occur, the ground becomes 12 or 150 colder than the air a few feet above it. Dew is deposited very unequally upon various substances; plants and vegetables, which need this sustenance, receiving the greatest abundance; but little is deposited upon the dry land, still less upon polished metalic bodies, and none at all upon the ocean. The deposition in these cases is proportional to the temperature, some bodies growing much colder than others when exposed to the same cooling influence; the surface of the ocean, as we have before remarked, remains at nearly the same temperature as the air incumbent upon it.

The surface of the earth is cooled by radiation of the heat it has received during the day, and thus prepared for the deposition of the dew. Hence dews are most abundant in a clear night when the heat radiated from the earth is not intercepted and thrown back from overhanging clouds. It is from this circumstance that the vulgar notion arises that the rays of the moon have a chilling influence. When the ground becomes cooled by the radiation of heat from its surface, below 320, the dew is frozen, and then takes the name of white or hoar frost.

It will be apparent from what we have said, that dew will occur most frequently when there is a considerable difference between the heat of the day and the night. It is on this account that we seldom have dews either in mid-winter, or mid-summer, i. e. at the solstices, but generally just after the vernal, and before the autumnal equinox, viz: in May, and August. Dew is most copious in those places which are sheltered from the wind, and high winds therefore are a sure preventive to its formation, and of hoar frost. Dew and frost occur most frequently in clear weather, when the radiations of heat from the ground are thrown up into the sky without being again reflected, hence the thinnest screen, or the shade of a tree is a protection, and a cloth thrown over delicate plants will preserve them from frost. The quantity of moisture precipitated from the atmosphere, depends upon a variety of circumstances; on the previous dampness of the commingled portions of the fluid, their difference of heat, the eleva tion of their mean temperature, and the extent of the combination which takes place. When the deposition is slow the very

minute, aqueous globules remain suspended. These are found to be made up of hollow vesicles, filled with air like a soap bubble; and as the air included, is rarefied by the latent heat when the vapor is condensed, the weight of these vesicles becomes less than the weight of an equal bulk of air, and therefore they rise or float, forming a cloud. When the air within these vesicles bursts, the drops of rain are formed, which are changed, according to the circumstances attending their formation, as regards rapidity and copiousness, or the state of the medium as to heat, into hail or snow. In order for the precipitation of moisture necessary to form rain, hail, or snow, contending currents must bring vast fields of air of different temperatures over a given spot, as for example, when a warm south-east wind encounters a cold northwester. In some parts of the world, as in Egypt, part of Chili, and Peru, it seldom rains, for there the winds usually blow in one direction.

Snow is formed by the crystalization of aqueous vapor instead of its formation into drops. It is thus converted into a white downy substance, which falls gently to the surface, forming in winter a warm covering, confining effectually the heat of the earth. The snow hats of the natives of Labrador are said to be quite warm. Below are figured some of the beautiful forms which the snow crystals assume in cold climates,

Hail is forme by drops of rain suddenly congealed during their fall, by passing through a lower stratum of dry and cold air. The

CLOUDS.

141 most violent hail storms are caused by a sudden transference of a body of warm air up beyond the term of perpetual congelation, where the drops of rain are frozen into hail stones. This is sometimes accomplished by means of whirlwinds, by which the hailstones being sustained for some time, are occasionally accumulated to a very large size. By referring to our figure of the curve of perpetual congelation, page 126, it will be understood why hail storms seldom occur in the equatorial regions, and most frequently in the temperate zones, and seldom or never in the polar regions. The term of congelation is too high at the equator for the hot air raised by a whirlwind to pass beyond, or up to it, and at the polar regions, the air seldom becomes as hot as is required to form hail storms. Different names have been given to the various forms of clouds, derived from their appearance and character, we will briefly notice them. First, we have, occurring at the greatest elevation, the Cirrus, or curl-cloud, which is a thin fleecy vapor, with a waving and striated appearance as shown in the engraving below. This cloud is frequently called the curl-cloud

from its flexuous form, and is not unlike a bunch of wool pulled out into fine pointed ends. After a continuance of fine weather the cirrus is often observed at great heights like a fine white line stretching across the sky. The peculiar form of cirrus shown in

G*

the engraving, called vulgarly, mare's tail, is thought to be an indication of violent winds, and the wind is generally from the quarter towards which the fine extended ends are pointed. When carefully observed, every particle of the cirrus cloud seems to be inmotion, though the whole cloud appears nearly stationary. This cloud under different circumstances, presents considerable variety of appearance. After a continuance of clear and fine weather a whitish line of vapor stretched out like a thread, may be observed at a very great height, the ends seeming lost in the horizon, this is often the first indication of a change from dry to wet weather. To this line of cloud others are added, or as it were propagated from the sides in an oblique or transverse direction the whole having the appearance of net-work.

The Cumulus, is a dense mass of rolling clouds rising from a horizontal base. The name denotes a heap or pile; it is sometimes called the stacken-cloud, since the masses of which it is composed seem stacked or piled together. This cloud is generally formed during the day, but is desolved at the approach of evening, it has hence been termed the cloud of day. The Stratus or fall-cloud is a low cloud seeming to rest upon the earth, hence its name stratus, a covering. This cloud is generally formed during the night, and is sometimes called the cloud of night, it is generally dissipated by the rays of the sun and in this case is

Among this variety of

considered indicative of fine weather. clouds are included those fogs and creeping mists that in summer evenings fill the valleys, remain during the night, and disappear in the morning. The formation of the cumulus is best viewed in fine settled weather, about sunrise or a little after. Small specks of cloud will be seen in the atmosphere, which seem to be the result of the gatherings of the stratus or evening mists, which rising in the morning form into small clouds whilst the rest of the sky becomes clearer. About sunrise two or more of these unite and form a stacken-cloud. In the evening it again subsides giving place to the stratus or fall cloud. In our engraving the cumulus is seen above and the stratus nearer the horizon. Some varieties of the cumulus are supposed to be closely connected with electrical phenomena. The hemispherical form is more

perfect in fine than in changeable weather.

The Cirro-stratus is often called a mackerel sky, and is seen in fine summer evenings, it is generallly called the wave-cloud on account of its frequent alterations of figure. It is formed at a great height and presents many varieties and is sometimes seen as a thin extensive sheet covering the heavens, and it is this form of the cloud in which the halos appear, which are thought to indicate rain, and when the sun sets apparently shrouded in a dense stratum of this cloud it is a sure indication of a wet morning. A