﻿4 ON CONSTRUCTING CANNON OF GREAT CALIBER. 



contemporaneous action of the ball, which passes from rest to motion, and through 

 every gradation of velocity up to 1,600 feet a second, and leaves the gun as our 

 historical period of T^oth of a second expires. 



The expansive force of gunpowder, which must be resisted by the strength of the 

 cannon, depends almost entirely upon the cucumstances imder which it is fired. 

 Count Rumford has shovra, by his experiments made about sixty years ago, that if 

 the powder be placed m a closed cavity, and the cavity be two thirds filled, the 

 force will exceed 10,000 atmospheres, or 150,000 pounds upon the square inch; and 

 he estimates that if the ca^ity be entirely filled Avith the grained powder, and re- 

 strained to those dimensions, the force will rise to 50,000 atmospheres. My own 

 experience, made in bursting wrought-iron cannon the strength of which was kno^\Ti 

 to me, leads me to believe that he has not over-estimated its power, although I 

 am aware that it is generally considered as excessive. If, following an opposite course 

 to that herein described, the powder be at liberty to expand upon any side, the force 

 thrown in the other directions is very small. Thus, if a charge be placed loose in a 

 gun, without shot or wad, the force upon the walls of the gun is very trifling ; — no 

 more than is produced by the restraint of the inertia of the charge itself, or the 

 fluid formed from it. If we would divest a charge of this property of inertia, and 

 fire it in a constantiy maintained vacuum, it would not rend walls made of cartridge- 

 paper, if a single end were left open for its escape. From the preceding statement, it 

 will be seen that gunpowder will take any force, from perhaps 50,000 atmospheres, 

 when confined to a close cavity, down to zero, if it be deprived of inertia and fired 

 in a vacuum constantly maintained. 



In artillery practice, the restraining power which causes the powder to act against 

 the walls of the cannon is derived principally from the inertia of the shot. This is 

 so much greater than the inertia of the powder itself, that the latter may be neglected 

 in the considerations that are to follow. Xow, bearing in mind what has been 

 already said, let us compare the difference of the force of poAvder as exerted upon a 

 small and a large gun respectively. It is perfectly well known, that, if we have a 

 pipe or hollow cylinder of say two inches in diameter with walls an inch thick, and 

 if this cylinder will bear a pressure from within of 1,000 pounds per inch, another 

 cylinder, of the same material, of ten inches in diameter, will bear tiie same number 

 of pounds to the inch if we increase the walls in the same proportion, or make them 

 five inches thick. A cross-section of these cylinders will present an area proportional 

 to the squares of their diameters, and if the pressure be produced by the weight of 

 plungers or pistons, as in the hydrostatic press, the weight required in the pistons 

 will be as the squares of the diameters, or as 4 to 100. 



