involved in the Construction of Artillery. 159 



with that observed by Captain Parry in his earlier Arctic voyages — viz., that 

 the astronomical instruments exposed to extrenaely low temperatures for long 

 periods, and quite undisturbed, did not contract to their extreme point until 

 after they had been subjected to some slight jar or blow, when the metal of 

 the instrument suddenly became reduced in volimie, and its dimensions again 

 stationary. 



40. The extreme slowness, continuing sometimes for months, with which 

 these molecular changes take place, due to the gradual adjustment of such inter- 

 nal strains, has been beautifully shown in a,memoir on the elastic properties of 

 soHds (Annales de Chimie, vol. xli. p. 61), by M. Savart, who found that plates 

 of sulphur cast into flat discs continued to change their state of molecular ar- 

 rangement for long periods after solidification. 



41. It follows from this that old guns that have long been bored and laid in 

 store are likely to be more trustworthy than those hastily cast, bored out, and 

 brought into service ; and this seems to be supported in some degree by ex- 

 perience. 



42. In general extension and support of tlie views I have advanced as to cast- 

 ings in iron becoming endowed with variable powers of resistance depending 

 upon external form and mode of casting, &c., the important memoir of M. Savart 

 above alluded to should be consulted. By refined and delicate methods of 

 investigation founded upon sonorous vibrations elicited, he has shown that nu- 

 merous bodies, such as zinc, lead, cast copper, glass, plaster of Paris, sealing-wax, 

 and others, though possessing apparently a perfectly homogeneous structure, 

 have it not ; but, on the contrary, all possess lines or planes usually crossing each 

 other at right angles, in which their resisting powers are enfeebled, and which 

 he has called awes of greatest and of least elasticity, and which he attributes 

 to the arrangement of their molecules assumed in the process of cooling. The 

 relations of these phenomena to the conditions of cooling and external form of 

 the body as affecting these, however, do not appear to have been perceived by 

 M. Savart, and the author of this paper believes have been here stated in a 

 distinct form for the first time. 



43. Besides the effects already referred to, due to the contraction of cast-iron 

 in becoming solid, another class of abnormal strains introduced by the consolida- 

 tion of one portion of a casting before another, must not be passed over, as often 



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