154 POPULAR SCIENCE MONTHLY. 



the construction of a heavy gun, if steel be employed, the utmost 

 care should be exercised to secure that of the highest grade pos- 

 sible, 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 projec- 

 tile, 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 diame- 

 ter. 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 com- 

 pressed 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, Arm- 

 strong, 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 superseded smooth-bore guns.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 wa& 

 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 



