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Show 170 SENSIBILITY OF TH~ AlR. smallest cause that can act upon it; and it is just as capable of obeying the most powerful. It surrounds all earthly things, and it regulates them all. If the 1 distance is not the thousandth part of a hair's breadth, or if it is "round about the pendent world," the " viewless wind'' is perfectly true to it.. . The principle upon which all that ts done ts an exceedingly simple one. The temperature of the air, its pressure when in its natural and. u.ncon~ned state, and its density, or the qum;tit¥ of 1t ~n a gtven space, are all, by the 'rery consbtutwn of tts nature, so nicely balanced and adapted to ~a~h other that the least chancre in any one of them IS mstantly followed by the ~orrespon~ing change .in the others ; and its freedom of motiOn enables It to make an instant adjustment by motion. · There are few or no causes of disturbance arising from pressure in the air itself; because the only pressure which it has in itself is its own weight, or pressure downwards towards t~e ear.th i. and the pressures of foreign substances mixed with It are considered as mere pressures, not of very much consequence. Thus the principal causes of disturbance or motion in the air are differences of heat : and, from general or local causes of heat, these are almost incessant. When it is said that, of all substances in nature, the air is the most sensible to heat, the meaning must not be misunderstood. Sensibility to heat does not mean being actually heated, but only being put into a state of action by ~eat; and from what has been said about the connexwn between apparent heat and resistance to motion, it will readily be understood that if the air were absolutely free to move, it would never show any increase of heat at all; but would expand and yi~ld. to the full extent of the heating cause. Nay, If It were allowed to expand faster than that cause operated, it would thereby be cooled. But light as the air is, even the smallest THE WEATHER. 171 portion o~ it. has some weight ; and softly though it moves, s~tlltt has some friction. Both of these offer S?m.e resistance t? the cause of heat, and thus the atr IS .always. a little warmed before it begins to move m obedience to the heat. The resistance is t~e .greatest where the pressure is greatest, and the air m consequence the most dense ; and that is one of the. reasons why the air is hotter in hollows than on hmghts. If one were to ascend till the air had only hal~ the pressure,. and consequently only half the density and the resistance to friction that it has at the ~eanlevel of the sea, then it would yield twice as easily to the heating cause ; and the same cause that would render it not only warm, but disagreeably hot at the level of the sea, would not bring it perhaps e.ven to the heat ~t which ice melts at the great elevatwns.. At less difference of height, the differ~ nce o[ resistance would be less than that ; but there IS .a dtfference even for the smallest difference of hmght, . although our observation will not reach the , very m~nute cases, any more than it will reach the v:ery mmutest ends in any department of observa- 1 bon .. We see as much, however, as may suffice to co~vm?e us t~at the law is general; and that is all whiCh IS ~eqmred for the purposes of knowledge. The daily and annual motions of the earth (and the atmosphere m~)Ves along with it) cause the heat ~f tJ:e sun to fall differently on every place, 011 every ay I? the year, and at every hour of the day; and t.he d~fferent effects of the heat of the sun on different. kmds. and forms of surface further increase the v~nety, tJ11 the relative portions of heat, at any considerable numb~r of places, for any one time cannot be calculated With any thing like certainty o; indeed at all. ' '' If that co~ld be done, we should all be perfect! weather-wise," and should be able to tell how th~ sky tppeared and felt in distant places, and how it wou d appear and feel at future times, with just as |