﻿ON CONSTRUCTING CANNON OF GREAT CALIBER. 17 



every discharge, until it becomes so deep as to deflect the ball upwards at the instant 

 of its flight, to strike the upper surface of the bore, where a second indentation is 

 made, considerably in advance of the flrst, and from this a thii-d, still more advanced, 

 upon the under side. These indentations go on increasing in number and size, and 

 at length bulges appear upon the outside of the gun, which becomes oval near the 

 muzzle, and is at last destroj^ed. 



The lodgment here described has been attributed wholly to the downward pres- 

 sure of the fluid when escaping through the opening of the windage, which is all upon 

 the upper side of the ball, the imder side resting by its weight in contact with the bore. 

 There must undoubtedly be a great escape, not only of the fluid, but of burning 

 powder in grains, through this passage, and the downward pressiu-e from these causes 

 may present an excess over the opposite pressure of the powder upon the under side 

 of the ball, capable of producing some impression upon the under surface of the bore. 

 I am inclined, however, to attribute the indentation mostly, if not entirely, to the com- 

 pression of the back hemisphere of the ball under the enormous blow of the explo- 

 sion, producing a corresponding enlargement of the ball in its diameter transverse to 

 the axis of the bore. The smith produces such a change of form in his bar of iron, 

 at pleasure, by the blows of a sledge applied to its end. The operation is called up- 

 setting. This enlargement must impress itself uj^ou the part of the bore upon the 

 under side upon which the shot rests, and is alone sufficient, in my mind, to ac- 

 count for the whole mischief 



This ^"iew of the subject is confirmed by the form of the lodgment, which 

 consists, at first, of a single narrow impression, exactly corresponding to a very small 

 segment of the ball, and not in the least in advance of the spot on which the ball 

 rests before the discharge. Now this would be the exact form and place of an im- 

 pression produced by a sudden enlargement of the ball, and an equally rapid recov- 

 ery of its true figure, which it would derive from its elasticity. But if the lodgment 

 were produced by the pressure of the fluid upon its upper surface, it ought to form 

 a long groove or channel, ceasing only with the diminished pressure of the fluid near 

 the muzzle. Furthermore, the lodgment is greatest when a hard oakum wad is used 

 behind the ball. Now such a wad miist prevent, in some degree at least, the escape 

 of the fluid, and therefore diminish the do^vnward pressure. But such a wad, driven 

 hardest against the middle of the ball, in its rear, would act most advantageously to 

 produce the lateral enlargement by upsetting it as before described. 



Hard cast-iron guns do not exhibit this indentation in so great a degree, because, 

 being unmalleable, they are incapable of a permanent change of form without fracture. 



VOL. VI. NEW SERIES. 3 



