64 HEAT OR CALORIC. 



It is most heated at the earth's surface, and in a rapidly decreasing 

 series, (perhaps even a geometrical one,) as we ascend, 

 (a.) Line of perpetual congelation. 



At a certain elevation in the atmosphere, it freezes in some part of 

 every day in the year ; and at a height not less than three miles, it 

 would freeze water, at all times, in every climate, that surrounds our 

 earth. 



(b.) Height of the line of perpetual congelation. 

 At the equator it is 15577 feet, as ascertained by Mr. Bouguer by 

 actual observation on Pinchinca, one of the peaks of the Andes ; in 

 lat. 45 it is 9016 feet; in lat. 70 it is 1557 feet; in lat. 80 it is 

 120 feet ; and at the pole, it is nearly coincident with the earth's sur- 

 face. Most of these numbers were obtained by calculation, upon a 

 principle explained by Mr. Kirwan.* 



(c.) Causes of the increase of cold in the higher regions of the at- 

 mosphere. The sun's rays do not heat the air while passing through it; 

 they heat the earth first, and this heats the air by actual contact. 

 As we ascend, the capacity of the air for heat increases in an arith- 

 metical, while its density diminishes in a geometrical ratio ; hence, it 

 requires more heat to produce a given temperature. Among the 

 minor causes, may be mentioned the absence, in a great degree at high 

 elevations, of animal and vegetable life, of fermentation, of combus- 

 tion, respiration and putrefaction, all of which generate heat. 



(d.) Snow is in every climate, perpetual on high mountains. 

 Because their tops pierce the regions of perpetual cold, and snow 

 once remaining the year round, will continue ; the sun cannot, in a 

 second summer, melt what it has failed to melt in a first. 



Any commencement of warming there, by the sun's rays, before the 

 first snow fell, would have been very transient, because ventilation 

 would soon begin, as the lateral columns of air, not over the moun- 

 tain ridges or top, would not be heated at that elevation, and being 

 heavier would rush in upon them on all sides, and therefore the sur- 

 face there would never become warm. 



(e.) Effect of the winds on the term of perpetual cold. 

 They raise it by mingling warm air with the cold ; if there were 

 no winds, perpetual cold would no where, be over a mile above the 

 earth's surface. Dr. Black's Lectures. 



* The mean temperature at the equator and in any parallel of latitude, being ascer~ 

 tained by observation, we take the difference between each of these two numbers and 

 the freezing point ; the height of the term of perpetual congelation at the equator 

 is also ascertained by observation ; the number to be found, is the height of the 

 same term in any parallel of latitude ; the proportion will be, as 52 (84 mean 

 temp. 32 = 52) the number at the equator, is to 15.577 the height of the term of 

 perpetual congelation there, so is 40. 3 the number at 28 of lat. (72.3 mean temp. 



32 40.3= third term,) to 12.072 the height of the term of congelation there, 



and so for any other latitude. Due allowance must of course be made, for the el- 

 evation of the country above the sea, for its mountainous or level surface and for va- 

 rious other causes, which would influence its climate. 



