114 Professor Piazzi Smyth's Experiments on the 



Experiments on the Thermotic Effect of the Compression and 

 ^ Expansion of Air, By Professor PiAZZi Smyth. Com- 

 municated by the Author. 



Having proposed some time since the thermotic effect of 

 the compression and rarefaction of air, as a means whereby 

 the atmosphere of rooms in tropical climates might be lowered 

 to a degree suitable to the health and comfort of the inmates, 

 I had felt the want of some actual experiments, which might 

 exhibit to practical men more convincingly than the results 

 of theory alone, the sufficiency of the principle for the purpose 

 proposed ; and as the theoretical quantity depends upon the 

 capacity of air for heat, a relation by no means well known, 

 and inquired into with much difficulty, it appeared worth 

 while for higher purposes to put these experiments on re- 

 cord, in the absence of any better ones. 



All that I could ascertain as having been done before was 

 on far too small a scale to give trustworthy results. Dr 

 Darwin, for instance, experimenting on an air-gun, and Mr 

 Dalton on a thermometer in an air-pump, and thereby having 

 manifestly to deal with such excessive losses by conduction 

 and radiation, that it is not to be wondered at that they should 

 have fallen far below the truth. 



I had had an apparatus made at the Cape of Good Hope 

 in 1844, and another in Edinburgh in 1846, but both being 

 too small to prove much more than the mere existence of the 

 principle, need no further mention here, as in 1849 I was 

 ^enabled, by the kindness of Mr Wilson of Kinneil, to operate 

 on the air of his blowing machines at the blast furnaces ; and 

 was thus placed under even more favourable circumstances 

 than if I had had the largest size of experimental apparatus 

 actually constructed. 



The furnaces in question are situated a few miles up the 

 Firth of Forth, and are supplied with air at an average pres- 

 sure of 3 lb. to the square inch, by a steam-engine working 

 two double-acting pumps, each about 4 feet in diameter, and 

 8 feet long. Here, therefore, was an admirable opportunity 

 for determining, sensibly free from all effects of heat of fric- 



