THE SCIENTIFIC PAPERS OF 



transmission of power by air was attended with such losses as would 

 render its application of doubtful advantage. This was a q uestion of 

 considerable interest, and one which could happily be dealt with in 

 a very definite manner. Having had occasion some time ago to 

 look into this question, the conclusion he had then arrived at was 

 that in the ordinary mode of transmitting power by compressing 

 air, cooling that air, and then letting it expand again, the attain- 

 able limit of the useful effect was about 50 per cent, of the power 

 exerted in the compression. In the least favourable of the practical 

 results obtained in the experiments described in the paper the 

 useful effect was only about 25 per cent, of the power, implying a 

 loss of as much as 75 per cent. The machinery, however, which 

 had been employed in this instance appeared to him to be very far 

 from perfect. Some mechanical imperfections in one of the 

 air-compressing engines had indeed been pointed out by the author 

 of the paper, the indicator diagram showing that the air was 

 neither able when compressed to get out of the cylinder freely, nor 

 before compression to get into it freely : two great evils which 

 could easily be remedied by a proper construction of the air- 

 valves. 



But there was another defect, which was independent of mere 

 mechanical construction. As there was no injection of cold water 

 into the compressing cylinder, the compression curve developed in 

 that cylinder was a dynamical curve, not following the simple 

 hyperbolic line of Boyle and Marriotte, but rising in a more abrupt 

 manner, owing to the accumulation of heat during the act of com- 

 pression. It was well known that in compressing air the whole of 

 the force exerted in the compression appeared in the form of heat, 

 and this heat expanded the compressed air ; so that a much larger 

 volume had to be expelled after compression had taken place, than 

 would have been the case if the temperature had been kept the 

 same as before compression. But the remedy for this loss was a 

 very simple one, consisting merely in injecting cold water in the 

 form of spray into the compressing cylinder, in sufficient quantity 

 to keep the temperature practically uniform throughout the stroke. 

 The saving of power thereby realized would be very considerable, 

 because the air when compressed to four times its original pressure 

 would be heated by 250 Fahr., and the consequent increase of 

 volume would be about as 2 to 3, involving a loss of power of 



