involved in the Construction of Artillery. 157 



every cavity of the mould, without risk of defect, is that at which a large cast- 

 ing, such as a heavy gun, ought to be " poured." As respects the rapidity of 

 cooling desirable, we shall be enabled presently to consider the conditions that 

 determine the extent to which it may be safely carried. 



32. A certain amount of contraction on becoming solid from the liquid state 

 occurs in all castings. It is well known to practical founders that for cast-iron 

 this is variable, and depends upon the mass of the casting, being greatest for 

 small and least for large castings of the same "make" of iron; but it is ob- 

 vious, and it follows from M. Bolley's researches, that the contraction also will 

 be greater in proportion as the metal is poured into the mould at a higher tem- 

 perature, although, from the expansion in the act of crystallizing, the specific 

 gravity of the solid mass may be less at the higher than at the lower tempera- 

 ture of " pouring." 



33. As, therefore, there are two conditions that principally affect the degree 

 of contraction — the total change of volume between the liquid metal and its 

 solid casting ; namely, the extent to which the fluid metal as entering the mould 

 has been expanded by elevation of temperature and the state of final aggrega- 

 tion of the crystalline particles — which we have seen depends much upon the 

 former — so there will be a determinate amount of contraction due to a determi- 

 nate thickness or mass of casting, irrespective of, though also related to, the coefii- 

 cient of contraction for any particular " make" of iron ; for there is no doubt that 

 different makes, cceteris paribus, contract somewhat differently. From whence 

 it follows, that different parts of the same casting, if differing materially in scant- 

 ling or mass, will have different amounts of final contraction ; and hence — 



34. Sudden changes of form or of dimensions in the parts of cast-iron guns, 

 besides the injury they do to the crystalline structure of the mass, introduce 

 violent strains, due to the unequal contraction of the adjoining parts, whose 

 final contraction has been different. 



How desirable is it, therefore, to introduce such alterations of the forms of 

 our ordnance as shall avoid those sudden and enormous (and often useless) 

 changes of adjacent mass, that we observe ; as for example, in the sea and 

 land service 13-inch mortars, where at the chamber (where the strain being as 

 D" is least) the thickness of metal suddenly approaches twice that of the chase — 

 a malconstruction the fidl evils of which we have yet to consider. 

 VOL. XXIII. r 



