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above which it never thaws, must of course depend on the summer temperature. This line coincides with the former only at the equator, and, instead of declining immediately therefrom, it even rises a little towards the tropics, because their maximum or summer heat exceeds the constant heat of the equator. Beyond these limits, however, the line of perpetual snow regularly descends, until in the central latitudes of Europe (on the Alps for example) it is reduced to 8000 feet, on the Norwegian mountains to 5000, and in the Arctic Regions it descends to the sea-level.

35. Thus we see that a certain rate of upward diminution of temperature is necessary to the atmospheric equilibrium; but this diminution of temperature produces no currents, because the upper strata, notwithstanding their coldness, have no tendency to descend, nor the lower and warm strata to ascend, because in changing their position they would also change their temperature. Currents can only be set in motion by a still greater inequality of temperature, by the lower strata becoming still warmer, and the upper still colder than the heat equilibrium requires. Now, there is a constant tendency towards such an increase of inequality, because the lower strata are constantly in the day time receiving heat from the earth or sea, made hot by the solar rays, while the upper strata are losing heat by radiation into space, the constant temperature of which is supposed to be 58° below Fahrenheit's zero.

36. From this view of the atmosphere we are able to understand the wonderful and beautiful contrivance by which the winds of the temperate zones are made to mitigate the extremes of temperature, both in the tropical and the polar regions. It might be thought that the prevailing winds next the earth's surface could

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not have this doubly beneficial tendency, but that they must always blow from the colder of two parallels into the warmer, as is the case with those currents which produce the trade winds. We must now inquire why this law is reversed in nearly all places between the latitudes of 30° and 60°, as in this country and all over Europe, for example, where the prevailing winds blow from warmer into colder latitudes. It has been already stated (32) that the immediate effect of a difference of temperature between two adjoining columns of air is to make the warmer column overtop and overflow the colder. In fact, all the upper half of the warmer column (or the half in respect of quantity, not height) is rendered denser and more elastic than the corresponding parts of the cold column; but this difference of elasticity, which is greatest at the top, diminishes downwards to a certain point where the pressures of the two columns are equal, and consequently there is no wind either way. Below this the difference is reversed, the colder column being the more elastic, and therefore flowing into the warmer more and more strongly as we descend. The trade-winds are thus accompanied by an exactly equal counter-current from the equator in the upper half of the atmosphere; the existence of such a current, exactly opposite in direction to that below, being abundantly confirmed by travellers who have ascended the peak of Teneriffe, or that of Owhyhee, the only insular mountains sufficiently high to reach the upper current. The summits of most of the West India islands, however, approach the neutral line of separation between the two currents. Indeed, the trade-winds appear to be so much diminished there as to be inappreciable above the level of 3000 feet, although the other current cannot be felt below 12,000 feet. The effects of the

upper current were strikingly displayed in the volcanic eruption at St. Vincent, in 1812, the ashes of which were carried to Barbadoes; and those of Cosiguina, a volcano in central America, have been conveyed to Jamaica in direct opposition to the course of the constant trade wind. Light clouds also, which float at a greater height within the tropics than elsewhere, frequently give evidence of the same fact. Now, it is obvious that this upper current, constantly losing heat by its exposure to open space, must, after travelling a certain distance, become cold enough to descend, and change places with the lower current. This change appears to take place usually about the 30th parallel of latitude in each hemisphere, which is accordingly the outer limit of the trade-winds. Beyond this the equatorial current is undermost, and continues to be so until it again becomes by contact with the earth sufficiently warmed to resume its original position above the colder current that proceeds from the polar regions. It will of course be understood that the number of these changes that may occur between the 30th parallel and the pole will depend on difference of season, the amount of radiation from the sun and from the earth, the screening effect of clouds, and many other local circumstances, and hence the proverbial inconstancy of the winds over the cooler half of each hemisphere; an inconstancy arising, not from the absence of exact laws (for these winds are regulated by laws as fixed as those which apply to any other terrestrial phenomena), but from the presence of so many disturbing causes, which, as they cannot be anticipated, so they cannot be taken into account in attempting to generalize the phenomena of these winds,

-a circumstance which clearly exhibits the folly of those half-informed persons who construct weather-almanacs for predicting atmospheric changes a year in advance.

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The superior return current from the equator having descended at about the 30th parallel commonly continues to be the lower current or prevailing wind until it approaches the polar circle, where it again rises and is replaced by the polar gales, which prevail in high latitudes. These are caused by the cold and dense air of the polar regions sinking and spreading in every direction, being overflowed by an ingress of warmer air from every side, which supplies a constantly descending cataract of air upon the pole the exact converse of the effect at the equator.

These effects are embodied in one view in Fig. 16,

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which represents a quadrant of the atmosphere between the pole P and the equator at E. At E the heated air ascends and forms the upper current until it reaches about the 30th parallel, where it descends and forms the lower current, which it continues to do until it approaches the polar circle, where it again rises. At the pole p the arrows are intended to represent the polar gales descending and forming the lower current. On leaving the polar circle they are displaced by the warmer return current from the equator, and, rising

above this, descend at about latitude 30° and proceed to the equator, forming the constant north-east tradewind in the northern hemisphere, and an equally constant south-east trade-wind in south latitudes.

37. But it may be said that this flow of air from the poles to the equator ought to produce a constant north wind in the northern hemisphere, and a constant south wind in the southern hemisphere, at least within a certain limited distance of the equator, probably below the latitude of 30°, or over the warmer half of the earth's surface. The reason why we do not find this to be the case is on account of the motion of the earth on its own axis from west to east. The earth being a sphere, the different parts of its surface must, of course, move with very different velocities. At the poles the motion is nothing, but at the equator it is 1042 miles an hour. In the latitude of 30° it is about 900 miles an hour, so that in the belt between the equator and latitude 30°, the average velocity may stated at 980 miles an hour, while the space lying between 30° and 40° does not move at a greater rate than about 850 miles an hour.

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Now, as the atmosphere may be regarded as an integral portion of the earth's surface moving round with it with the same velocity, it follows that the cold current of air which sets in from the temperate zones towards the equator not only has a motion north and south in that direction, but also a velocity of about 850 miles an hour due to that parallel of latitude from which it is withdrawn; the equatorial regions, however, are moving to the eastward at the average rate of 980 miles an hour; the cold air arriving into these regions at the slower rate would, on its first arrival there, be left behind; or, in other words, the surface of the earth would travel faster to the eastward than the air

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