involved in the Construction of Artillery. 



26.5 



preceding fracture, tends to be bent with curves of contrary flexure into a 

 hollow, whose section is thus compressed atyi b, and h', and extended at the 



other sides of the plate opposite to these letters. "We say tends only to bend, 

 because, on the principles already stated, time is not given it to bend ; but the 

 extensions and compressions occur in the plane of the plate, the same as if 

 bending did take place by the directions in which the striking out of the frag- 

 ments takes place in the way already described. Thus, then, we have pressures 

 instantaneously propagated through the mass, radially from the point of impact 

 of the shot, and in the plane of the plate, forming a circle of compression, at 

 the struck side, whose centre is the first point of impact, and its boundary 

 evanescent, and surrounded by an extended annulus, and the exact reverse of 

 all this at the back or opposite side of the plate, the extensions and compressions 

 being all radial to an axis passing through the plate at the point first struck. 



234. The effects on the crystallization of the iron is precisely analogous to 

 those described further on in breaking a bar. Lines of maximum and of 

 minimum pressure are produced within the plate ; its crystals, all probably 

 lying originally parallel to the plane of its surfaces, and to that of its original 

 lamination, are instantly changed in direction ; their principal axes are reversed, 

 and assume the directions of minimum pressures within the mass, which are 

 those of tangents to the circumferences of the circles of extension and com- 

 pression, and such are just the directions in which we find them. The iron, 

 therefore, breaks short and brittle, because of the velocity of the blow, and 

 the relation of this to its elasticity and elastic range (0) ; and it would do this 

 whether the character of the fracture were altered (as produced by breaking 

 slowly) or not, but the alteration of the fracture from fibrous to crystalline is 

 due to the sudden compressions and extensions visited upon the internal parts 

 of the plate. It is, therefore, only another case of our general law of crystalline 



