Page:Our big guns.djvu/18

( 12 ) in two ways; the one that the shot is subjected to the pressure for a longer period, and that therefore a less intensity of pressure suffices; the other, that the number of expansions are increased, and more duty is got out of the powder.

But it will be found that to give in the 5-inch gun the low average pressure, and the number of expansions obtaining in the 8-inch gun, in their entirety, a very undesirable piece of artillery, would have to be made. Assume, as we have, that the spherical shot of the 8-inch gun had a travel of 80 inches or 10 calibres, the travel to be given to the 5-inch projectile to obtain the conditions of pressure and of powder duty obtaining in the 8-inch gun must be as much as 205 inches = to 41 calibres of that gun. Such means, therefore, of obtaining the desired total pressure on the smaller area can only be partially followed. The length cannot for practical purposes be increased to anything like this extent, and therefore a higher average pressure must prevail, involving the employment of more powder, and powder of a nature that will keep up a sustained pressure; a continued shove, rather than a blow.

We have seen, that in order to reduce the rate of diminution of the muzzle velocity, and in order to concentrate the striking energy, it is necessary to employ an elongated projectile, but the use of such a projectile lands us in another difficulty, requiring another remedy, accompanied of course by its own attendant difficulties: the elongated projectile would, if fired without rotation being imparted to it, very soon "turn over," as it is called, and would lose its true direction, were it not given the spinning motion, obtained by rifling the gun.

This rifling involves wounding the bore of the gun, by making in it a number of grooves, each one of which is an invitation to the commencement of a crack, and it also involves the absorption of part of the energy of the powder, in producing the rotation of the projectile.

And with respect to this question, of the whole energy of the powder not being spent in propelling the shot, I trust I may