2 7 8 Jnalyjis of the Ah. 



is fed by a cool air, is evident from hence y 

 that when the Sun ihines on a Fire, and 

 thereby too much rarefies the ambient air, 

 that Fire will not burn well; nor will a 

 fmall Fire burn fo well near a large one, as 

 at fbme diftance from it. And e contra, it 

 is a common obfervation, that in very cold 

 frofty weather, Fires burn moft briskly ; the 

 reafon of which feems to be this, that the 

 elaflick expanfion of the cold condenfed air 

 to a rarefied ftate, when it enters the Fire, is 

 much brisker than that of an air already 

 rarefied in a good meafure by heat, before it 

 enters the Fire; and confequently a conti- 

 nued fuccefiion of cold air mud give a brisker 

 motion to the Fire, than the like fucceffion 

 of hot air : And fuch colder and more con- 

 denfed air will alfo (as Sir Ifaac Newton ob- 

 ferves qu. 11) by its greater weight check 

 the afcent of the vapours and exhalations of 

 the Fire, more than a warmer lighter air. 

 So that between the action and re-action of 

 the air and fulphur of the fuel, and of the 

 colder and denfer circumambient air, which 

 rarefies much upon entring the Fire, the heat 

 of the Fire is greatly increafed. See Vol. II, 



/•3 2 9- 



This 



