318 THE SCIENTIFIC PAPERS OF 



or 90. If you wanted to go to the freezing point, or below it, 

 you must select a liquid that would boil at a point considerably 

 above the freezing point of water, under the reduced pressure 

 maintained by the air-pump. He should have liked the air- 

 machines to have been more discussed, as they were well worth 

 attention. There was a well-known method of producing a low 

 temperature by compressing atmospheric air, cooling it, and then 

 allowing it to expand ; and some years ago his attention was 

 directed to a machine of that description invented in America, 

 which he found laboured under a most egregious error in its 

 conception. The air was compressed by a very excellent machine, 

 and was cooled by a well-arranged system of tubes, but it was 

 then allowed to expand through a throttle-valve, under the idea 

 that depression of temperature would thus take place. But this 

 was an entire misconception of the facts. Heat was nothing but 

 force, and the reason why air in expansion became lowered in 

 temperature was simply because it developed force in so doing, 

 and if no force were developed in its expansion, no depression 

 could take place. Therefore, if air were compressed to a hundred 

 pounds to the square inch, and then were expanded through a 

 small orifice, there would be precisely the same temperature in the 

 expanded air, as there was before, but if the same air were 

 expanded between the same limits in an expansive engine, a pro- 

 portionate loss of heat would take place, and the machine would 

 give back a considerable amount of the power expended in com- 

 pressing the air. Where the problem was simply to cool the air, 

 this kind of machine was, therefore, well worthy of attention. 

 Professor Gamgee had complained of being misled by engineers, 

 and he feared he was not yet quite out of the hands of the 

 Philistines, for the rotary-pump he referred to would not, he 

 thought, be equal to an ordinary honest cylinder and piston. 

 However, as it was not particularly described, he would not 

 condemn it altogether. 



