240 



ATMOSPHERE. 



plains of Mexico, high as that is, and having certainly 

 an increase of latitude which with surfaces similar and 

 similarly surrounded, would make it the colder coun- 

 try of the two. has yet its surface so much more suscep- 

 tible to the direct action of the sun, that the heat which 

 it resists receiving from the atmosphere, and that 

 which it gives out to that fluid, together suffice to 

 raise the line of congelation in the neighbouring moun- 

 tains three quarters of a mile higher than in Mexico. 



These two cases are worthy of being borne in mind, 

 not only as they are in themselves singular and strik- 

 ing, but as showing to what an extent the action of 

 the atmosphere, in the general economy of nature, 

 may be modified by differences in the surface of the 

 earth. This is the more necessary, because the actions 

 of all the different surfaces that are met with on the 

 earth, being matter of very ample and minute detail, 

 some of which are yet but imperfectly known, cannot 

 be developed in a general article. 



Still the subject is one, without a notion of the ge- 

 neral principles of which no man can either merit the 

 name or enjoy the pleasure of being a naturalist, be- 

 cause it is the one which is pre-eminently concerned 

 in " putting the wheels of life in motion," and causing 

 them to work differently in different places, so as to 

 produce that rich variety of contrasts which forms 

 the grand bond of union and incentive to intercourse 

 between nation and nation, and makes the inhabitants 

 of different zones or hemispheres act as whetstones 

 in polishing and sharpening each other. 



The general law is, that the heat communicated to 

 the atmosphere acts most upon those surfaces where 

 its action is the most beneficial ; and, where its action 

 cannot be beneficially exerted, it mounts up into the 

 higher strata, and is wafted to, and confers its genial 

 influence upon, other regions. Thus the heat which 

 cannot find fit employment on the thirsty heights of 

 Central Asia, speeds southward to the slopes of the 

 Himalaya ; and in consequence the rustics tend their 

 flocks, reap their simple harvests, and live in compa- 

 rative plenty, where, according to the theory, the 

 surface should be shrouded under fathoms of perpe- 

 tual snow. 



This, it must be admitted, is saying very little ; 

 and yet, in a general way, it is almost all that can be 

 said, only it may be said in different words. Gene- 

 rally then, the heat communicated to the atmosphere 

 by the sun acts more powerfully on humid surfaces 

 than on dry ones. Not that it heats them more, but the 

 reverse, for the heat is rendered not so sensible on the 

 humid surface, in consequence of the increased action 

 which it performs there. Dry surfaces which are 

 pale in their colour return heat by reflection, and do 

 not get so speedily heated as those of a darker tint, 

 which after they are heated give out heat by radia- 

 tion. Of humid surfaces, it is not the surface of pure 

 water that most completely absorbs or occupies the 

 action of heat. When still, the surface of pure water 

 reflects back to the air much of the heat which falls 

 upon it; and the more in proportion as it falls the 

 more obliquely. Hence, sheltered places that have 

 a clear lake or tranquil river on the south, or inclining 

 to the south-west, are very warm, especially upon 

 sunny days in winter ; but places which are situated 

 the reverse way to the river or the lake are propor- 

 tionally cold. The contrast of such places, even 

 when at no great distance from each other, is often 

 very striking. Strand-on-the-green and Mortlake, 

 both on the Thames, and not above a mile apart 



afford an instance. On a clear winter day, with the 

 wind a little sharp at north, the contrast between the 

 river side at Strand and that at Mortlake is almost as 

 great as that between Dorset and Caithness. These 

 are local actions, however, and have little or no effect 

 beyond their limited locality. 



When the surface of water is in a state of agitation, 

 it retains much less of the heat which falls on it than 

 when it is tranquil ; and water which is shallow, and 

 partially covered with reeds or other tall aquatic 

 plants, returns less heat to the atmosphere than per- 

 haps any other description of surface. All surfaces 

 indeed which are covered with luxuriant vegetation, 

 in a state of vigorous growth, consume much of the 

 heat that falls on them, though there is no doubt that 

 some portion of heat is set free, or become sensible 

 in the process of growth, because in that process 

 there is an expansion of the substance of the plant 

 acting against the resistance of the air, and there is 

 also the change of substances from a more fluid to a 

 more solid state. 



The general cause of the absorption of heat,'or the 

 apparent cold of humid surfaces, whether of the water, 

 the moist earth, or of growing vegetables, is under- 

 stood to be evaporation, or the conversion of water 

 into invisible vapour, which rises and spreads in the 

 atmosphere. This vapour may be considered as con- 

 sisting of pure water, whether it is raised from pure 

 water, water impregnated with other substances, the 

 humid surfaces of growing vegetables, or any other 

 source. At all events, if there are other substances 

 accompanying it at the first, they are not raised to 

 any great height, or carried to any great distance. 

 Fogs and drizzling rains from the sea, especially if 

 they come from low sandy beaches where the water 

 is apt to get heated, sometimes tinge the hedges and 

 trees with salt to the distance of a mile or two inland, 

 but they do not reach far ; and though the miasmata 

 of marshes are said to produce ague, fever, and other 

 diseases, in those who dwell in their immediate vici- 

 nity, the rains which result from vapour raised out of 

 them are perfectly pure and limpid. It seems, there- 

 fore, that water is purified as well as raised and dis- 

 tributed invisibly through the air, in the process of 

 evaporation. 



This process of evaporation is perhaps the most 

 important, in a natural history point of view, of all 

 those in which the atmosphere is an agent, because 

 it is the one in which the grand circulation of water 

 originates, and without water there could be no life 

 or growth upon land. 



The process of evaporation is as general and com- 

 mon as it is important, and yet it belongs to that 

 class of natural actions of which no very satisfactory 

 account can be given. Its importance in a natural 

 history point of view is so great that we shall hazard 

 a few remarks expressly upon it in an article EVAPO- 

 RATION ; but we may remark in the meantime that 

 there are two theories, each of which has some plau- 

 sible arguments in its favour, anJ both of which are 

 liable to objections of so much cogency as to render 

 the truth of both very questionable. One of these 

 theories is that evaporation is produced solely by the 

 action of heat upon the water, and that the air has 

 no concern in it, the vapour being buoyed up by its 

 own elasticity. In proof of the theory, there is ad- 

 duced the well-known fact that evaporation takes 

 place very copiously in a receiver exhausted of air, 

 in which case the air can have nothing to do in its 



