Page:Popular Science Monthly Volume 51.djvu/164

154 the construction of a heavy gun, if steel be employed, the utmost care should be exercised to secure that of the highest grade possible, in order to withstand the enormous tension due to explosion. As soon as this tension becomes equal to the limiting measure of elasticity for the steel, the wall must yield, even if the thickness of the gun were infinite. Since the breaking limit, or ultimate tenacity, of cast steel has just been seen to be, on an average, at least five times that of cast iron, it follows that, with the same diameter and thickness of metal and the same weight of projectile, a steel gun warrants the use of a charge of powder of the same quality five times as great.

Professor Treadwell showed in 1856 that, if we assume a gun to be made up of a large number of uniform, cylindrical, concentric layers of metal, then the resistance of each layer to the bursting force of explosion will vary inversely as the square of the diameter. The stress, therefore, decreases at a rate very similar to that of the radiation of heat or light. If the wall of the gun be under no initial stress of any kind, its inner portion must have great resisting power, and very little is gained by thickness of wall much in excess of the diameter of the bore. Treadwell therefore proposed a plan of construction by which a cast-iron tube of only moderate thickness should be re-enforced by a series of layers of encircling wrought-iron hoops. These should be shrunk on while hot, so that, after cooling, the cast iron tube is strongly compressed while the wrought-iron hoop becomes stretched. The force of compression is thus added to the ordinary strength of the cast iron to resist explosion. With various modifications this plan has been carried out by most gun constructors during the last forty years. During the civil war it was applied with great success by R. P. Parrott, of West Point, and by Blakely, Armstrong, and Whitworth in England.

It is perhaps impossible to say what inventor was the first to introduce the use of rifled cannon. They have now entirely superseded smooth-bore guns. The Parrott rifled cannon, made of cast iron according to the Rodman plan and re-enforced around the chamber with a hoop of wrought iron, was the most generally serviceable gun employed during the late war, more than two thousand of them coming thus into use. The largest of these was twelve feet in length, with a bore ten inches in diameter, its weight being about twelve tons. A charge of twenty-five pounds of powder was employed to project a shot weighing two hundred and fifty pounds. The cost of its construction in 1863 was forty-five hundred dollars.

These details are given for the sake of subsequent comparison with the rifled cannon of to-day. For twenty years after the close of the war there was a period of stagnation in America, so far