Page:Our big guns.djvu/17

( 11 ) acting on the area of the base of the shot throughout its travel in the gun, and if there are two shots of equal weight and equal length of travel in the gun, it is clear that, whatever their form, or whatever the area of the bore of the gun, i. e. of the base of the shot, the total average pressure must be the same. Let us see how this operates in the cases we have before considered, that of a spherical projectile of 68 lbs. weight in an 8-inch gun, and that of a cylinder of 68 lbs. weight, but 15 inches long, in a 5-inch gun. The respective areas of these calibres are as 64 to 25; if therefore the total pressures are to be the same, the pressure per square inch must be increased from a mean of 25 in the case of the 8-inch gun, to a mean of 64 in the 5-inch gun. That is, to give a 5-inch diameter long projectile of 68 lbs. weight the same muzzle velocity as a 68 lbs. spherical projectile of 8-inch diameter, after traversing the same length of barrel, the mean pressure must be 2&middot;72, or nearly 2¾ times as much.

Further, it will be found that in the 5-inch gun, the same weight of powder will no longer do the same work on the shot, as it would in the 8-inch.

A gun is, after all, only an engine, with its piston (the shot) moving at a very high velocity, and the powder gases act on the shot very much in the same way, in which steam acts upon the piston, in an expansive steam engine, after the admission from the boiler is shut off, and expansion has begun. We know that in these engines the useful effect of the steam increases, in a certain ratio, as the number of expansions increase. Now if, for example, a shot travel of, say 80 inches were given alike to the projectile of an 8-inch gun, and to that of the 5-inch gun, and an equal quantity of powder be used in each case, the powder occupying a cubic contents of, say 500 inches, the expansion in the instance of the 8-inch gun would be 9 times, while in the 5-inch gun it would be only 4&#x2155; times.

A remedy for this condition of things, is to be found in lengthening the barrel of the 5-inch gun, so as to give a greater travel, under pressure, to the shot. This operates