184 Mr. Mallet on the Physical Conditions 



total heat generated, that is, communicated to the gun in the discharge. Its 

 value can only be determined by experiment, and in continued firing, will 

 decrease at each successive discharge, the gun gaining less heat as it becomes 

 warmer from each flash. A medium 12-pounder field-gun weighs 18 cwt., and 

 the service charge is 4 lbs. = jj^ of the weight of the gun. If the whole of the 

 heat from one discharge were absorbed by the gun, its temperature, assumed 

 uniform, would be raised about 34° Fahr. The whole heat, however, is not 

 absorbed, nor is that which is retained by the gun uniformly diffused. 



14. — Explanation of'-'' Drooping at the Muzzle" 



95. In this case (that of heating by quick firing) the interior expansion is 

 not almost limited, as in the former, to a ring in tlie immediate neighbourhood 

 of the shot, but extends to the whole length of the chase or bore, so that the 

 whole gun becomes lengthened by the "end on" strain of its expanded interior. 

 In bronze guns the coefficient of expansion is so great (greater than that of 

 cast-iron in about the ratio of 20 to 11), and the resilience, or power of elastic 

 recovery of form, so small, that in extreme cases the extension due to the end 

 on strain surpasses the elastic limit of recovery, and the gun becomes perma- 

 nently lengthened. When in this state the firing is continued, and the metal 

 becomes much heated throughout (though still hottest in the inside), heat is 

 carried olF by convection from the lower side of the gun, by the ascending 

 currents of the air around it,so fast, that the upper side of the gun is relatively heated 

 and expanded more than the lower side ; and when the cross strain thus produced 

 has bent the metal beyond the limit of its elastic recovery, the gun " droops at the 

 muzzle," as it is called, an effect vulgarly ascribed, and even by writers on ar- 

 tillery, to the "softening of the metal by the heat," a condition that could not 

 happen until a temperature should have been attained, at which no cartridge 

 could be placed in the gun without its instantly exploding ; in fact, more than 

 a " red heat," 1800'^ Fahr. (Daniell.) 



I am not aware that this explanation of the cause of drooping at the muzzle 

 has been before given. It sufficiently indicates the severity and injudiciousness 

 of the proof test, by " quick firing," formerly applied to all field-guns, and still 

 said to be used by some foreign governments. 



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