involved in the Construction of Artillery. 305 



made by the American methods of trial, viz., by the depth to which a given 

 prism can be forced by a given weight into the material, must lead to entirely 

 fallacious results, — uniformity and chemical relation to combustion being as 

 much elements of wear as hardness. 



311. The extent of rifling, or actual scoring away of the metal, at each dis- 

 charge, for shot of the same caliber and velocity, can only be determined by expe- 

 riment ; but inasmuch as the mechanical part of the reaction is of the same nature 

 as that producing mere friction between surfaces, there cannot be much doubt 

 that the loss of metal at each discharge of cast-iron spherical shot, of equal 

 calibers and velocities, from guns of the same length, but of diiferent materials, 

 will by abrasion only, be some function of the coefficient of friction of cast- 

 iron upon each of these several materials. Now, Morin's experiments, though 

 not embracing exactly what we require, assign the following values for the 

 friction of cast-iron, wrought-iron, and bronze, in terms of the pressure, when 

 in movement on each other : — 



Cast-iron on cast-iron, . . . 0'15 

 Cast-iron on bronze, .... 0'15 

 Cast-iron on wrought-iron, . . 0'16 



The latter is doubtful, as the wrought-iron appears to have been the moving 

 body. We have no corresponding results for steel. These figures, then, would 

 indicate that the loss of material by abrasion only, will not greatly differ in any 

 of these cases. 



312. It may appear, that no analogy holds, between friction in which the 

 pressure is kept within the limits of sensible abrasion, and that of scoring out 

 by the rapid shave of a shot in traject; but all friction consists in abrasion, only 

 reduced in degree, and Morin's experiments proved that the resistance to motion 

 produced by it varied directly as the pressure, and was wholly independent of 

 the velocity. 



313. It follows, however, that for guns of the same material, and with the 

 same velocity of shot, the wear from this cause, i. e. the weight of metal shaved 

 off at each discharge, will increase with the weight of the shot, or as D^ ; while 

 for different velocities it will vary as F^ upon the accepted principles of vis viva. 

 With variable charges of powder, and the same shot, the grooving and enlarge- 



