255 
of Edinburgh, Session 1878 - 79 . 
the opposing plug, which offers but a very feeble resistance to the 
internal pressure. These facts, thoroughly well authenticated, have 
not, to my knowledge, received a satisfactory explanation, though a 
clear idea of the conditions of the case is all that is required to 
explain this, at first sight, anomalous behaviour. 
The explanation lies in the fact that the charge travels along the 
bore of the gun, if not with the same velocity as, at least with a 
velocity comparable to, that of the transmission of pressure through 
the air, i.e., the velocity of sound. Thus, as the charge advances 
along the barrel it is continually compressing the air immediately in 
front of it ; but this pressure gets no relaxation by expansion into 
the front part of the barrel. The compression, of course, generates 
heat in the air, which increases the velocity of sound through it. 
But this does not affect the question in its general bearings. It is 
sufficient to notice that the snow, &c., is driven out with the full 
velocity of the charge (neglecting the weight of the snow-plug com- 
pared with that of the charge). But before the plug can be driven out 
with this great velocity the pressure behind it must be very great. 
Let m = the mass of the snow-plug. 
g = the force of gravity. 
v = velocity of the bullet or wad when close to the plug 
(i.e., on leaving the gun). 
p = the pressure of the air driving out the plug. 
A = the sectional area of the bore. 
b = the length of the snow-plug. 
p = the density of the snow -plug. 
The work done in giving to the mass m a velocity = v is 
1 2 
w = -mv. 
2 
But w is performed by the pressure pA acting through the dis- 
tance ^ . 
1 a b 
pAb = m?; 2 . 
mv 2 9 
P= Ab= pV - 
Thus, the pressure at the muzzle of the gun is independent of the 
diameter of bore and length of plug. 
