642 TRANSACTIONS OF THE AMERICAN INSTITUTE. 



ber of the cabinet; in fact, they have in every instance failed. After them 

 came a series of bronze guns; then cast-iron was tried for a wliile, after 

 which wrought-iron built up guns, fabricated with more skill than had pre- 

 viously been bestowed upon them; yet they failed, for 1,476 Armstrong 

 guns, or parts, were returned for repairs up to July, 1862. Then cast-iron 

 again cast solid, the same as the Dahlgren system, which was in turn aban- 

 doned for cast-iron banded guns, (Parrott's system.) The English govern- 

 ment burst or rejected 356 of this kind, and they were abandoned as old 

 iron in 1862, follov/ing after or coeval with which came guns cast hollow 

 as by Colonel Rodman. Not one of these systems has stood the test of 

 battle, and most of them not that of proof 



SCUD CAST GUNS. 



11. The Dahlgren gun has exhibited better endurance in proof than any 

 other large gun, yet Dahlgren only allowed limited charges of powder fired 

 from them, and that behind spherical shells; the^^ could not stand a charge 

 commensurate with the size of the gun, with a solid shot, especially under 

 rapid firing. Although the gun is made so as to secure a quality of poro- 

 sity to the interior metal, which can be heated inside without being expan- 

 ded to sufficient extent to break the dense surrounding metal, under slow 

 firing, as used in proving and ordinary practice: for example, the finger 

 may be inserted into a sponge without rupturing the outside, while a needle 

 cannot be inserted into a cake of ice without causing radiating cracks. 

 The effect on this gun under heavy firing would be similar to the effect of 

 powder in rock blasting. The rock is somewhat compressible and of brittle 

 texture; the pressure of powder enlarges the diameter of the hole by com- 

 pressing the material immediately surrounding it; then first, suppose the 

 bore two inches in diameter to be so enlarged as to start two cracks on 

 opposite sides to a depth of two inches, the gases of powder enter these 

 cracks, acting then upon a surface six inches wide. If the pressure in the 

 bore, two inches in diameter, was before sufficient to induce cracks two 

 inches wide each side, when the pressure acts upon six inches, the cracks 

 will be increased six inches in addition on each side, making eighteen 

 inches width of sui'face upon which the pressure acts to continue the frac- 

 ture further. (See fig. 1.) 



NKCESSITY FOR INITIAL TENSION. 



12. " If we make equidistant circular marks on the end of an India-rub- 

 ber cylinder, and stretch it, we can see plainly how much more the inside 

 is strained than the outside or even the intermediate parts. Tlie spaces 

 between the marks will become thinner, each space becoming less thin than 

 that outside of it, and tlie inner space much thinner than the others, show- 

 ing that when the inside is strained almost to breaking, the inteimediate 

 parts ai'e doing much less work, and those far removed almost none. 



13. "In the first volume of the 'Transactions' of the Institute of Civil 

 Engineers, p. 133, there is a paper by I^rofessor Peter Barlow, F. R. S., on 

 the Strength of Cylinders." The law he deduces is, that "in cylinders of 

 m-'al the power exerted by different parts varies inversely as the squares 

 o' tlie distance of the parts from the axis. Thus, in a 10-inch gun, when 



-.e inside, which is 5 inches from the axis, is fully strained, the metal 2 



