Mr. Ivory 07i the Combination of Heat with Elastic Fluids. 165 



of solstitial observations, from 1791 to 1816, to determine the 

 obliquity of the ecliptic. On comparing them with those which 

 were executed in 1750, by Bradley, Mayer, and Lacaille, it 

 will be observed that the obliquity undergoes a diminution of 

 44<" in every century. 



The last arrangements of this great astronomer furnished 

 fresh proofs of his love for science. He bequeathed his li- 

 brary and his apparatus to the observatory of Palermo, adding 

 an annual sum for the education of a pupil. Piazzi enjoyed 

 a just reputation, acquired by his innumerable and important 

 labours. He was director - general of the observatories of 

 Naples and Palermo, president of the Academy of Sciences 

 of Naples, member of that of Turin, Gottingen, Berlin, and 

 Petersburg, foreign associate of the French Institute, of the 

 Royal Society of London ; ordinary member of the Italian 

 Society, and corresponding member of the Institute of Milan, 

 &c. 



XXXV. Continuation of the Subject relating to the Absoj-p- 

 tion and Extrication of Heat in a Mass of Air that changes 

 its Volume. By J. Ivory, Esq. M.A. F.R.S.* 



TN treating the subject of this article in the last Number of 

 -*- this Journal, care has been taken to avoid all assumptions 

 merely hypothetical, and to ground the reasoning on acknow- 

 ledged facts. My chief purpose at present is to show that the 

 conclusions which have been obtained lead to a very direct 

 and simple solution of the important problem concerning the 

 velocity of sound in the atmosphere, which has hitherto been 

 investigated in a manner rather complicated and circuitous. 

 1 shall also be able to correct some inaccuracies that have 

 crept into the mathematical part of this research, and which 

 have arisen from an obscure and imperfect knowledge of the 

 relations that subsist between the quantities concerned. But 

 before entering on these topics it may not be improper to re- 

 capitulate briefly, the main grounds on which the theory pro- 

 ceeds. 



In the first place, when heat is applied to a mass of air un- 

 der a constant pressure, the variations of volume are propor- 

 tional to the quantities of absolute heat which produce them. 

 The same thing in effect may be enunciated by saying in the 

 usual phrase, that the specific heat of air under a constant 

 pressure is the same at all temperatures. The proposition 

 must not be understood absolutely and indefinitely : we ob- 



* Communicated by the Author. 



tain 



