SOLAR INFLUENCE.

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proportion in a rude way. The earth can at no time prevent the sun's light from reaching any portion of the atmosphere, except a circular portion, directly opposite to the sun, and of the same breadth as the section of the earth. We do not know the height of the atmosphere, and therefore cannot state the quantity; but from the properties of a sphere, the portion of the atmosphere always acted on by the sun is very considerably more than the half of it.

When the sunbeams act as heat upon the atmosphere, they always expand it, or swell it into greater bulk, and make it lighter. When made lighter, it ascends, for the same reason that warm air escapes with a simmering noise from hot water. Being made lighter, it ascends higher above the earth's surface; but as it ascends, it gets colder. The atmosphere is all over a spring, and therefore, when any cause disturbs its balance, it instantly reacts, and endeavours to return to its former state.

This great susceptibility of change in the atmosphere, by any disturbing cause, however slight, and its equally great tendency to revert back again, in proportion as the disturbing cause slackens, and to assume the same condition as at first, is the first and greatest means of the revival of nature in the Spring. It should therefore be thoroughly understood by every one who endeavours to obtain a rational knowledge of the beautiful operations of nature. If the air of the atmosphere has perfect freedom to obey the action of heat, it exhausts that action in the expansion of its substance, and never becomes sensibly warmer. On the other

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ATMOSPHERIC ACTION.

hand, if the expansion is by any means resisted, the air is sensibly heated, because its own condition does not occupy the whole action of the heat which is applied to it.

Atmospheric air is a very light substance as com→ pared with most others with which we are conversant; still its weight is considerable, amounting to about fifteen pounds on every square inch of the earth's surface at the mean level, or supporting on the average a column of about twenty-nine inches of mercury in the common barometer. This weight diminishes, at such a rate, that, if heights are taken in the proportion of 1, 2, 3, 4, and so on, the diminution of weight is in the proportion of 1, 4, 9, 16, and so on.

same as the

It is the resistance offered by this weight of the atmosphere which is the valuable part of it, and enables it to apply to the earth generally, and to animals and vegetables particularly, that warmth which is the most essential element of their Spring revival. At the mean level of the earth, which is about the surface of the wide and free oceans, this resistance is greatest; and it diminishes at that rapid rate which we have mentioned, as greater elevations are arrived at. In consequence of this, the places which are lowest are the first to obey the reviving influence of the Spring. As we ascend, the obedience to this influence becomes less and less; and at every point of the earth's surface there is a certain height at which Spring, Summer, and Autumn are never known, but where Winter holds the surface mantled up under a covering of perennial snow.

DIFFERENT ELEVATIONS.

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There is no portion of the British islands where the snow can be said to be perennial, that is, where the average temperature of the hottest day is as low as that at which water freezes. But on the top of Ben Nevis, and that of several other mountains in Scotland, there are very few nights entirely without frost; and in the horseshoe-formed precipices which are very generally found on the north-east or shaded sides of such mountains, snow, and also ice upon the small lakes at the bottom of the snow, are found throughout the year. These are small miniatures of the snow-clad summits, and the glaciers which are found at the foot of them, on a far more splendid scale on the Alps.

This property of the atmosphere, by means of which its obedience to the action of heat decreases so rapidly, as the elevation above the mean level of the earth increases, is a very important point in the seasonal economy of the earth, and in all those appearances and results to which the seasons give rise. For, in consequence of it, a country under the equator, whose plains or vallies, at the mean level of the sea, are constantly exposed to a high summer temperature, enjoys, if its mountains are high enough, every degree of climate, from the equator to the pole. Equatorial countries, possessing lofty mountain ridges, are thus a sort of miniatures of the whole globe; and valley and moun tain work to one another in the Spring and other seasons, in the same manner, though not of course to the same extent, as the equatorial and polar regions of the whole globe.

What have been now stated are the leading pro

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ACTION AND RESISTANCE.

perties of the atmosphere, which are so beautifully the ministers of Almighty goodness in bringing about the Spring, and carrying the action of the Spring forward to the full maturity of the year, so that all the children of nature may be fully supplied with the bounty of their beneficent Creator. There are many minor circumstances, essential to a complete understanding of this most instructive and most extensive subject. They would, however, require more space than we can afford for the whole subject.

The susceptibility of the air to solar action, as we have endeavoured to explain it, consists of two distinct kinds: the one, motion in the air, or a transfer of it from place to place, both upwards and downwards and laterally; the other, a heating of its substance when the solar action is increased, and a cooling of the same when that action is diminished. The first of these is the modification of the influence which tends to produce the Spring, considered in itself as a season; and the second of them tends to produce that revival of plants and excitement of animals to which their Spring actions are owing. We have seen, that the first of these results from the extreme susceptibility of the air to alter its volume with every variation of temperature; and that the other results from the resistance which the weight, or gravitation of the air offers to the former property.

The gravitation, though it is the measure of the abstract quantity of matter in the air, cannot be said

to be exerted by the air itself.

It results from the

mass of the earth; and in this way those solid parts of

ACTION AND RESISTANCE.

57

the earth, not only deeper than mines ever dug, but really all the way to the centre, perform a portion and by no means an unimportant part in the economy of the earth's surface.

In order to show the value of the great mass of matter in the earth as affecting its surface economy, it may be proper to state the relative gravitation, or absolute quantity of matter in a section of the earth, having a base of a square inch at the surface, terminating in a point at the centre, and being in fact a square pyramid, having an inch for its base, and four thousand miles for its length or height. This four thousand is about the mean distance from the surface to the centre. The average weight of the matter of the earth is four and a half times that of water; and a cubic foot of water weighs one thousand ounces. We have thus the elements of a very easy calculation; and the result is, that making allowance for the fraction which the radius, or distance from the surface to the centre of the earth, is less than four thousand miles, the weight of this section of an inch from the surface to the centre, is thirteen millions of pounds.

We stated already that the pressure of the atmosphere upon an inch of the earth is fifteen pounds, which is only a nine-hundred thousandth part of the weight of that portion of the earth on which it rests. Nine hundred thousand to one is an immense proportion; and yet this is the power by which the attraction of gravitation in the earth restrains the atmospheric air, by controlling its elasticity. But the air remains elastic, and therefore its elasticity must be more powerful