244 F. A. P. Barnard on the Explosive Force of Gunpowder. 
as compared with the original volume of the powder; thirdly, 
the volume of the fixed products, both at the temperature of 
of leaving the gun; and computing, according to recogni 
principles of physics, the initial pressures which would be neces 
sary to produce such velocities, 
- In making such a computation, one assumption must be made 
{at least in the first instance) which is not true; and which, in 
so far as it is not true, will have the effect to make the compu 
maximum less than the real maximum pressure. This assum) 
tion is, that the powder is completely fired before the project! 
begins to move. The same assumption was made by Robins and 
by Hutton, and it is implicitly involved in all the velocity formule 
which are found in treatises on artillery or on ballistics, at the 
present time. It being assumed, then, that all the gas which 
the powder is capable of producing is set free before the ball 
begins to move, we require to know, in order to determine the 
velocity it will generate in the projectile, the following particu-~ 
lars, viz: the original bulk of the gas, its initial temperature, 
its bulk at the atmospheric t t 1 pressure, its capacity — 
_ for heat both at constant pressure and at constant volume, the 
length of the bore of the gun, the part of this length occupied 
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