316 A method of cooling the Air of [No. 10, new serii:s. 



of 7*2 inches of mercury — less than £ of an atmosphere — raising the 

 temperature of air 29° Fahr. or from 63° to 92°; and on being 

 allowed to escape and expand from that pressure freely into the 

 atmosphere, the fall of temperature is also 29 ° . 



This result was immensely above what any friends to whom I 

 had mentioned the matter had anticipated, and they would have 

 been inclined to doubt, had not these experiments been so unex- 

 ceptionable in the huge scale of the pumps employed. The work- 

 men at the place were well aware of the heat of the air in the com- 

 pressed vessel and pipes, and the instant that the hand was laid 

 on the large reservoir into which we bored the hole, the great in- 

 crease of heat was perceived. The men had very absurdly, but 

 very confidently, been in the habit of attributing the heat to the 

 friction of the air in the pipe ; but, in the first place, the air was 

 almost stationary in that pipe, which was some five feet in diame- 

 ter ; and, in the next place, when the air was allowed to escape 

 through a 1 inch pipe, and so produce incomparably more friction, 

 the fall of 29° was obtained, instead of any further increase. 



Professor W. Thomson being employed on the theory of heat 

 about this time, and being engaged in preparing an account of 

 Carnot's theory, I applied to him to know what the increase of heat 

 would be, if air at 70° Fahr. was to be compressed £ of an atmo- 

 sphere, the barometer being 30 inches. He replied, that some of 

 the elements required for the calculation were not exactly known, 

 but that, as near as he could compute it then, it would be 30° 

 Fahr., which is a remarkable confirmation of the 29° for 7*2 inches 

 derived from experiment. 



Mr. W. Macquourn Rankine, C. E. who last winter produced 

 his mathematico-mechanical theory of heat, states, that it gives the 

 same result as the above ; so that, for practical purposes and small 

 pressures, we may take very safely 4° Fahr. as being the rise in 

 the temperature of air for 1 inch pressure of mercury, and 30™ 

 Fahr. for 7*5 inches, or \ of an atmosphere ; and Mr. Rankine 

 further computes that a theoretical Horse-power working one hour 

 will be sufficient to lower 9,000 cubic feet of air 20° Fahr. without 

 any deduction for friction. 



Making a very liberal allowance for friction, for loss of effect by 

 radiation of heat, and for imperfect cooling of the compressed air, and 



