176 Mr. Mallet on the Physical Conditions 



to being fireil. The shot lies in coatact with the lower side of the chase, and 

 so heats the adjacent portions of metal of the gun, by direct contact, conduc- 

 tion, and radiation ; at every other point round the interior circumference of 

 the chase it is separated by a lunaric interval due to the windage, which is 

 widest at the upper side of the shot. All round this the interior surface of the 

 gun is heated almost wholly by radiation from the hot shot. The heat thus 

 communicated to the gun at its interior surface travels slowly, in pulses, out- 

 wards through its metal, and is still more slowly carried off and dissipated in 

 the suri'ounding air from its exterior. It also moves in pulses by conduction 

 through the metal, in the direction of the length of the gun ; but the main volume 

 uf heat communicated to the gun is accumulated closely in the neighbourhood 

 of the shot when rammed home. Each heated shot in succession adds some 

 increment of heat to that already acquired by this part, provided the interval 

 between one discharge and the next be not sufficient to enable the gun to cool 

 down to its former temperature, which can scarcely happen in service, or unless 

 it be cooled artificially. The gun is also heated powerfully in the whole length 

 of its interior by the flame of the powder, but the heat due to this also produces 

 its greatest effect close to the seat of the cartridge and shot when rammed home. 



79. The result is, that in continued firing (whether with cold or hot shot) 

 the interior of the gun is hotter than the outside, and that the parts of the gun 

 nearest the breech are the hottest, and that the point around the interior circum- 

 ference here, which is the hottest of all, is the lowest point ; and furthermore, 

 that when red-hot shot are fired, all the conditions are greatly exaggerated 

 under which heat is communicated to the interior. Now heat is dissipated from 

 the exterior in two ways — by radiation, which, although not always strictly 

 equal all round, may be assumed commonly to be so, and by evection; that is 

 to say, by currents of air, which may act in either of two directions, vertically 

 or horizontally, but which generally act together. Lateral or longitudinal cur- 

 rents due to wind carry off a portion proportionate to the low temperature and 

 velocity of the air in motion. 



80. Vertical and ascending currents are at the same time produced by the 

 rarefaction of the air immediately in contact with or adjacent to the heated gun 

 (which we assume to be nearly horizontal). These ascend, and give place to 

 fresh portions of colder air, which, impinging first upon the lowermost side of the 



