Mr. Graham on the Finite Extent of the Atmosphere. 109 



strata of the atmosphere are of a tenuity incomparably 

 greater than that of the vapours whose condensation we ge- 

 nerally witness. But this tenuity has been arrived at, at 

 the expense of the previous absorption of much more heat ; 

 or in other words, the latent heat of vapours, which is emitted 

 upon their condensation, is in proportion to their tenuity. 

 Hence it is probable, that the condensation of the elastic air 

 into solid particles would be attended with the emission of 

 accumulated stores of light and heat. Would not air, too, it 

 might be asked, emit light upon its complete condensation and 

 loss of physical state, while it may be made to do so by mere 

 mechanical compression ? Here, perhaps, we have the cause 

 of that degree of luminosity which is generally associated 

 with the upper regions of the atmosphere, and which has in- 

 duced Professor Leslie, with that daring originality which fre- 

 quently characterizes his beautiful speculations, to attribute to 

 them a phosphorescent property. 



These luminous appearances will be more frequent and 

 striking at the polar regions, from the temperature, there, ap- 

 proaching more closely to the condensing point of the gaseous 

 substances constituting the atmosphere. Their pi-oper sites will 

 be the thermal poles, or points on the earth's surface of lowest 

 temperature. From late observations, the thermal poles of 

 the earth appear to coincide with its magnetic poles. Let us 

 suppose a determination to condensation to take place in the 

 superior regions of the atmosphere at the thermal pole. The 

 surrounding elastic air would rush in, and expand, to fill the 

 vacuity occasioned by the condensation. But this rarefaction, 

 with its attendant fall in temperature, would frequently be 

 productive of condensation and deposition in these masses of 

 air themselves. In this way, the tendency to condensation, 

 originating perhaps at the thermal pole, would be widely and 

 rapidly propagated ; and the attending streams of light would 

 appear to shoot from that point. Here we recognize the bril- 

 liant phaenomena of the aurora borealis. 



It evidently follows from this theory that the atmosphere 

 will be of different altitudes over different parts of the earth, 

 according to their temperature. Within the tropics it will be 

 higher than over the polar regions. Hence the higher parts 

 of the equatorial atmosphere will tend to fall back upon the 

 poles, — a disposition which will cooperate with the inferior 

 current in an opposite direction, to produce a grand circula- 

 tion of the atmosphere, and to impress a general character 

 upon winds. 



XXV. On 



