VOL. LXXXVII.] PHILOSOPHICAL TRANSACTIONS. 145 



tending the discharge of an air-gun. The quantity of powder used in the experi- 

 riment was indeed very small, not amounting to more than -f part of what the 

 chamber was capable of containing; but having so often had my machinery 

 destroyed in experiments of this sort, I began now to be more cautious. 



Having found means to confine the elastic vapour generated in the combustion 

 of gunpowder, my next attempts were to measure its force: but here again I met 

 with new and almost insurmountable difficulties. To measure the expansive force 

 of the vapour, it was necessary to bring it to act on a moveable body of known 

 dimensions, and whose resistance to the efforts of the fluid could be accurately 

 determined; but this was found to be extremely difficult. I attempted it in various 

 ways, but without success. I caused a hole to be bored in the axis of one of the 

 screws, or breech-pins, which closed up the ends of the barrel just described, and 

 fitting a piston of hardened steel into this hole, which was -^ of an inch in 

 diameter, and causing the end of the piston which projected beyond the end of 

 the barrel to act on a heavy weight, suspended as a pendulum to a long iron rod, I 

 hoped, by knowing the velocity acquired by the weight, from the length of the 

 arc described by it in its ascent, to be able to calculate the pressure of the elastic 

 vapour by which it was put in motion : but this contrivance was not found to an- 

 swer, nor did any of the various alterations and improvements I afterwards made 

 in the machinery render the results of the experiment at all satisfactory. It was 

 not only found almost impossible to prevent the escape of the elastic fluid by the 

 sides of the piston, but the results of apparently similar experiments were so very 

 different, and so uncertain, that I was often totally at a loss to account for these 

 extraordinary variations. I was however at length led to suspect, what I after- 

 wards found abundant reason to conclude was the real cause of these variations 

 and of all the principal difficulties which attended the ascertaining the force of fired 

 gunpowder by the methods I had hitherto pursued. 



It has generally been believed, after Mr. Robins, that the force of fired gun- 

 powder consists in the action of a permanently elastic fluid, similar in many 

 respects to common atmospheric air; which being generated from the powder in 

 combustion, in great abundance, and being in a very compressed state, and its 

 elasticity being much augmented by the heat, which is also generated in the com- 

 bustion, it escapes with great violence, by every avenue; and produces that loud 

 report, and all those terrible effects, which attend the explosion of gunpowder. 

 But though this theory is very plausible, and seems on a cursory view of the sub- 

 ject to account in a satisfactory manner for all the phenomena, yet a more careful 

 examination will show it to be defective. There is no doubt but the permanently 

 elastic fluids, generated in the combustion of gunpowder, assist in producing 

 those effects which result from its explosion ; but it will be found, I believe, on 

 ascertaining the real expansive force of fired gunpowder, that this cause alone is 

 quite inadequate to the effects actually produced; and that therefore the agency of 

 some other power must necessarily be called in to its assistance. 



vol. xviii. U 



