4^8 Relations of Air y Heat, and Cold. 



terially their intimate properties. Thus, common air, on 

 being condensed 30 times, has its capacity for heat reduced 

 to one half; and, iF suddenly compressed to 20 limes its 

 ordinary df;nsity, it will disencajre so much heat as to show 

 an elevation of temperatuie tqual tagOO degrees bv Fahren- 

 heit's scale, and sufficient for ihe intlanimation of most 

 bodies. On thi? property is founded a pretty contrivance 

 lately made, the stroke of a small condensing syringe being 

 employed to set on fire a bit of tinder. An opposite effect, 

 when air is suddenly rarefied, take* place ; a certain quan- 

 tity of heat being now absorbed, or an apparent cold pro- 

 duced. 



" The increased capacity of rarefied air is the true cause 

 of the cold which prevails in the higher regions of the at- 

 mosphere. From the unequal action of the sun's rays and 

 the vici'^situdes of day and night, a perpetual and quick 

 circulation is maintained between the lower and the upper 

 strata ; and it is obvious, that, for each portion of air which 

 rises from the surface, an equal and corresponding portion 

 jnust also descend. But that which mouius up, acquiring 

 an enlargement of capacity, has its temperature propor- 

 tionally diminished; while the correlative mass falling 

 down carries likewise its heat along with it, and, con- 

 tracting its capacity, seems to diffuse warmth below. A 

 stratum at any given height in the atmosphere is hence 

 alike affected by the passage of air from below, and by the 

 return of air from above, the former absorbing beat, and the 

 Jalter evolving it. But the mean temperature at any height 

 in the atmosphere is still on the whole permanent, and 

 consequently those disturbing causes must be exactly ba- 

 lanced, or the absolute measure of heat is really the same' 

 at all elevations, suffering merely some external modifica- 

 tion frou) the difTerence of capacity in the fluid with which 

 it has combined. That temperature is hence inversely as 

 the capacity of air possessing the rarity due to the given 

 altitude. Having therefore ascertained, by some delicate 

 experiments, the law which connects the capacity with the 

 rarity of air, it was not very difhcult to trace the gradations 

 of cold in the higher atmosphere, and even to mark the 

 precise limit where the reign of perpetual congelation must 

 conmience. Thus, I find that, under the equator, the 

 boundary of the frozen region begins at the altitude of 

 15i07 feel, in the parallel of 45° at 7G71 feet, in the lati- 

 tude of London at 5950, and in that of Stockholm ai 3818, 

 while towards thf; pole it comes to graze along the sur- 

 face." The 



