160 Mr. Mallet on the Physical Conditions 



producing results of the most important character in artillery. This will be 

 more readily understood by immediate reference to example. When a large 

 gun, or, still more, a large mortar, is cast solid, and the metal cools in the ordi- 

 nary way, tlie external portions solidify long before the interior has ceased to 

 be liquid, and the process of solidification is propagated, as it were, in parallel 

 " couches" from the outside to the centre of the mass. The lineal contraction 

 of any one couche assumed of indefinite thickness is in the direction of its 

 circumference directly proportionate to that circumference; and so it would 

 seem (at first) that the contraction of the whole assemblage should be at every 

 point proportionate to its distance from the centre, and that so the solid, 

 when all cold, should be left iu a state of molecular equilibrium. This is not 

 the case, however; for no sooner has the first couche or thickness of solid 

 crust formed on the exterior, than it forms a complete arch all round, so that 

 the contraction between fluidity and solidification of each subsequent couche 

 is accommodated (the continuity of the mass remaining unbroken throughout) 

 by portions of matter withdrawn radially from the interior towards the still 

 cooling exterior ; that is to say, from a smaller towards a larger circumference. 

 The final effect of this, propagated to the centre of the mass, is twofold : — 



1st. To produce a violent state of internal tension in the molecules of the 

 metal, in radial lines from the axis of the gun viewed as a cylinder, tending to 

 tear away the external portions of the mass from the internal nucleus ; a force 

 which is zero at the axis and at the exterior, and a maximum between and 



probably at a point of the radius somewhere between R and -^ from the ex- 

 terior. 



2nd. To produce about the centre or along the axis a line of weakness, and 

 one in which the texture of the metal is soft, porous, of extremely low specific 

 gravity, with coarse and frequently, distinctly separated crystals, and often 

 (notwithstanding the precautious of the founder in "feeding" the "head of 

 the casting" — that is to say, in slowly adding fresh quantities of hot and fluid 

 metal while ever it is possible to get it introduced into the centre of the solidi- 

 fying mass), leaving actual cavities in the centre of the casting. 



44. In a casting of two or three feet or more in diameter, it is not unusual 

 (with the founder's best care) to find a central portion of from 6 to 8 or more 



