22 Prof . Leslie on Hedt and Climate. [July, 



greater than what is actually observed. Upon sueh a supposi- 

 tion, the regions of the north would have been for ever divided 

 from those of the south, and each parallel of latitude inhabited 

 by a particular race of beings. Among the many beneficial 

 purposes to which the element we breathe is subservient, this 

 undoubtedly is one of the most important,— that it cherishes the 

 polar regions by heat conveyed from the tropics, which, in 

 return, it refreshes by northern gales ; thus labouring assiduously 

 to maintain a balance of temperature on the surface of the globe. 

 Such a view of the subject corroborates the general presumption, 

 that the planets and their satellites have all of them atmospheres. 

 It is a theory very generally received at present, that the 

 solar heat is first evolved at the surface, from which it again rises, 

 and dissipates itself through the atmosphere. Conjectures have 

 been formed on the law of this ascent, and conclusions thence 

 drawn concerning the rate with which the temperature dimi- 

 nishes at different altitudes. But were the principle accurate, 

 those countries should be hottest which enjoy most abundantly 

 a vigorous sun-shine. Our own island is confessedly warmer, on 

 the whole, than the same parallels on the Continent ; yet these 

 regions are generally blessed with a clear and joyous heaven, 

 while our " weeping sky" is shrouded by far the greater part of 

 the year. Besides, the rays of the sun act most fiercely on the tops 

 of mountains, where it surely is not warmer than in the valleys 

 below. But the whole theory is founded on an erroneous as- 

 sumption, which ought long since to have been banished from 

 science ; I mean the proposition, that light is transmitted 

 through a transparent medium without any obstruction what- 

 ever. On the contrary, near one half of all the light which falls 

 at the Equator is absorbed in its passage through the atmosphere, 

 and, in the high latitudes, a much larger proportion. Inquirers 

 have commonly committed another oversight, in supposing that air 

 conducts heat like a solid body. But were that fluid completely 

 Confined, its conducting quality would be found incomparably 

 inferior even to that of glass. In fact, that share of the heat which 

 is communicated by the slow pervading of the aerial mass, may 

 be totally neglected in every computation. It is by its motion 

 alone that the air transfers and disseminates warmth ; * and for 

 this purpose- it is admirably fitted by its extreme volubility, and 

 by the great variations produced on its density by changes of 

 temperature. If that fluid had possessed these properties in a 

 much higher degree, an almost uniform warmth would have pre- 

 vailed over the earth, The wisdom of the Great Geometer + of 

 the universe is displayed in the choice of the exact proportions, 

 which, while they temper the system, preserve the variety of 

 climate, and the grateful vicissitude of season. 



• t need scarcely observe that I was not yet acquainted with the pulsatory 

 energy which belongs to the gaseous fluids. — A. 

 t I allude to the famous expression of Plato, ©£»c au ytv(MTp£n. 



