257 
the Force of fired Gunpowder. 
rated in the combustion of gunpowder, and by which the 
barrel was burst, to the pressure of the atmosphere, it is ne- 
cessary to determine the area of a longitudinal section of the 
bore of the piece. Now the diameter of the bore being £ of 
an inch, and its length (after deducting 0.15 of an inch for 
the length of the leathern stoppers) 2 inches, th^ area of its 
longitudinal section turns out to have been \ an inch. And if 
now we assume the mean pressure of the atmosphere = 15 lbs. 
avoirdupois for each superficial inch, this will give 7-j for that 
upon a surface = \ inch, equal to the area of a longitudinal 
section of the bore of the barrel. 
But we have just found that the force actually exerted by the 
elastic vapour in bursting the barrel, amounted to 412529 lbs.; 
this force was therefore 55004 times greater than the mean 
pressure of the atmosphere ! For it is as 74- lbs. to 1 atmosphere, 
so 412529 lbs. to 55004 atmospheres. 
Thinking it might perhaps be more satisfactory to know the 
real strength of the identical iron of which the barrel used in 
the before mentioned experiment was constructed, rather than 
to rest the determination of the strength of the barrel upon 
the decision of the strength of iron taken from another parcel, 
and which very possibly might be of a different quality, since 
writing the above, I have taken the trouble to ascertain the 
strength of the iron of which the barrel was made, which was 
done in the following manner. Having the one half of the 
barrel still in my possession, I caused small pieces, 2 inches 
long, and about -t- of an inch square, to be cut out of the solid 
block, in the direction of its length, with a fine saw ; and these 
pieces being first made round in their middle by filing, and 
then by turning in a lathe with a very sharp instrument, were 
