108 Mr. Graham on the Finite Extent of the Atmosphere. 



atmosphere decreases as we ascend, and that with consider- 

 able regularity. The observations which we possess upon this 

 subject indicate a decrease of 1 degree, for every elevation of 

 about 300 feet. This brings us rapidly to a limit to the height 

 of the atmosphere. Supposing the temperature of the surface 

 of the earth 32°, the air would lose its elastic state at a height 

 which would be less than 480 times 300 feet, or under 27'27 

 miles. However, without questioning the continuance of this 

 decrease of temperature at great elevations, it is probable that 

 in the higher regions of the atmosphere it is by no means so 

 rapid as in the lower regions, where the law has been verified 

 by observation. For the great source of the heat of the atmo- 

 sphere is its contact with the surface of the earth, and not in 

 the calorific rays of the sun which it arrests in their progress. 

 Hence the lower strata of the atmosphere will possess a com- 

 paratively high and extraordinary temperature, and the fall 

 of temperature as we ascend will appear for some time rapid. 

 But at a certain elevation, the effect of this adventitious sup- 

 ply of heat will be greatly diminished. 



The increase of capacity for heat in gases, attendant upon 

 increase of bulk, accounts in a satisfactory manner for reduc- 

 tion of temperature in a mass of air as it is elevated and less 

 compressed. The superior stratum of the atmosphere we may 

 suppose to expand, from its imrestrained elasticity : its tem- 

 perature is thereby lowered, till at last it arrives at that point 

 which involves the loss of its elastic state. As the liquid state 

 is a physical state of bodies, which implies pressure and a 

 power to maintain the evolution of vapour (certainly in all non- 

 metallic bodies), the cooled and uncompressed superior air 

 w'dl be at once reduced from the gaseous to the solid state. In 

 this way may temperature occasion a limit to the diffusion of 

 the atmosphere. 



From the length of time during which the sun's rays con- 

 tinue to be reflected back upon the earth by the superior 

 parts of the atmosphere, after he has sunk beneath the hori- 

 zon, there is reason to believe that the atmosphere extends in 

 a state of great tenuity to a very considerable height above 

 the surface of the earth, and therefore that the theatre of this 

 condensation is considerably removed. Let us suppose that 

 it is so, and inquire whether its existence would be indicated 

 by any notable effect. 



We know well that in ordinary cases, the reduction of a 

 body from the gaseous to the liquid or solid state is attended 

 with a considerable extrication of heat. Light, loo, has been 

 observed in condensation following sublimation, particularly 

 in the case of benzoic acid. Now, the superior and condensing 



strata 



