Page:Our big guns.djvu/23

( 17 ) I have shown you what the simple passage of the gas of but 13 lbs. of powder can do in the way of erosion, and I will ask you to consider the result of a gas-leak, when there are hundreds of pounds of powder seeking escape.

Moreover, this joint has not only to be rapidly made, and to be absolutely tight, but when the gun has been fired the joint has to be as rapidly unmade; it must be a joint, therefore, that even under the enormous pressure, has not swelled out so as to stick fast, and thus prevent the ready opening of the breech.

These, I think it will be agreed, are no light problems, and yet they have to be solved before a working breech closure is obtained.

We will now consider the structure of the gun. It may be said, why not select the metal—cast iron, bronze, or steel—and having determined on the pressure to be resisted, and the factor of safety to be allowed, and knowing the resisting power of the metal you have selected, make the walls thick enough, and then all that is needed has been attended to. Happy would it be for the gun-maker, if a gun could be produced of reasonable strength, combined with manageable weight, by such simple means; but the following consideration will show that this construction is not one suited for Big Guns.

Assume that I have, interposed between my hand, and a pound weight I desire to lift, a spiral spring, say 1 foot long, and of such a character that it must extend 1 inch before it can raise the weight; and assume that I have another spiral spring, precisely similar to the first one, except the length, which is to be 2 feet: it is clear that if I employ this second spring to raise a pound weight it must extend 2 inches before it would do this. In other words, the percentage of extension must be equal. Now, suppose that one has a gun, of say 1 foot bore, with the walls 6 inches thick, making the external diameter 2 feet: it is obvious that, in order for the imaginary circle of metal at the outside of this two feet to be as effective in resisting the powder pressure as is the imaginary circle of metal of 1 foot diameter close to the bore, the increase of diameter of that outer circle should be twice that