The air of

mists, which are caused in the same way. seaside towns is naturally impregnated with moisture, hence the mists which hang over some of them during certain months of the year.

The moisture from the atmosphere may be deposited in the form of dew, rain, snow or hail. Dew is formed at the surface of the earth; the heaviest deposits occur on the warmest nights. The night air, laden with moisture, comes in contact with the surfaces of plants, which have lost their heat by radiation, and which are therefore colder than the air: at once follows the deposition of moisture in the form of dew.

The form of rain may be originated either by the cool surface of the earth, or by cold strata of air existing in the higher regions of the atmosphere. Mountainous regions receive more rain than lowland districts, and maritime places more than inland towns. The rule is "that the annual rainfall decreases as you proceed from the coast to the interior of a country, and as you proceed from the Equator to the Poles." We, who live in the Temperate Zone, have more rainy days in the year than people who live in Tropical Regions, but they receive heavier rainfalls than we do.

We are so accustomed to see water in a sensible form, either fluid or solid, as in rain, ice, hail, snow, fog, and the like, that every one is surprised when he is made conscious for the first time, that water may really be found in the condition of a perfectly invisible vapour. Yet, whoever has seen a bottle brought out of a cellar on a warm day, or observed the effect produced when the windows of a carriage are drawn up, and particularly those persons wearing spectacles, the glasses of which are suddenly dimmed by steam upon entering a heated room, must have noticed enough to convince him that such is the case. In such instances the colder surface of the glass condenses the vapour of water, previously invisible in the atmosphere, and thereby renders it sensible. All the great changes of sunshine, cloud and storm, the various hues of the rising and setting sun, the haloes which occasionally surround the sun and moon are all

influenced or occasioned by the vapour of water diffused throughout the atmosphere.

The vapour of water, however, in its simplest form is perfectly invisible. It exists mixed with the other gaseous matters which compose the atmosphere, and diffused over all parts of the earth's surface. Every substance which contains water, is capable also of permitting it to evaporate.

Not only large masses of water-as seas, lakes, and rivers, as well as ice-but every portion of vegetation, all soils, even those which appear driest, are continually permitting some portion of watery vapour to escape from them. The quantity of vapour in the atmosphere at any given time is influenced by a variety of causes; but the presence of such a vapour is most important for many purposes. Dew, which is formed by the condensation of the vapour of the water upon the leaves and other parts of plants affords nourishment to vegetation when no rain falls; and a certain quantity of vapour of water is essential to the health of man. In some hospitals when they were first warmed by heated air, it was found that the inmates suffered from their skin cracking and peeling off, as in very hot climates; but the inconvenience was immediately removed, when vessels of water were placed in several parts of the building, which by evaporation, supplied the requisite quantity of moisture to the air. The quantity of evaporation going on constantly is far greater than is usually conceived. In a hard frost, a lump of ice or snow will be observed sensibly to diminish, especially if a brisk wind is blowing over it. This is quite independent of the wasting of the frozen substance by thawing. In fact, snow or ice may totally disappear without any perceptible thaw, simply by evaporation. It has been computed from actual experiment, that an acre of snow evaporates four thousand gallons of water in twenty-four hours. All plants exhale vapour, and some much more than others. Thorn hedges exhale seven times as much as those of holly: and a cabbage perspires six or seven times as much as a man from the same quantity of surface,

There is, however, a limit to the power of evaporation, and this limit is fixed by the temperature of the climate, so that if, on the coldest day of winter, the air contains as much moisture as possible, or is, as it is called, saturated with vapour, it can then receive no more vapour unless its temperature is increased. But as the temperature of the air increases, more and more vapour may be mixed with it: yet still, as the heat of the air never exceeds a certain degree, the quantity of vapour also is limited.

Such a limitation is necessary for the well-being of all plants and animals: either a perfectly dry air, or an atmosphere overcharged with vapour, would be inconsistent with their existence in a state of health. As the atmosphere is now constituted, there is found in every part a certain quantity of vapour ready to make its presence sensible, whenever any change of circumstances causes it to be condensed.

One of the most common effects thus produced is that of clouds. The well-known experiment, mentioned above, of the condensation of vapour on a cold surface such as glass, shews that if the temperature of the air be by any means lowered, the quantity of moisture, which it will retain in the state of invisible vapour, will be diminished. In cold weather, this is made very evident by the condensation of the breath of animals. The air which comes from the lungs contains within it a quantity of watery vapour which would be quite invisible, if it were breathed out into an atmosphere of the same or nearly the same temperature, as that of the animal's body. But when the air is much colder, some of the vapour is instantly condensed, and forms very small drops. The same effect is seen on a large scale when steam is discharged from a steam-engine. Where, then, any change takes place in the temperature of the atmosphere, from any cause, there is a probability that the vapour in the atmosphere will be condensed and become visible.

Thus, suppose the air perfectly serene and clear, and that it contains in every part just as much vapour as it

is then capable of containing; if a stream of colder air be now made to pass through a part of this atmosphere, the temperature of the two portions of air when united will be lower than that of the first portion was before, and the vapour in it will be partially condensed forming a cloud of greater or less density according to circumstances.

If the condensation goes on, the very small particles of water which float in the atmosphere-or, after descending a little way, meet with a warmer temperature, and are again turned into invisible vapour-will unite in drops of a sensible magnitude and fall in rain. Should they meet with a still greater degree of cold, the drops freeze in their descent, and appear as hail; or, if the congelation takes place while the particles of water are still very small, snow or sleet will be formed. By the same means all the different appearances of fog and mist are occasioned. During the heat of a summer's day, evaporation goes on with great rapidity, as has beer already noticed, from water, from all vegetable bodies, and even from the earth. But at sunset, heat begins to be lost by radiation, and some of the vapour is immediately perceptible, especially where evaporation has been most copious, as along a river or over meadows. The course of a river may sometimes be distinctly traced, for a long distance, even when the water itself is not visible, by the fine cloud formed by such congelation. On the other hand, when the atmosphere is charged with visible moisture, an increase of heat converts the water into invisible vapour. A very beautiful instance of this effect is often seen in Autumn. At sunrise the whole atmosphere appears full of floating particles of water, forming a dense mist, the minute drops of which are distinctly visible. As the sun rises above the horizon, the air is gradually warmed, the fog begins to disperse, at first rising a little into the form of clouds, but soon totally disappearing.—Saturday Magazine.

THE CLOUD.

1 BRING fresh showers for the thirsting flowers, From the sea and the streams;

I bear light shade for the leaves when laid
In their noonday dreams.

From my wings are shaken the dews that waken
The sweet birds every one,

When rocked to rest on their mother's breast
As she dances about the sun.

1 wield the flail of the lashing hail,
And whiten the green plains under;
And then again I dissolve it in rain,
And laugh, as I pass, in thunder.

I sift the snow on the mountains below,
And their great pines groan aghast;
And all the night 'tis my pillow white,
While I sleep in the arms of the blast.
Sublime on the towers of my skyey bowers
Lightning, my pilot, sits;

In a cavern under, is fettered the thunder,
It struggles and howls at fits;

Over earth and ocean with gentle motion,
This pilot is guiding me,

Lured by the love of the genii that move
In the depths of the purple sea;

Over the rills, and the crags, and the hills,
Over the lakes and the plains,

Wherever he dream, under mountain or stream,
The spirit he loves, remains;

And I, all the while, bask in heaven's blue smile, Whilst he is dissolving in rains.

The sanguine surprise, with his meteor eyes,
And his burning plumes outspread,

Leaps on the back of my sailing rack

When the morning star shines dead,