found on the roofs and walls of mines and caves; and it is probable that in its first origin, the first spring of its being as it were, no plant and no animal can bear the light. Afterwards, however, by far the greater number of vegetables require light, otherwise they do not acquire the requisite strength of substance, or put on those colours which are characteristic of their healthy state. It is interesting to observe how some plants will struggle to reach the requisite enjoyment of this influence. A geranium or other plant trained fan-wise in the window of a room which is not very light, turns the whole of its shoots, and even its leaves, away from the room, and toward the window; and if it is turned round, after a time they will all bend over the other way. In the tower of the old cathedral of Dunkeld, in Scotland, now in great part a ruin, we once witnessed a remarkable effort after light in a common potato. This potato had been left among the rubbish on the floor; and the only light which found its way into the tower was through a small window at the height of forty or fifty feet. The potato had found its way to the angle of the tower, and climbed up there until the topmost shoots had reached within a few yards of the window at the time when we observed it. If this potato had been planted in the open fields, the stem of it would not in all probability have exceeded a foot or a foot and a half; but here the effect of the light upon it had been such, that it shot up full twenty-five times as much as it would have done in the open air. owing to the potato itself,

How much of this was and how much to the

light, is not easily determined; out it shows that in the economy of the vegetable, world light is a very important element.

We have said already, that, considered as light, the beams of the sun are much more easily refracted, or turned into a new direction, than the same beams considered as heat. This quality has not much seasonal effect; but it is exceedingly useful in nature. Were it not for this, every shadow would be absolute darkness; and both the rising and the setting of the sun would be so sudden and so striking, as hardly to be borne; whereas the scattering of light by atmospheric refraction, often renders the shade more agreeable to the sight than the direct sunshine, and makes the day and the night pass gradually into each other, through the evening twilight and the morning dawn. There is, however, as we have said, little seasonal influence in the light; only the twilight of morning or evening is always longest when the sun's apparent daily path makes the smallest angle with the horizon. Consequently it is longer at midsummer and midwinter than at the equinoxes, and longer in high latitudes than in places near the equator.

The waters are, next to the atmosphere, the portion of the earth and its appendages upon which the solar influence has the greatest though not the most palpable effect. The sunbeams have more direct influence in sensibly heating the waters, than in producing the same effect on the air; because water is a body of much greater weight and consistency than air is, and therefore cannot escape so readily from any agent which

tends to alter its condition. Water is also much less susceptible of changes of bulk from different degrees of heat than air is; and below a certain temperature, at which it is still liquid, it begins to expand. There is therefore much less transfer of water from place to place, by the returning of the Spring, and the declining of the Autumn, than there is of the air, though the influence of the celestial bodies, the sun, and more especially the moon, does occasion a small motion of the waters, seasonally, monthly, and daily.

When we look at a correct representation of the earth, at an artificial globe or a map, for instance, we perceive, that, independently altogether of fresh-water lakes, rivers, and marshes, which in some countries occupy a considerable extent, full seven-tenths of the earth's surface is covered by oceans and seas, all connected with each other, and forming one vast expanse of water. From the great extent of this-more than double that of the dry land—we must conclude that water performs a most important part in the economy of the earth; and when we examine the facts, they show us clearly that such is the case for there is scarcely any operation carried on amid all the varied works and workings of nature, in which water, or one of the elements of water, is not concerned. As it is thus general in its distribution and its use, we would naturally conclude that water must be in some way or other far more obedient to the varying degrees of heat in the different seasons, than it is in mere transfer from one place to another.

When we come to examine the matter with a little care,

we find that this action of the solar influence on water is equally essential to the labours of the Spring, and to all the seasonal labours of the year, as the effect of the same influence on the air. The two are, however, very different in their mode of operation. The air, as we have already stated, expands in volume with increase of heat, and diminishes in volume when the heat is diminished; when it does this only to a very limited extent. But the water is dissolvable into very minute portions by the action of heat; and in consequence of this, there is constantly ascending into the air from the surface of the waters, and from that of every moist substance, that supply of vapour, which again descends in showers, liquid or frozen, without which showers the Spring would return in vain. Water is just as essential to growing and living nature as air is, and indeed it is more so; for humidity reaches plants and animals in that very early stage of their being, when the light, and generally speaking the air, is excluded from them, as being inconsistent with the tender state in which they then are.

The air is the vehicle into which this vapour of water is received; and when it is once there, it is of course carried upon the currents of the air, or, as we say, the wings of the wind, in what direction soever those currents may set.

It will be borne in mind, that we stated, that the tendency of the currents is from the colder place to the warmer; and thus the air, laden with refreshing moisture, must accompany the sun's apparent declination northward in the Spring, and southward again in the Autumn.

WETTING AND DRYING.

67

It does not follow, however, that the times when those currents set most strongly either way, are those at which there are the greatest discharges of water from the atmosphere, whether in the liquid or the solid state. It is not the quantity of water which may happen to be in the air at any time which, of itself, determines the quantity of rain or snow. The disposition of the air to part with it forms an element of this; and an element to which it is of the utmost consequence to attend. The warmer that air is, it can retain the more; and it can do the same the rarer that it is, or the less of it that there is in an equal volume.

These are beautiful provisions. The season of springing and growing is the season when watering is most required; and yet long continued rains, during this season, would defeat the purpose of wild nature, and also that of him who cultivates. Therefore, at this season, the air is most abundantly charged with moisture, and also in the best condition for taking up moisture by evaporation. In this state of things, changes of heat produce more immediate effects, both on the vapour in the atmosphere and on the moist surface of the ground than they do at any other season. Nature is, as one may express it, sooner wetted, and sooner dried; and this alternating action of wet and dry at comparatively short intervals, is by no means the least important part of nature's action during the Spring.

There is still another beautiful result of this adaptation to each other of atmospheric air and the vapour of water. This enables the atmosphere to deposit the