ATMOSPHERE. 



ont moisture. Whether they give out moisture by 

 the same pores, or other parts of their surface, by 

 means of which they at other times absorb it, is, 

 though probable, not proved, and perhaps not capable 

 of proof ; but it is certain, that when they have an 

 abundant supply, they absorb more than is necessary 

 for the ultimate purposes of vegetation. This, in 

 particular situations, and at particular seasons, they 

 give out in greater quantity than the evaporative 

 power of the atmosphere can receive ; and thus they 

 are found humid and dripping, in cases where there 

 can be little or no condensation of atmospheric 

 moisture upon them. Besides giving out to the 

 atmosphere a portion of that moisture which they 



receive from the soil by their roots, they, under other 

 circumstances, appear to give out to the soil by their 

 roots a portion of that which they receive from the 

 atmosphere by means of their other surfaces; and 

 when the roots have penetrated into strata not capa- 

 ble of retaining any moisture, the plant may be 

 parched while there is humidity in the air, just as by 

 hot and dry air it may be parched while there is 

 humidity in the soil. All these circumstances tend 

 to show that plants excite a more powerful action in 

 the atmosphere, or engage it in more powerful action 

 than bare surfaces of any description, and that, there- 

 fore, surfaces thickly covered with them preserve a 

 more uniform temperature amid atmospheric changes, 

 whether diurnal or seasonal, than any other surfaces 

 of the land. 



These are but a very few out of a vast multitude of 

 circumstances, but they may afford some assistance 

 in forming a notion of the general principle ; so that, 

 bearing them in mind, and bearing in mind also that 

 the atmosphere is always taking up moisture by 

 evaporation, and that heating expands it and gives 

 it a tendency to ascend, and cooling produces the 

 opposite effects, we are in a condition for forming 

 some notion of its general action in the economy of 

 nature. 



The sun is the grand agent which puts the whole 

 in motion ; and the peculiar motions of the earth so 

 vary the sun's action with days and seasons, that the 

 details of this part of the subject, even without 

 reference to the variety of surfaces upon which the 

 sun acts, render this single part of the subject exten- 

 sive as well as complicated. 



It may hardly be necessary to state, that the rota- 

 tion of the earth from west to east causes the appa- 

 rent revolution of the sun from east to west in twenty- 

 four hours ; and the path of the point where the sun'is 

 vertical shifts northward from our midwinter to our 

 mid-summer, and southward the other half of the 



243 



north and soufti, because the heating cause has passed 

 over the parts to the east, but is approaching those 

 to the west, and the westward motion causes that 

 trom the north to come as from the north-east, and 

 that from the south as from the south-east These do 

 not meet by a considerable space, because, as thev 

 approach the centre of heat they begin to feel its 

 influence and ascend. But if the globe had a sur- 

 face uniformly susceptible to heat, they would play 

 round it constantly, shifting northward and south- 

 ward with the apparent place of the sun. The surfaces 

 of the great oceans clear of the land are nearly 

 uniform ; and these currents play over them, and are 

 well known as the trade-winds. Of course these 



, , . . *-' vumse muse 



winds, which are nowhere very violent, diminish as 

 stance of that which is their cause increases 

 so that they are not felt to any very great distance 

 either northward or southward. They however pro- 

 duce, m the upper strata of the air, winds in the 

 opposite directions, because the air cools as it ascends 

 and it is urged onward by the ascent of a fresh supply 

 from the surface. As that returning portion recedes 

 trom the central cause of heat, it descends and gives 

 put heat ; and thus the atmosphere distributes the 

 heat of the sun northward and southward. 



The land interferes with the regular motion of 

 these winds, by intercepting them by its elevation 

 and also by affecting the heat of the air by its surface 

 Ihus the wind which is a surface-wind over the 

 desert in northern Africa, is too rare for bein- a 

 surface-wind over the Atlantic. Therefore it ascends 

 instead of moving westwards; and a considerable 

 portion of the sea is calm or without wind. But the 

 air which was so much heated carries with it much 

 more moisture than it can keep suspended in the 

 more elevated part of the atmosphere to which it 

 ascends ; and the result is, that those tranquil por- 

 tions of the sea are visited by frequent and heavy 

 falls of rain so much so, that they are popularly 

 called " the rains." These calms and torrents of rain 

 do not take place immediately upon the African 

 shore, but so far out at sea as to allow the stagnant 

 portion of the atmosphere to be compressed laterally 

 by the currents from the north and south east. 



On the western coast of America, at some distance 

 from land, there are similar calms and rains ; but 

 as the portion of the American continent off which 

 they are found is higher and not so dry as the 

 northern part of Africa, they are neither so marked 

 nor so constant in their appearance. In both oceans 

 these calms and rains are to the north of the 

 equator. There are two reasons for this first, the 

 parallel of maximum heat is a little north of the 



TT . j ., , . ,. ----- . |wirc ui lucuuuiuui ueai is a nttie nortn ot tne 



His daily change is smaller at the beginning equator, as our summer half-year, being the one 

 and end ot each halt-year, and greater at the middle which ommioe the, ./,<./; -~- ..e ri, ._u,_ 



and end of each half-year, and greater at the middle 

 of each, which are the times of the equinoxes. 



point in the apparent path of the vertical sun where 

 the heat is greatest, is the centre of the solar action ; 

 and in the course of each year it travels in the manner 

 which has been stated. At that point the atmosphere 

 has a tendency to expand and ascend, greatest at the 

 surface, and greater in proportion as that surface 

 is susceptible of being heated. The air from the 

 adjacent places presses in to supply the room of that 

 which is heated and ascends ; and as the centre of 

 heat is apparently in rapid motion westward, the 

 motion of the air towards it is turned in that direction. 

 The chief motion of the air is of course from the 



which occupies the aphelion portion of the earth's 

 orbit, is longer in absolute time than the other ; and 



rp, . i .1 i * lungei in ausoiutc time tnan tne other; ana 



The point where the sun is vertical, or at least that secondly, the land immediately under the equator 

 unt in the aimarent oath ot tha vertical sun whoro I onH o*r f/%- rt *:^ ui_ j- ,_._ ,1 



J ' inav.vAic4.Hji y UltUCI tllC t>vJ LIclLUl 



and even for a considerable distance to the northward 

 of it, is not favourable for their production. Guinea 

 is a fertile country, with luxuriant vegetation near the 

 shores, and rising into mountains in the interior ; and 

 Peru, the greater part of the narrow isthmus in 

 America, and also Mexico till considerably northward, 

 are mountainous. 



The places which meet the current of the trade- 

 wind, or receive it from the oceans, are, as may be 

 supposed, the very reverse of those at which it com- 

 mences. They are the oriental seas and the Malay 

 peninsula on the west of the Pacific, and Brazil, the 

 Z2 



