MECHANICS AND USEFUL ARTS. 97 



applied to its whole length than is required to burst it if applied to 

 only a portion of its length ; hence the pressure should diminish as 

 the shot recedes from the breech, but not nearly as rapidly as the 

 experiments show that it does diminish even with the largest-grained 

 powder. 



As the shot starts very slowly at the breech, and moves with con- 

 stantly accelerated velocity in its course through the bore, in order 

 to make the pressure uniform throughout, the gases should be evolved 

 from the burning powder with a corresponding acceleration. But if 

 the powder is granular the combustion commences on the surface of 

 the grains and proceeds inward, constantly reducing the grains, and, 

 consequently, the extent of the burning surface. Thus the rapidity 

 with which the gases are evolved is retarded instead of being acceler- 

 ated. Capt. Rodman conceived that if the powder was formed into 

 hollow cylinders, to be fired wholly from the inside, the burning sur- 

 face would be enlarged as the combustion progressed, and, conse- 

 quently, the rapidity with which the gases were evolved would be ac- 

 celerated. In order to confine the combustion to the interior of the 

 cylinders, he moulds them together into octagonal cakes, from one to 

 two inches in thickness, which are perforated with small holes. 



The cakes are submitted to a powerful pressure in a cylinder, the 

 plunger being armed with wires to form the holes. In practice, the 

 axes of the cylindrical holes are parallel to that of the bore. 



Capt. Rodman says that the increasing rapidity of the evolution of 

 gas may be regulated so as to give any pressure desired along the 

 bore, by establishing the proper relation between the number and 

 diameter of cylindrical holes, and the thickness of the walls between 

 them. 



" The initial burning surface, and the ratio of the maximum to the 

 mean pressure, may also be varied by varying the number and thick- 

 ness of the cakes in a given weight of charge ; the initial burning sur- 

 face and the maximum pressure both increasing with the number of 

 cakes, since the burning surface extends over the whole surface of the 

 cakes. 



" The thickness of walls between the cylinders should be such as 

 to be burned through, or consumed, before the projectile leaves the 

 gun ; and for ordinary velocities we should economize in weight of 

 charge, by making the walls of such thickness as to burn through by 

 the time the projectile has traversed two-thirds or three-fourths of the 

 bore, and allowing the gas to act expansively from there to the 

 muzzle. 



" It will readily be seen, from the foregoing, that this form of car- 

 tridge gives us entire control over the rate of combustion of the 

 charge a fact the importance of which can hardly be overrated ; 

 for, taken in connection with the hollow mode of castino 1 cannon, it 



^5 ' 



removes all limit, as regards safety, to the calibre, of which even cast- 

 iron guns may be made." Scientific American. 



DOREMUS'S COMPRESSED POWDER. 



The idea has been suggested by Prof. R. O. Doremus, the well- 

 known chemist of New York, that gunpowder for projectile purposes 



9 



