242 



ATMOSPHERE. 



view of the atmosphere opens a very wide field, and 

 one in which the judicious inquirer has still much 

 wisdom to win ; but it is one upon which we cannot 

 in the meantime enter. Our more immediate business 

 is with the atmosphere as the recipient of the vapour 

 of water, and the distributer of that water to the 

 general surface of the earth in rain, or to particular 

 substances for silent absorption. 



This is the simple and general view of the matter 

 which is most essential to the proper understanding of 

 natural history ; and it is of the utmost consequence 

 that our views of it should be clear and accurate, 

 because it is a subject which, if we study nature at all 

 in its connexion, we are constantly meeting. We are, 

 then, to understand that the air takes up, from all 

 humid surfaces, moisture in the state of invisible 

 vapour ; that this is taken up by the corpuscular attrac- 

 tion of air and water, and the action of heat, jointly, 

 though the heat may act sometimes as electricity as 

 well as generally as ordinary heat ; that the moisture 

 so taken up is distributed through the air, and sus- 

 tained there by the same agencies which raise it in 

 vapour ; but that, though dispersed and invisible, it is 

 not combined, but always ready to be given out with- 

 out chemical effort. 



The heat which the atmosphere receives, whether 

 from the direct rays of the sun, from reflection, or 

 radiation from the earth, or any other cause, in like 

 manner enters into no chemical combination with that 

 fluid, so as to produce v> hat may be its final action there, 

 If the surface of the earth is not of a nature to receive 

 it, and profit by it, then it acts in expanding the at- 

 mosphere ; and the part so expanded ascends, carry- 

 ing with it the water which it may hold in a state of 

 vapour. The action of heat, therefore, puts the at- 

 mosphere in motion, and enables it to waft from place 

 to place the water which it holds as vapour. 



The capacity of retaining water in the state of va- 

 pour increases with the increase of heat, and dimi- 

 nishes with its decrease, but the two do not vary at 

 the same rate. In theory, the power of retaining 

 moisture should increase and decrease faster than the 

 heat, because it is promoted by two causes, the action 

 of the heat, and the diminished density or pressure of 

 the expanded air. In practice, the facts agree with 

 this theory ; and if they did not, it is not easy to see 

 how the atmosphere could part with the moisture 

 which it has once held as vapour. Those east winds 

 which frequently injure vegetation on the low-lying 

 grounds upon the east side of England, often arrive 

 on the coast in a state of complete saturation, and 

 ready to deposit their humidity in rain ; and, where 

 bleak and elevated districts abut upon the shore, they 

 produce rain. But the low and warm grounds, in Essex, 

 Suffolk, and Norfolk, for instance, radiate, or other- 

 wise give out as much heat as converts what upon the 

 bleak and elevated land would be a moist and raining 

 wind into a drying one ; and this will go on for weeks, 

 till the fields are. as dry as a summer path, and the 

 vegetation is perished and shrivelled ; nor is there in 

 the places which those east winds scourge any 

 means by which to stop their progress, or even miti- 

 gate their action. But when these winds reach the 

 high grounds in the midland counties, where they 

 have a colder surface, and also a greater elevation, 

 they are no longer able to retain the whole of the 

 humidity with which they are charged, but deposit 

 part of it in rain. The evaporation of that rain cools 

 the surface and the air over it ; the east wind is, in 



consequence, checked in an earlier part of its course ; 

 the rain falls more easterly, and may conduct the east 

 wind to the coast, while the south-west wind follows in 

 the rear ; or the opposition of the westerly wind mav 

 produce a similar result, in a manner to be afterwards 

 explained. 



The kinds of surface which the globe, at different 

 places, presents to the sun, are various beyond any 

 enumeration shorter than separate descriptions of the 

 whole, and the action of the sun upon them is just as 

 varied. This holds not only with regard to the land, 

 but in some instances also with regard to the sea, 

 the cold produced by evaporation from which, and 

 the action of its currents, are not jointly able to 

 maintain a uniformity of temperature, either in the 

 water itself or in the atmosphere over it. Unlike 

 the atmosphere, indeed, the sea has very little ten- 

 dency to be thrown into currents by the different 

 action of heat upon it several parts. The cause is 

 evident : the expansion of water by heat and its con- 

 traction by cold are very limited ; and the evapora- 

 tion where the action of the sun is greatest, is not 

 sufficient to produce a sensible motion even of the 

 mere surface. Where the water is comparatively 

 shallow and clear, and the bottom of a light colour, 

 the heat of the perpendicular sun returned from the 

 sea, is indeed not much less than that returned from 

 the surface of dry land. This is severely felt by 

 navigators in Torres' Strait, in a considerable portion 

 of the sea on the north coast of New Holland, and 

 in other places, where one can hardly bear to look 

 down on the water, and the air over it has a tem- 

 perature of from 1 10 to 120 of the common ther- 

 mometer. There, from the heat of the air and that of 

 the water, the quantity of evaporation must be sup- 

 posed to- be very great; but the cold which it 

 ? reduces is not increased in the same proportion, 

 n the polar seas, on the other hand, the effect of 

 evaporation adds so much to the natural cold of the 

 air, that while by the action of the direct and reflected 

 rays of the sun, the pitch on the sunny side of a ship 

 melts and runs down like water, the frost under the 

 shade is more intense than when the sun is obscured. 

 We have approximations to that in some of the 

 colder parts of Britain, where, though the direct sun 

 feels warm, the cold in the shade is more intense 

 than on a cloudy day. 



The heat from reflected light over the sea, if none 

 is returned from the bottom, is least where the sun is 

 perpendicular, and increases as the direction becomes 

 more oblique. It is also greatest when the surface 

 is smooth and glassy, and the cold produced by eva- 

 poration is then also the least, so that a curl upon 

 the water is always a means of cold. When the 

 water is deep, the heat never has so much influence 

 upon it, so that, even under the vertical sun, there is 

 a cooling effect in the sea. 



The surfaces of the land are much more varied, 

 and all that can be said of them is, that with an equal 

 action of the sun, or of any other heating cause, the 

 surface which is the most humid, and parts with its 

 humidity with the least action, is always the coolest. 

 The leaves of plants which are growing vigorously 

 and cover the ground with their shade, and which 

 have at the same time an abundant supply of water 

 at the roots, present, perhaps, the most cooling of all 

 surfaces. They not only multiply the actual surface 

 upon which they grow many times over, but they 

 aid the evaporation by the tendency they have to give 



