190 Prof. J. Le Coute on the Influence of 



and all in the presence of artillery officers, who were furnished 

 with the most accurate methods of measuring time. They amply 

 prove the fact that combustion is retarded at considerable ele- 

 vations. 



Thus a variety of well-established facts concur in fortifying 

 the conclusions to which we are led by a priori reasoning, — 

 namely, that the process of combustion is retarded by diminution 

 of the density of the air, while it is acceleraiedhy its condensation. 

 It has long been a matter of common observation, that ordinary 

 wood-fires burn more freely when the barometer is high ; but 

 Mr. Marcus Bull and others maintain* that this result is not 

 owing to the augmented density of the air, but to the greater dri/- 

 ness of the atmosphere. The facts brought forward in this paper 

 are strongly opposed to this explanation ; for there are not the 

 slightest grounds for supposing that there was less than the ordi- 

 nary amount of aqueous vapour present in the condensing cylin- 

 ders of M. Triger, or more than the usual quantity mixed with 

 the air at the elevated stations in India. On the contrary, phy- 

 sical considerations lead us to precisely opposite conclusions. 



2. Temjjerature of the Air. 



In relation to the influence of the temperature of the air on 

 the rate of combustion, our information is still more meagre. The 

 experiments of Grotthuss and Sir H. Davy on the "Effects of 

 Rarefaction by Heat on Combustion and Explosion," give con- 

 tradictory results t ; but as they i-elate exclusively to the influence 

 of temperature on the ignition of explosive mixtures of gases, 

 they test its eflfects on combustibility, and are obviously inappli- 

 cable to the question under consideration. The well-known 

 effects of the " hot blast " in increasing the temperature of fur- 

 naces, cannot be applied as a test of the influence of warm air 

 on the rate of combustion under ordinary circumstances. First, 

 because the air of the " hot blast " is not in its natural state of 

 density ; and secondly, because the augmentation of temperature 

 observed in such cases probably arises from its greater availability, 

 growing out of the fact that less heat is carried off in the pro- 

 ducts of combustion, rather than an absolute increase in the 

 rapidity of burning. 



In the absence of direct experimental evidence, it may be 

 admissible to apply general reasoning based upon well-known 

 physical principles. So far as an increase of temperature influ- 

 ences the density of the air, it is sufficiently evident that its 



* Vide Trans, of Amer. Phil. Soc. 2nd series, vol. iii. pp. 55, 56. Phi- 

 ladelphia, 1830. 



t Phil. Trans, for 1817, p. 53. 



