MECHANICS A1STD USEFUL AETS. 53 



the shorter gun, it must be concluded that it was done by virtue of a 

 greater initial pressure and an earlier action upon the shot. That 

 necessarily implied a greater strain upon the gun at the first explo- 

 sion, and that would necessitate the employment of stronger guns. He 

 should have expected a smaller velocity by a shorter gun, for the ac- 

 tion of the gas was necessarily shorter than in a longer gun. The heat 

 question, however, was to him the greatest puzzle of all. How they 

 could have the propelling power without heat in the gas, and if they 

 heated the gas, how they escaped heating the gun, he could not under- 

 stand. Prof. Pole said he was quite unable to give any explanation of 

 the difference of recoil. If the shot left the gun with the same velocity 

 as when fired with gunpowder, it was natural to suppose that there 

 must be the same quantity of recoil. Mr. Siemans, having briefly spo- 

 ken on the dynamical question involved in the matter, suggested that 

 the greater heat imparted to the gun in the case of gunpowder might 

 be owing to the greater amount of solid matter, which, taking up the 

 great heat of the gases under a pressure of some four hundred atmos- 

 pheres, imparted a portion of the same by radiation to the side of the 

 gun, while in the case of gun-cotton gases only were produced, which 

 could only impart heat to the gun by the slower process of conduction, 

 and left a larger margin of heat to be developed in force by expansion. 

 Admiral Sir E. Belcher thought that the reason the gun was not heat- 

 ed by an explosion of gun-cotton might be because the gases had not 

 time to heat-the gun, owing to the rapidity of the explosion, which was 

 slower in the case of gunpowder ; or that it might arise from the great- 

 er amount of fouling in the case of gunpowder. Mr. Scott Russell then 

 said he would endeavor to clear away the many difficulties which at- 

 tended this very difficult subject. How was it that in gunpowder and 

 in gun-cotton, where there were equal quantities of gas put in, the gas 

 in the case of gunpowder was raised to an enormously high tempera- 

 ture, and came out at an enormously high pressure, showing that they 

 had gas enormously expanded by heat ; whereas in the case of gun-cot- 

 ton the gas came out quite cool, so that you might put your hand upon 

 it, and the gun itself was quite cool ? He (Mr. Russell) had a theory. 

 Steam was a gas, and steam expanded just by the same laws as other 

 gases did. A great deal of the gas of gun-cotton happened to be steam. 

 Let them conceive one hundred pounds of gun-cotton shut up in a cham- 

 ber that just held it. They had got there all the gases that had been spo- 

 ken of, but they had also got twenty -five pounds of solid water about 

 one-third of a cubic foot of water in that chamber. What did they do 

 with it ? They put fuel, they put fire to it. They heated the whole 

 remaining pounds of patent fuel. If, then, they considered the gun-cot- 

 ton gun as the steam-gun, they got rid of two difficulties. They would 

 have, first, the enormous elasticity of steam ; and secondly, they would 

 get the coolness of it. They all knew that if they put their hand to 

 expanded high pressure steam, it had swallowed up all the heat and 

 came out quite cool. He believed that the gun-cotton gun was neither 

 more nor less than Perkins's old steam-gun, with only this difference, 

 that you bottled up the fuel and water, and let them fight it out with 

 each other. They did their work, and came out quite cool. He hoped, 

 however, that it was understood that he did not dogmatize. He put 

 all he had said with a note of interrogation upon it. Prof. Tyndall 

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