Page images
PDF
EPUB

FORMATION OF AERIAL CURRENTS.

51

and thus a constant circulation of currents is established until the whole of the water attains the boiling point. But, when heat is applied at the surface, no such currents are established, the upper layer becoming heated without communicating much, if any, heat downwards.

31. It may be supposed that such an experiment as this does not apply to the atmosphere heated by the warm rays of the sun. It does, however, apply with the greatest strictness. The rays of the sun are not warm in passing through transparent media such as air; they give out no appreciable heat until they are arrested by some body capable of receiving them, and then, and not until then, is any warmth experienced. Even at the equator the air receives comparatively no heat from the powerful vertical rays of the sun above, but is heated almost entirely from below by the surfaces on which it rests, which are made hot by the rays which have passed through the clear air without any heating effect. This beautiful provision is necessary to give motion to those horizontal currents which produce wind, and those ascending and descending currents which mingle all the particles of air, and tend to preserve its purity, and to equalize the distribution of heat throughout the atmosphere. If the atmosphere were heated from above instead of from below, it would arrange itself in layers as fixed and determinate with respect to each other as those of a sedimentary rock, only increasing in density by night and expanding by day; all then would be a universal calm; there would be no cool gales from the temperate zones to mitigate the heat of the torrid or the cold of the frigid regions; the one would be desolated by heat and the other by cold, and both alike uninhabitable.

32. But as the solar rays are vertical at only one spot at a time, and become more and more oblique

as we recede from that spot, every gradation of temperature is experienced from 120° above to 50° below zero. If we suppose two spots on the earth's surface, one situated vertically under the sun, and the other receiving his rays obliquely, and consequently with less heating effect, the greater expansion of the air over the former spot will cause it to extend altogether to a greater height; but this will not influence the barometer, because, although there is a greater height of air on the warmer spot, there will be the same absolute quantity on both. But, as it is impossible for a fluid to remain thus heaped up in one spot without tending to assume a level surface, the heap of air above the warmer spot will overflow and spread over the colder, thus causing an accumulation of more air on the colder place than on the warmer, and the barometer will rise at the one place and fall at the other. But, as the lower stratum at the cold spot will thus be more elastic than that at the warm spot, the former will immediately rush into the latter until their elasticities are equalized. This motion constitutes wind. The equatorial regions being constantly more heated than the polar, the atmosphere above the former (independently of centrifugal force) must always extend higher than over the latter, and its upper portions must always be overflowing and tending to produce a level surface which is never attained. But this upper current from the equator is supplied by another current at the earth's surface, flowing with equal constancy towards the equator. Indeed, these contrary currents exist on a small scale in every room in which a fire is burning; but they may be best illustrated in two adjoining rooms, in one of which is a good fire, while in the other there is none. If the door between the two rooms be thrown open, the cold air will enter the heated room in a strong current; at the same

ACTION OF HEAT UPON AIR.

53

time the heated air of the warm room ascends and passes the contrary way into the cold room at the upper part of the same doorway, while in the middle of this opening, exactly between the two currents, the air appears to have little or no motion. On holding a lighted candle near the bottom of the doorway where the air is most dense, the flame will be strongly drawn towards the heated room; and, if held near the top of the door, it will be drawn towards the cold room with somewhat less force, while midway, between the top and bottom, the flame will scarcely be disturbed. Cases of this kind are illustrations of the convection of heat, similar to that which takes place in a boiler; and a similar process is carried on in the great aerial ocean, the whole mass of which is kept in perpetual circulation by the partial application of the sun's heat. An upper current of warm air is constantly flowing from the equator towards the poles to supply the place of the lower current, which flows in the contrary direction near the earth's surface, and forms, in certain latitudes, what are called the Trade-winds.

33. The tendency to the equalization of elasticity and of heat in the atmosphere, or, in other words, to equality of pressure in all parts of the same horizontal stratum, and to equality of heat in all parts of the same vertical column, is the cause of all winds, however they may be modified by local circumstances. Equalization of heat, however, does not imply an equal temperature. The quantity of heat required to effect a given change of temperature in a given portion of air depends on its bulk rather than on its weight, because the specific heat or capacity of air becomes greater the more it is rarefied; so that by suddenly rarefying a portion of air its temperature is instantly lowered, and when suddenly condensed its temperature rises; the new temperature

thus acquired being, however, in each case only momentary, because the equilibrium of temperature is immediately restored by the surrounding bodies. But this change of temperature is permanent when produced by the removal of a portion of air from a denser into a rarer stratum, or vice versa. Thus if a portion of the earth's surface could be transported to any height, so as to be relieved of a portion of the atmospheric pressure, its consequent diminution of elasticity would be more rapid than that of its density, because this diminished elasticity is due, not only to diminished density, but also to diminished temperature: the result is an increased capacity for heat, whereby it requires more heat than before to preserve its temperature unchanged; but it obtains no additional supply, because all the surrounding air is as cold as itself. The normal or equilibrial state of the atmosphere as regards heat is not, therefore, a state of equal temperature throughout its height, but a temperature gradually diminishing upwards, according to a simple mathematical law, the fall of temperature being equal for every equal height ascended, and on an average it is very nearly 3° of Fahrenheit for every 1000 feet.

This explains the changes of climate so beautifully exhibited in miniature on the slopes of mountains, where within a few miles are brought together those varieties of nature usually spread over many degrees of latitude. Thus on the sides of the Andes or other intertropical ridges the scenery of every zone is found, and the traveller in the course of a couple of days passes in review the whole scale of vegetation; from matted forests impervious to the sun, filled with the aroma of gums and balsams, and resounding with the din of animal life, he passes by imperceptible gradations into the peaceful woods of a temperate region,

LINE OF PERPETUAL SNOW.

55

where the trees, as he continues to ascend, become smaller and less crowded; these are gradually succeeded by shrubs, low herbage, mosses, and lichens, until at length he enters upon those awful solitudes of snow, where organic life appears extinct.

34. This striking succession of changes is explicable if we bear in mind that the mean temperature of the equatorial atmosphere (which at the sea level is 82°) is diminished 1° for every 333 feet of ascent; so that at the height of 50 times 333, or 16,667 feet, the mean temperature is reduced to 32°, that is to say, it is as often below as above the freezing point; and, as there is no variety of seasons near the equator, it of course freezes every night at this elevation; so that, while there is an eternal summer in the plains, there is an eternal winter on the mountain tops. Indeed, it is found that no summit exceeding 15,700 feet is ever free from

snow.

The distinct line formed by the lower boundary of the snow on mountains furnishes us with a natural register thermometer on a stupendous scale, as Professor Daniell appropriately names it, a scale of which each degree occupies more than 300 feet measured vertically. Except at the equator, this line of course rises in summer and falls in winter, the difference produced by seasons being increased the farther we advance towards the poles. The lowest position of the snow line depends, of course, on the winter temperature, which diminishes immediately from the equator, very slowly at first; so that this limit, which at no place within the tropics descends much below 15,000 feet, afterwards declines so much as to meet the sea-level at about the 40th degree of latitude, while, in all latitudes above this, frost and snow occur in winter, even at the sea-level. But the highest position of the snow-line, or that limit

« PreviousContinue »