ATMOSPHERE. 



239 



form in principle ; there is a diminished temperature as 

 we ascend, and as temperature is one of the elements 

 of living action, whether animal or vegetable, there 

 must be a corresponding diminution of that action as 

 we ascend above the mean level. 



When, in addition to these, which may be consi- 

 dered as some of the effects of the atmosphere, con- 

 sidered merely as passive or neuter, upon the action, 

 of plants and animals, we add the operation of exter- 

 nal causes, we find results of much greater import- 

 ance. The heat of the sun is unquestionably one of 

 the most powerful agents in the production of life and 

 growth, and the grand medium through which that 

 agent acts is the atmosphere. Clear notions of the 

 mode of the action of heat through that medium are 

 therefore absolutely necessary to anything like a ra- 

 tional understanding of the active parts of nature, and 

 without that, natural history is a mere detail of dead 

 specimens, unworthy the name of science, of small 

 value in the arts, and affording neither occupation nor 

 pleasure to a rational mind. 



From the very nature of the atmosphere, or indeed 

 of an organ of any kind, we might infer the general 

 action of heat upon it, without any observation or ex- 

 periment. The gaseous, or aerial state, is owing to 

 the action of heat, which subdues the cohesion of the 

 substance, and in proportion as it is rarefied, or has its 

 cohesion farther subdued, the action of heat, which 

 goes to the support of it in that state, and which is 

 consequently rendered insensible, or incapable of be- 

 ing given out to other substances without a corre- 

 sponding condensation of the air, must be increased. 



Hence, generally, when air (atmospheric or any 

 other) is heated, it expands, and when cooled it con- 

 tracts or condenses ; and if the expansion or con- 

 traction ever take place freely in the exact propor- 

 tion of the action of heat, which is communicated in 

 the one case or abstracted in the other, the air may 

 pass through a considerable range of density without 

 having its sensible heat altered. But, in consequence 

 of the differences of pressure and density, air at dif- 

 . ferent heights of the atmosphere can neither expand 

 nor contract to the same extent with the same addi- 

 tion or abstraction of the action of heat. The pres- 

 sure and density of the lower strata both tend to re- 

 sist expansion, and the pressure at least tends to fa- 

 vour contraction ; while, in the upper strata, the di- 

 minished pressure and the rarity tend to favour ex- 

 pansion, and the diminished pressure at least tends to 

 hinder or diminish condensation. Wherefore, there 

 is in the lower stratum of the atmosphere a tendency 

 to give out to other substances any heat that may be 

 communicated to that stratum, and this tendency di- 

 minishes as the height above the mean level of the 

 sea increases. The effect of this must depend upon 

 the degree of the action of heat which is communi- 

 cated ; but still, if we suppose that the whole of the 

 atmosphere is equally exposed to any heating cause, 

 the heat which is resisted by the atmosphere, and 

 becomes sensible, and is given out to plants and 

 animals, must be greatest at the level of the sea, and 

 diminish as the height rises above that level. But, as 

 heat is one of the most essential agents in the economy 

 of all the living and growing productions of the earth, 

 this tendency in that part of the atmosphere which is 

 ' at the level of the sea to give out heat most readily, 

 and the diminution of this tendency as the height in- 

 creases, must again render the lowest places the most 

 fertile, and productive of life of all kinds, and the more 



elevated ones less so, in the proportion of their ele- 

 vation that is, on the supposition that the soil and 

 geographical position are the same, or similar. 



But further : the grand heating cause to which the 

 atmosphere is exposed is the rays of the sun, acting 

 either directly, by refraction, by reflection, or by ra- 

 diation ; and in all these ways the solar light may be 

 considered as .acting upon the particles, or ultimate 

 component parts of the atmosphere, although these 

 parts are, individually, by far too small for being per- 

 ceptible by our senses. This being the case, the action 

 of the solar heat, or indeed of any other heat, come 

 from whatever cause, and act in whatever manner it 

 may, must be in proportion to the density of that 

 portion of the atmosphere upon which it acts. Air twice 

 as dense contains of course twice as many particles 

 in the same space ; and as these must all be equally 

 acted upon by an equal heating cause, the heat com- 

 municated to one of double density must be double, 

 and so on for other proportions. If the surface of 

 the ground, or the objects upon that surface, have a 

 lower temperature than that which the stratum of air 

 next the ground receives from the heating cause, the 

 heat will be, in great part, given out to them, and in- 

 crease their temperature, and consequently their 

 action in so far as that can be promoted by heat ; but 

 if the ground, or what is upon the ground, has a 

 higher temperature, the effect of the heat communi- 

 cated to the atmosphere will be to expand the portion 

 next the ground, and cause it to ascend in conse- 

 quence of the diminution of weight which the same 

 volume of it sustains by the expansion. 



Much, therefore, depends upon the kind of surface. 

 If that be of a nature capable of receiving the heat 

 which is imparted to the atmosphere, that heat will 

 be communicated to it ; and it will, according to cir- 

 cumstances, produce growth, or drying, or parching, 

 and burning up. But if the surface is not in a condi- 

 tion to receive the action of heat, that action will be 

 carried upward in the expanding air, and produce its 

 effects in the atmosphere. 



It was mentioned that, in the mountains which rise 

 over the dry and warm table land of Mexico, the line 

 of congelation is a quarter of a mile higher than the 

 theory would lead us to suppose ; and that on the 

 north side of the Himalaya it is fully a mile higher. 

 The latter would, to common observation, appear the 

 more singular, not only on account of the greater dif- 

 ference, but because the Himalaya are in a more 

 northern latitude than the Mexican mountains, and 

 the situation where the line of congelation is highest 

 is on their northern slope, or that which is turned 

 away from the sun, while on the southern, or sunny 

 slope, it is lower, though still considerably above what 

 the theory from partial data would give. Now the 

 plain of Mexico, though warm and dry, and of a cha- 

 racter to have its temperature raised, by the direct ac- 

 tion of the sun, so high as to receive no heat from the 

 air, is yet not a barren plain, but in many places 

 under natural plants of considerable growth, and in 

 others under culture. Mexico is also at no very 

 great distance either from the eastern or the western 

 sea, and it is, in consequence, subject to occasional 

 rains. The country to the north of the Himalaya is 

 not so well known ; but there is reason to believe that 

 considerable portions of it are barren desert, and, 

 parts of it, at least, are very rarely visited by rain. 

 Accordingly, the country to the northward of the 

 Himalaya, though probably more elevated than the 



