involved in the Construction of Artillery. 173 



8. — Causes of Liability to Bursting in f ring Red-hot Shot. 



71. Amongst the causes assigned by artillerists for the frequent bursting of 

 guns in firing red-hot shot, have been these, that the windage is reduced by the 

 expansion of the shot enlarging its great circle, and hence the stress upon the 

 gun increased ; and that by the conjoint effect of the heat of the shot, increas- 

 ing its diameter, and that of the expansion of the gun (produced by the heat of 

 the shot) in an internal direction, or towards the axis, diminishing its caliber, 

 the shot becomes wedged in the gun. That this solution is erroneous is not 

 difficult to prove. 



72. Sir H.Douglas (Naval Gunnery, p. 88) states that at a white heat a 241b. 

 shot expanded y\j ; a 16-pounder, y^j^ ; and a 6-pounder, ^ of their respective 

 diameters. These figures are probably erroneous, inasmuch as if the coefficient of 

 expansion and the temperature be the same for each shot, the ratio of expansion 

 to diameter should be the same for all. 



Professor Daniell's experiments with his own pyrometer gave the lineal ex- 

 pansion of cast-iron between 62° Fahr. and its melting point at -X^ part, very 

 nearly ; it is almost certain that this largely overrates the amount, as the re- 

 sults of practice in iron-founding have constantly proved that the contraction 

 of fluid cast-iron in becoming solid seldom reaches j,}q of the lineal dimensions, 

 and never exceeds ^. 



The figures given by Sir H. Douglas, however, may, perhaps, be accounted 

 for in this way — that the three diameters of shot were all cast from different 

 qualities of iron, and that, being cast in iron moulds, a certain amount of in- 

 ternal strain might remain permanently upon them, by the rapidly chilled sphe- 

 rical crust compressing the interior, which is relieved when the shot is again 

 heated red-hot, and which compressive strain would vary with the diameter, 

 and be least for the largest shot. The 6-pounder shot being small, and pro- 

 bably equally hard all through, this would not apply to it. 



73. However this may be, it is certain that shot, cast in iron moulds, must 

 have a constant strain of the exterior upon the interior, and hence a powerfully 

 increased tendency to split and fly to pieces on striking any hard object, such 

 as a wall ; and this suggests that shot intended for battering purposes would 



VOL. XXIII. 2 A 



