PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 463 



original form of bombs and shells, was spherical ; and that form 

 is still used in all smooth-bored pieces. They are made of differ- 

 ent sizes, up to 15 or 20 inches in diameter, and filled with such 

 materials as the peculiar exigencies of the service may suggest as 

 most destructive or disagreeable to the enemy. For firing into 

 open forts, they are generally filled with balls, or sharp pieces of 

 iron, sometimes six-sided or cubical, with fragments of bottles, 

 and similar missiles. But in firing into casemated fortifications, 

 it is often far more disastrous to the enemy to stifle them out, by 

 filling the shells with substances which will produce an intensely 

 disagreeable smoke. And for firing into wooden structures, like 

 a ship, or a fort with wooden buildings, portfire, a material which 

 will burn very intensely even under water, is used with great 

 advantage. The thickness of shells is made such as to avoid 

 breaking with the concussion of firing, and at the same time to 

 break up into a large number of pieces with the explosion of 

 their contents. Shells for rifled cannon are made of the same 

 general form with the solid shot. The Hotchkiss' shells are 

 arranged in the same manner as the solid projectiles, Mdiich bear 

 his name, and were described at a former meeting, namely, with 

 a belt of lead to be expanded a certain amount by the impulse of 

 the discharge so as to fill the grooves of the cannon. In firing 

 spherical shells, there is always so much windage that enough 

 gas passes the shell to fire the fuse, which is necessarily at or 

 near the front of the shell ; but with rifled cannon we have not 

 this advantage. It was supposed, until within a few years, that 

 the shell could only be fired from a mortar, which is so inaccu- 

 rate in its aim as to be rarely serviceable except in firing into a 

 city, or in similar cases. But it is now found that they can be 

 fired from ordinary cannon with great accuracy of aim. They 

 can therefore be used in firing at ships, and unless the ship is so 

 near that they pass completely through, they are much more 

 destructive than the solid ball. Although the gas cannot get 

 by in the rifled cannon until after the shell leaves the mouth of 

 the cannon, when the shell is moving at its highest velocity, yet 

 the fuse will generally be fired. But for greater certainty, the 

 Hotchkiss' shell has channels left in the side for the gas to pass 

 through. 



The instant of explosion may be regulated in two entirely dif- 

 ferent ways. We may either use a fuse which shall burn for a 

 certain number of seconds, or we may cause the explosion by the 



