198 



THE SCIENTIFIC PAPERS OF 



the compressing cylinder is practically unavoidable, the question 

 arises, how great is the necessary expenditure of engine-power, or, 

 in other words, the difference between the power absorbed in the 

 compressing cylinder and the power obtained in the expanding 

 cylinder ? The difference of power arises in consequence of the 

 abstraction of heat from the compressed air, and is directly pro- 

 portionate to the number of degrees of heat abstracted or generated 

 in compression and afterwards lost in expansion. The quantity of 

 heat lost in the expanding cylinder is, however, proportionate to 

 the power produced in expansion, and is entirely independent of 

 the limits of expansion which, as has just been shown, determines 

 the degree or quality of cold produced. It follows that low rates 

 of compression and expansion of the air are favourable to the 

 production of cold with the least expenditure of engine-power, 

 and I find by calculation that the power value of a given amount of 

 heat abstracted (or cold produced) increases in the ratios of the 

 squares of the number of degrees of reduction of temperature, re- 

 quired. 



" It is therefore highly advantageous to compress and expand 

 large volumes of air to a moderate extent, consistent with the 

 requisite energy of action of the machine. 



"Adhering for the present to the rate of compression obtained in 

 working your apparatus, namely 55 Ibs. per square inch effective 

 pressure, the following result might be obtained if no incidental 

 losses of any kind were obtained, and taking the speed at 15 revo- 

 lutions per minute. 



H.P. indicated in compressing cylinder 

 ,, expanding cylinder 



engine .... 



Units of heat produced in compression 

 minute ....... 



Injection water raised in compression cylinder 



Units of heat produced in expansion 



Ice produced per hour from water of 60 



600 gallons of water cooled per hour 



The diameter of expanding cylinder Lo be . 



per 



35-0 



25-2 

 9'8 



1,500 

 2-5 

 10-80 

 381 Ibs. 

 10-8 

 17| ins. 



In comparing these results with those actually obtained, it will 



