to confine the gases, and the 

 pressure was then assumed to be that represented by the weight. 



It seems probable that Rumford's erroneous determinations were 

 mainly due to two causes : — 



1st. To the weight closing the barrel being lifted, not by the mere 

 gaseous pressure, but by the products of explosion (produced, it will be 

 remembered, from a very ' brisante ' powder and considerably heated by the 

 red-hot ball) being projected at a high velocity against it. In such a 

 case the energy acquired in traversing the barrel would add notably to 

 the pressure due to the density of the charge ; and it is again remarkable 

 that the augmentation of pressure, due to this cause, was clearly indicated 

 by an experiment designed for the purpose by Robins. 



2nd. To the gases acting on a much larger area than was allowed for 

 in his calculations ; and this view appears to be confirmed by the resume 

 he gives of his experiments. 



No attempt was made for very many years either to corroborate or 

 amend Count Rumford's determinations ; but, in 1845, General Cavalli 

 endeavoured indirectly to arrive at the pressure developed by different 

 kinds of powder in a gun of 16 cm. calibre. His method consisted in 

 drilling holes in the gun at right angles to the axis, at different distances 

 from the base of the bore, in which holes were screwed small barrels of 

 wrought iron, so arranged as to throw a bullet which would be acted on 

 by the charge of the gun while giving motion to the projectile. By ascer- 

 taining the velocities of these bullets he considered that the theoretical 

 thickness of the metal at various points along the bore could be deduced. 

 His experiments led him to some singular results. 



