Mr. J. Gill on the Dynamical Theory of Heat. 489 



The ratios of both differences are 



1-224 ,0-536 .- 



0959 =13and 0^277 =1 ' 9; 

 therefore the ratio 



At x ~ Ab^ 

 is nearly constant. 



From the preceding computation of nearly 238,000 observa- 

 tions made at Prague during 1840-68, we may deduce that 



(1) When the obliquity of the lunar orbit reaches its highest 

 value the average annual temperature reaches its lowest, and the 

 pressure, on the contrary, its highest value. 



(2) The variations of the averages of temperature and pres- 

 sure are periodical ones, and their period is half a lunar year (of 

 9^ solar years). 



(3) The variations of temperature produced by the lunar in- 

 fluence are more sensible during the winter than during the 

 summer months ; for the former are nearly six times the latter. 



(4) There must be also another cause, producing a constant 

 increase of temperature, independent of the moon's motion, which 

 may perhaps result from a similar action of the sun on the at- 

 mosphere during the period of the change of the obliquity of 

 the ecliptic. The amount of it calculated for a hundred years 

 would be l°-8 to 2° R. 



Prague, April 24, 1868. 



LI. On the Dynamical Theory of Heat. 

 By Joseph Gill, Esq. 



[With a Plate.] 

 To the Editors of the Philosophical Magazine and Journal. 



Gentlemen^ 



FOUR years ago you were so good as to allow me to state in 

 the Philosophical Magazine some objections to the Dyna- 

 mical Theory of Heat which I then entertained from the appa- 

 rent results of numerous experiments on the practical working 

 of steam-engines. At that time I ventured to think that the 

 specific heat of air might possibly vary with its density, though 

 the contrary had been proved by the celebrated experiments of 

 Regnault. Since that time I began again the study of Reg- 

 nault's " Experiences," particularly respecting the specific heat 

 .of gases under different pressures ; and while I felt fresh admi- 

 ration for the wonderful sagacity and fertility of resource dis- 

 played in his experiments, and could see no fair reason to doubt 

 jiis results as to the calorific capacity of air at different densi- 



2 G2 



