28 



The other animals described in this communication were, a spe- 

 cies of naked-eyed Medusa, for the reception of which the authors 

 found it necessary to establish a new genus, Plancia {Plancia gracilis.) 

 Seven new species of Medusae, referable to the genera Oceanea, Slab- 

 heria, Hippocrene, and Thaumantias^ were also doscribed. 



The communication was illustrated by coloured drawings. 



2. Account of Experiments on the Tliermotic Effect of the 

 Compression of Air, with some practical applications. By 

 Professor C. Piazzi Smyth. 



3. Theoretical investigations into the same by W. Petrie, 

 Esq. Communicated by Professor C. Piazzi Smyth. 



Having brought before this Society in April 1849, a plan for 

 cooling the air of rooms in tropical climates, the author was anxious 

 to determine by actual experiment on a very large scale the practi- 

 cability of the principle involved, viz., the thermotic effect of the 

 compression of air. He had had a small apparatus made in 1844, 

 which, though not sufficiently large to give exact numerical data, at 

 least showed that the plan was in the bounds of possibility. 



But in December 1849, Mr Wilson, of the Kinniel Ironworks, 

 having kindly allowed him to experiment on the compressed air in 

 the reservoir tubes of the furnaces, Professor P. S. proceeded there 

 in company with Mr Stirling, C.E., and Capt. Gosset, R.E., with 

 an apparatus which was exhibited on the table. 



Thirty-four different experiments were made, in as varied a way 

 as possible to insure accuracy, and the mean result was, that the 

 air being at 63° Fahr., and the barometer at 30* inches, and the 

 pressure guage indicating 7' 2 inches of mercury, the rise of tem- 

 perature of the air on being made to enter the compression-chest, 

 was 28°' 9, and the fall on escaping therefrom was 26°-9. 



Professor W. Thomson, from Carnot's theory of heat, and Mr 

 Macquorn Rankine from his own, deduced nearly the same quantity, 

 but with some uncertainty, as the specific heat of air was involved. 



Mr Petrie, however, without taking up any theory of heat, but 

 merely the mechanical nature of a compressible fluid, and the well 

 known quantity of the expansion of air from heat, deduced a formula 

 which represented the above observations as well as could be ex- 

 pected. And pursuing his formula to its ultimate consequences, he 



