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On tliis subject captain Blakely, R. A., in an article published in the London Artisan, states that " a 32-pounder is the limit of cast iron guns of the present shape, any larger than that being unsafe with, a full charge." In reference to cannons of large caliber, the shot can be carried to a greater distance, and do more execution than small balls, because the weight of the ball is greater in proportion than the surface of resistance to the air. Thus a 16-inch.shot presents sixteen times the surface of resistance of a 4-inch shot, but it weighs sixty-four times as much. Large guns, however, require to be made stronger than small ones, large shot taking a longer period of time to acquire its velocity, therefore the pressure of the powder on the gun remains longer. The time that great pressure is exerted on any material is an important element, to which too little attention has been paid in submitting bodies or instruments to severe tests of strength. A body may bear a certain pressure for one second, which if conT tinued for one minute would destroy it. This is doubtless the case with cast iron, of which material cannon are made. Captain Blakely recommends that cannon of large caliber (say 10-inch) be formed of the same shape they are at present, but that the outside, at the breech, be strengthened with two layers of thin wrought iron cylinders put on at a bright red heat and hammered. One gun of this description made by him stood 447 rounds with double charge, and 158 rounds loaded to the muzzle. R. Armstrong, of Newcastle, England, has made a cannon of a solid steel center, with bar iron coiled round it and welded, which has stood thousands of rounds. Captain Blakely believes that, for very large cannon, a good plan of construction would be with a cast iron cylinder center, and either rod iron wound round it at a great heat and welded layer over layer, each in cooling taking a permanent strain, or else substitute strong iron wire wound round it at a high heat, each layer having a greater initial strain than the one under it. In this manner all the fiber is laid in one direction, andthe outside takes its share of the strain. The subject of heavy ordnance is now exciting much attention among engineers of gunnery and others. The foregoing views, in our opinion, deserve general attention from all interested.
