involved in the Construction of Artillery. 303 



and velocity, will be inversely proportionate to the hardness of the material of 

 the gun. 



In this respect cast-iron can offer but very slight pretensions, if any, over 

 wrought-iron, while both are immeasurably superior to bronze, and steel to all 

 of them. 



307. A third condition of wear, different from either, has been ascertained 

 by careful examination of the heavy guns of the United States Government, 

 known as Columbiads, after sustained firing, and is both novel and interesting. 



It was found that a considerable enlargement of bore took place, all upon 

 the upper part of the interior of the chase, just above, and in advance of, the 

 position of the shot, when rammed home. 



The greatest enlargement being at about an inch in front of the centre of 

 the shot, and extending as far as three or four inches forward of that point ; the 

 surface of the bore here was cut into ridges and furrows, while the opposite 

 side, under the ball, for three or four inches in length, was smoothed and bur- 

 nished as if the shot had rubbed forcibly over it. 



The explanation of this is very instructive. The ball, rammed home, and 

 resting on the lower side of the chase, leaves the whole of the windage open, 

 as a lunaric area, greatest at the upper side, as in diagram, sect. 77. At the 

 moment of explosion, and before the ball's inertia has been overcome, so that 

 it begins to move, as well as during the first instant of its motion, the flame, 

 and perhaps some unignited portions of the powder, are driven out through this 

 lunaric aperture, and past the ball, with enormous force and velocity, both 

 almost reaching the possible maximum, for fired powder. This tremendous 

 blow-pipe, acts upon the interior of the bore at each discharge, precisely in the 

 same way as the issue from the vent, acts in enlarging it, in cast-iron guns, 

 burning away the graphite first, as the most ignitible material, and then burn- 

 ing and blowing away, as oxides and sulphurets, the intervening finely divided 

 fragments of the iron itself 



308. The wear is not uniform, but in ridges and furrows, and for precisely 

 the same reason that the enlargements of the vents, of which numbers have 

 been accurately figured (see "Experiences faiths a Gavre, en 1836, sur les 

 Bouches a Feu," &c., Paris, 1837), are all in irregular curved, triangular or mul- 

 tiangular forms. 



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