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the Force of fired, Gunpowder. 
more than part of what the chamber was capable of con- 
taining; 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 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 ~ 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, sus- 
pended as a pendulum to a long iron rod, I hoped, by know- 
ing 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 pre- 
vent 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 afterwards found abundant 
