20 ON RIFLED GUNS. 



Again, in the Parrott missile, the sudden rotation is produced by a force of torsion 

 applied at one end; it is resisted by the inertia of the projectile which it develops, and to 

 which it is equal. At the mouth of the piece, in the case of the 100-pounder, that resistance 

 is equal to about thirty-six hundred pounds. If the shell had an incipient defect, this strain 

 would not fail to develop it, and probably open an avenue for the inflamed gas to enter. 



Taking the velocity of the projectile at one-third the distance from its starting point to the 

 mouth to be equal to half its initial velocity, and denoting the radius of the inner surface of the 

 shell by p 1 , we shall have for the velocitj' of rotations of this surface, by making ?/ = | ?, Eq. (5), 



whence, 



Giving at this point a rotary velocity of about fourteen feet a second. 



It is understood to be the habit of Mr. Parrott, latterly, to coat the interior of all his 

 shells with a mastic, which presents a very smooth and yielding surface. This has the effect 

 to lessen the friction, close any very fine apertures arising from imperfect casting, and to 

 diminish greatly the chances of explosion, if it do not wholly remove them. 



There is, however, another source of premature explosion from excess of mechanical 

 action. Every sudden compression is attended by development of heat, and there can be no 

 doubt this may be sufficient in degree to explode powder. Can it, in any case of practice, be 

 enough to fire the exploding charge of the shell? A knowledge of the law which connects 

 the velocity of the projectile with the distance of the latter from its starting point would 

 enable us, by the aid of the equation next preceding that numbered (14), to find the pressure 

 upon the powder of the shell at any point within the gun, and therefore the maximum pressure. 

 But, as before remarked, this law is unknown. It is easy, however, to find the average 

 pressure in any case, from the initial velocity. 



Denote the weight of the exploding charge of the shell, in pounds, by W ; the initial 



velocity by V; the length of the projectile's path in the gun by /, and the average pressure 



upon the powder by p; then will 



W V 3 



For the 100-pounder, 



AV = 5 



V = 1250 



7=10,S33 



v 332) g = 32; 



