376 PROCEEDINGS OF THE AMERICAN ACADEMY 



I shall pass, therefore, from Captain Rodman's instrument to an 

 examination of his gunpowder. When the newspapers, some time ago, 

 gave the story of Captain Rodman's experiments with his 15-inch gun, 

 there was a great account made of the strange kind of gunpowder used, 

 the grains of which were said to be as big as pigeons' eggs. But Cap- 

 tain Rodman's book gives us the reason for the use of this very coarse 

 powder. It is to avoid the great stress laid upon the gun by the rapid 

 development of the gas or fluid, as produced by powder of common 

 grain, from its quick combustion. Now, Captain Rodman proposes, by 

 the slow combustion of these large lumps, not only to avoid the shock 

 of the first explosion, but to continue the development of the force 

 through the whole length of the bore. This would certainly make it 

 much easier work for the breech portion of the gun ; but the fact of 

 resorting to it shows the suspicion entertained by Captain Rodman of 

 his gun's strength. 



Let us, then, first see how far the idea of equalizing the pressure is 

 obtained by increasing the size of the grain. To do this, I must resort 

 to the measures given by Captain Rodman's instrument, which, though 

 worthless as absolute measures, may yet give some rude approximation 

 to comparative values when used upon the same gun and upon the 

 same place, but with different grained powders. Turning to his book, 

 then (p. 203), we find that a charge of 8 lbs. of powder grained to .1 of 

 an inch diameter, gave against a 43-lb. shot a force of 51,800 lbs., 14 

 inches from the breech, and 6,700 lbs. at the distance of 84 inches, — 

 these being the greatest and least forces ; while a charge of 8 lbs. of 

 powder of .4 inch grain, or G4 times the former in size of grain, 

 gave against the same shot 31,950 lbs. for the greatest, and 5,150 lbs. for 

 its least force. The greatest force of the fine powder, then, was about 

 7 times that of the least force ; while the greatest force of the coarse 

 powder was about G times that of the least force. Surely this is no 



doubt of that, but uses it, under the name of the inverse square,) have possessed a 

 tenacity of 150,000 pounds to the inch. Not only will no cast-iron bear this strain, 

 but no metal, nor any other substance whatever yet known to the art of man, is 

 capable of bearing it. Even cast-steel, the strongest body known, when wrought 

 down by the best hammering, will not sustain 150,000 pounds to the inch. In- 

 deed, if a mass of iron gun-metal, though the strongest ever made, were cast as 

 large as the Capitol at Washington, and then bored through with a caliber of 1 1 

 inches, it would, according to Barlow's formula, be split by a fluid pressure of, not 

 150,000, but 50,000 pounds to the inch. 



