involved in the Construction of Artillery. 233 



174. The alteration in constitution in the vertical direction, by which, when 

 cold, there is more copper in the lower part of the column of the gun, appears to 

 be due simply to the effects of gravitation acting upon the denser metal (tlie 

 copper), and partially eliquating it from its extremely loose combination with 

 the tin. It would be contrary to all chemical analogy to suppose that the 

 increased statical pressure towards the base of the column acted in inducing 

 the descent of the copper, — pressure tending usually to increase aiTinity and 

 promote the stability of compounds, — unless, indeed, we presume that the 

 affinity of tin for copper varies with pressure, and, as this increases, causes the 

 tin of the lowermost parts of the column to rob copper from the superior por- 

 tions of tlie mass. The simplest and first explanation seems the most probable. 

 The fact, however, is certain. Thus, Dussausoy found that of a prism of gun- 

 metal, of only 13 French inches high and 3 inches square, the copper varied 

 l^etween the extreme lower and upper ends in the ratio of 99'9 : 92 9. 



175. From these two distinct modes of segregation of the alloy in vertical 

 column, then it is obvious that, the "head of metal," above the intended muzzle 

 of the gun, plays a very different part in cast-iron and in gun-metal guns ; in 

 the former, consolidating and condensing the crystalline mass by pressure only 

 of tlie liquid head, but in the latter case not only doing this, but by extending 

 the total length of the column of liquid metal, giving greater uniformity of com- 

 position to the segment cut off from the lower end to form the future gun. For 

 it is obvious that with fixed limits of variation between the composition of the 

 two extreme ends of the vertical column, the longer the column itself is, the 

 less will be the total difference between the compositions of the extreme ends of 

 any segment cut from it. It will, therefore, be good practice to extend to the 

 utmost, in all cases, the " head of metal" (matelotte), or " dead head," above the 

 upper part of the gun in the vertical mould. 



176. The more slowly the column of metal cools, the more complete and 

 injurious will be the process of segregation in the mass of alloy, and with the 

 same methods of moulding, the cooling will be slower in proportion as the 

 temperature of the metal poured into the mould is higher. The same general 

 conclusion that we arrived at in the case of cast-iron guns applies, therefore, 

 here, though for a different reason ; the lower the temperature at which the vietal 

 remains sufficiently fluid perfectly to fill the mould, the belter will he the gun when 



