188 



Mr. Mallet on the Physical Conditions 



greater cohesive force than cast-iron ; in fact, the element of ex 

 tension, upon which the value of Te the coefficient which M. Pon- 

 celet denominates " de la resistance vive d'elasticite," or the "mo- 

 dulus of resilience" of other writers, referred to hereafter, depends, 

 is much greater for gun-metal, and hence a given force produces 

 a greater proportional distortion of form. 

 3\ As already remarked, the gun-metal has a coefficient of expansion, 

 by equal heat, far beyond cast-iron, or as 1541 : 893, or nearly as 

 151 to 9. Hence, equal inequalities of temperature will produce 

 nearly double the distortion in the gun-metal. 

 4°. A very moderate increase of temperature above that of the atmo- 

 sphere (say 50°) greatly reduces the cohesion (and probably in a 

 far higher ratio the stiffness) of gun-metal, as the researches of 

 Baudrimont have rendered nearly certain, while it produces a 

 directly opposite effect on iron, and, we may conclude, on cast- 

 iron. 

 102. Baudrimont has ascertained that the relative cohesive powers (coefficient 

 of rupture) of copper and of iron at the three temperatures, 0°, 100°, and 200° 

 Cent., are in the ratios of the following numbers,* which, as respects both, agree 

 pretty well with the results of the experiments of the Franklin Institute : — 



If, therefore, gun-metal and cast-iron follow the same law, as it can scarcely 

 be doubted they do, a gun-metal gun heated from the freezing point of water 

 to 200° Cent., loses resisting power in the ratio of about 20 : 25g, while a cast- 

 iron gun gains resisting power in the ratio of about 21 : 2O5, having, however, 

 an intermediate weak point at 100°, where its resistance diminishes to nearly 19, 

 a fact which indicates that cast-iron guns are safer in this respect when strongly 

 heated, than when heated less. 



5°. The rough black surface of a cast-iron gun enables it (on principles 



* Anu. dc C. T. .\xx. p. 304, and Pogg. Annal., Lxxxii. p. 156. 



