the Force of Fired Gunpowder. 107 



ing; 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 at- 

 tempts 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 upon 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 -j^ of an inch in diameter), and causing the 

 end of the piston which projected beyond the end of the 

 barrel to act upon a heavy weight, suspended as a pen- 

 dulum 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 answer, 

 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- 



