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Whatever particles of other matter (not endued with that repellancy) are supported in air, must adbere to the particles of air, and be supported by them; for in the vacancies there is nothing they can rest on.
Air and water mutually attract each other. Hence water will dissolve in air, as salt in water.
The specific gravity of matter is not altered by dividing the matter, though the superfices be increased. Sixteen leaden bullets, of an ounce each, weigh as much in' water as one of a pound, whose superfices is less.
Therefore the supporting of salt in water is not owing to its superfices being increased.
A lump of salt, though laid at rest at the bottom of a vessel of water, will dissolve therein, and its parts move every way, till equally diffused in the water; therefore there is a mutual attraction between water and salt. Every particle of water assumes as many of salt as can adhere to it; when more is added, it precipitates, and will not remain suspended.
Water, in the same manner, will dissolve in air, every particle of air assuming one or more particles of water. When too much is added, it precipitates in rain.
But there not being the same contiguity between the particles of air as of water, the solution of water in air is not carried on without a motion of the air so as to cause a fresh accession of dry particles.
Part of a fluid, having more of what it dissolves, will communicate to other parts that have less. Thus very salt water, coming in contact with fresh, communicates its saltness till all is equal, and the sooner if there is a little motion of the water. ***
Air, suffering continual changes in the degrees of its heat, from various causes and circumstances, and, consequently, changes in its specific gravity, must therefore be in continual motion.
A small quantity of fire mixed with water (or degree of heat therein) so weakens the cohesion of its particles, that those on the surface easily quit it and adhere to the particles of air.
Air moderately heated will support a greater quantity of water invisibly than cold air; for its particles being by heat repelled to a greater distance from each other, thereby more easily keep the particles of water that are annexed to them from running into cohesions that would obstruct, refract, or reflect the light.
Hence, when we breathe in warm air, though the same quantity of moisture may be taken up from the lungs as when we breathe in cold air, yet that moisture is not so visible.
Water being extremely heated, is e., to the degree of boiling, its particles, in quitting it, so repel each other as to take up vastly more space than before, and by that repellancy support themselves, expelling the air from the space they occupy. That degree of heat being lessened, they again mutually attract, and having no air particles mixed to adhere to, by which they might be supported and kept at a distance, they instantly fall, coalesce, and become water again.
The water commonly diffused in our atmosphere never receives such a degree of heat from the sun or other cause as water has when boiling; it is not, therefore, supported by such heat, but by adhering to air.
A particle of air loaded with adhering water or any other matter, is heavier than before, and would descend.
The atmosphere supposed at rest, a loaded descending particle must act with a force on the particles it passes between or meets with sufficient to overcome, in some degree, their mutual repellancy, and push them nearer to each other.
Every particle of air, therefore, will bear any load inferior to the force of these repulsions.
Hence the support of fogs, mists, clouds.
Very warm air, clear, though supporting a very great quantity of moisture, will grow turbid and cloudy
on the mixture of colder air, as foggy, turbid air will grow clear by warming.
Thus the sun, shining on a morning fog, dissipates it; clouds are seen to waste in a sunshiny day.
But cold condenses and renders visible the vapour: a tankard or decanter filled with cold water will .condense the moisture of warm, clear air on its outside, where it becomes visible as dew, coalesces into drops, descends in little streams.
The sun heats the air of our atmosphere most near the surface of the earth; for there, besides the direct rays, there are many reflections. Moreover, the earth itself, being heated, communicates of its heat to the neighbouring air.
The higher regions, having only the direct rays of the sun passing through them, are comparatively very cold. Hence the cold air on the tops of mountains, and snow on some of them all the year, even in the torrid zone. Hence hail in summer.
If the atmosphere were, all of it (both above and below), always of the same temper as to cold or heat, then the upper air would always be rarer than the lower, because the pressure on it is less; consequently nghter, and, therefore, would keep its place.
But the upper air may be more condensed by cold than the lower air by pressure; the lower more expanded by heat than the upper for want of pressure. In such case the upper air will become the heavier, the lower the lighter.
The lower region of air being heated and expand ed, heaves up and supports for some time the colder, heavier air above, and will continue to support it while the equilibrium is kept. Thus water is
supported in an inverted open glass, while the equi. librium is maintained by the equal pressure upward of the air below; but the equilibrium by any means breaking, the water descends on the heavier side, and the air rises into its place.
The lifted heavy cold air over a heated country, becoming by any means unequally supported or unequal in its weight, the heaviest part descends first, and the rest follows impetuously. Hence gusts after heats, and hurricanes in hot climates. Hence the air of gusts and hurricanes is cold, though in hot climates and seasons ; it coming from above.
The cold air descending from above, as it penétrates our warm region full of watery particles, condenses them, renders them visible, forms a cloud thick and dark, overcasting sometimes, at once, large and extensive; sometimes, when seen at a distance, small at first, gradually increasing; the cold edge or surface of the cloud condensing the vapours next it, which form smaller clouds that join it, increase its bulk, it descends with the wind and its acquired weight, draws nearer the earth, grows denser with continual additions of water, and discharges heavy showers.
Small black clouds thus appearing in a clear sky, in hot climates portend storms, and warn seamen to hand their sails.
The earth turning on its axis in about twentyfour hours, the equatorial parts must' move about fifteen miles in each minute; in northern and southern latitudes this motion is gradually less to the poles, and there nothing.
If there was a general calm over the face of the globe, it must be by the air's moving in every part as fast as the earth or sea it covers.
The air under the equator and between the tropics being constantly heated and rarefied by the sun, rises. Its place is supplied by air from northern and southern latitudes, which, coming from parts wherein the earth and air had less motion, and not suddenly acquiring the quicker motion of the equatorial earth, appears an east wind blowing westward; the earth moving from west to east, and slipping under the air.*
Thus, when we ride in a calm, it seems a wind against us: if we ride with the wind, and faster, even that will seem a small wind against us.
The air rarefied between the tropics, and rising, must flow in the higher region north and south. Before it rose it had acquired the greatest motion the earth's rotation could give it. It retains some degree of this motion, and descending in higher latitudes, where the earth's motion is less, will appear a westerly wind, yet tending towards the equatorial parts, to supply the vacancy occasioned by the air of the lower regions flowing thitherward.
Hence our general cold winds are about northwest, our summer cold gusts the same.
The air in sultry weather, though not cloudy, has a kind of haziness in it, which makes objects at a distance appear dull and indistinct. This haziness is occasioned by the great quantity of moisture equally diffused in that air. When, by the cold wind blowing down among it, it is condensed into clouds, and falls in rain, the air becomes purer and clearer. Hence, after gusts, distant objects appear distinct, their figures sharply terminated.
Extreme cold winds congeal the surface of the earth by carrying off its fire. Warm winds afterward blowing over that frozen surface will be chilled by it. Could that frozen surface be turned under, and warmer turned up from beneath it, those warm winds would not be chilled so much.
The surface of the earth is also sometimes much
* See a paper on this subject, by the late ingenious Mr. Hadley, in the Philadelphia Transactions, wherein this hypothesis of explaining the tradewinds first appeared.