remove the notion that dew is a kind of fine rain, San from the atmosphere.

Place a few bricks upon a grass-plat, to form a smal wall, about three feet in height; at the foot of this spread twenty grains of dry wool, and some feet distant place a similar weight of the same substance; let this arrange ment remain during the night, and upon examining

sunrise next morning, always providing no mis tare fallen,) the partially sheltered wool will be found to have increased only a few grains in weight, whilst the exposed portion eight or ten times as many grains in weight

Fig. 36.

Gravel walks, and stone pavements, radiate Leat and acquire dew, less readily than grass-plats, Lence, wood placed on the former has its temperature less depressed than on the latter, and therefore obtains less dew; and this effect is by no means referable to the capillary attrac tion of the fibres of the wool, for it ensues equally well if the wool be placed in a clean saucer; nor is it referable to hygrometric attraction of water from the air, be cause wool, placed upon a board, elevated at some feet from the earth, during a cloudy night, will acquire only a very minute increase of weight; the effect is entirely dependent upon radiating power.

Before the cause of dew was discovered, scientific

inquirers were at a loss to account for the principle upon which a thin mat, employed by gardeners, protected tender plants from cold; it appeared scarcely possible, that such a slight covering could have any effect in preventing them from attaining the temperature of the external air, and yet common experience proved its utility.

When the discovery was made, that bodies on the surface of the earth become much colder than the atmosphere, by radiating heat during a still and serene night, the true reason of the gardener's homely practice of covering plants with matting, was fully evident; it prevented radiation from taking place.

In order to acquire precise information upon this subject, the following simple arrangement was made: the experimenter "fixed perpendicularly in the earth of a grass-plat, four small sticks, and over their upper extremities, which were six inches above the grass, and formed the corners of a square, whose sides were two feet long, a very thin cambric handkerchief was tightly drawn.

"Therefore nothing existed to prevent the free passage of air from the exposed grass, to that which was sheltered, except the four small sticks, and there was no substance to radiate downwards to the latter grass, except the cambric handkerchief."

"The sheltered grass, however, was found nearly of the same temperature as the air, while the unsheltered was 5 degrees or more colder; one night the fully exposed grass was 11 degrees colder than the air, but the sheltered grass was only 3 degrees colder."

Hence we see the power of an awning, infinitely more slight in texture than that of matting, in averting or lessening the coldness of the ground that would otherwise occur, and be injurious to plants.

It is for this reason that a tent, or marquee, although merely constructed of thin canvas, and by no means airtight, becomes a comparatively warm shelter during a cold night; no dew will be found upon the plat of grass shielded by the tent, but abundance upon that exposed to the open air around.

A piece of matting, that has covered a plant during an autumnal night, will be found copiously bedewed, whilst the plant remains dry; the matting having presented its radiating surface, and parted with heat, which otherwise the plant would have done; nay, even the matting occasionally performs this office with such energy, that its surface bcomes covered with hoar-frost, from which it suffers no injury, as it has no vitality, whilst it shields the living plant, that otherwise would be compelled to radiate, and perish, by the degree of cold induced by its own power.

In some countries, the temperature never falls to the freezing point of water, and yet ice may be procured by taking advantage of the radiation of heat from surfaces, in the following manner.

A layer of dry straw, or stubble, is spread in an open situation during a calm and cloudless night, and upon this surface are placed shallow earthen pans, filled with

water; radiation immediately commences from the straw and the water; both lose heat so rapidly and so effectually, that the water can no longer remain liquid, and therefore becomes covered with a thin crust of ice.

It has been already stated, that water hastens the putrefaction of dead vegetable substances; the same remark is applicable to those of animal origin; in warm climates, more especially, the deposition of water, in the form of dew upon these, induces putrefaction with great rapidity; and as this generally happens only during clear bright nights, it was anciently supposed that moonlight favored animal corruption.

The moon, when at full, reflects upon the earth only about one three-hundred-thousandth part of the light of the sun; and the lunar rays, even when concentrated by a powerful lens, and the focus directed upon the bulb of a delicate thermometer, do not affect it in the slightest degree; hence the phrase, "the pale, cold moon," is not only poetically beautiful, but philosophically correct; and as the light alone will not induce putrefaction, and as there is no heat, the phenomenon appears to be solely referable to the radiating power of animal substances inducing a copious deposition of dew upon their surfaces, for water is essentially requisite in all cases of putrefactive changes.

If a thermometer be placed upon a grass-plat, it will very frequently indicate a temperature of fifteen or twenty degrees colder than one suspended over the grass, at the height of three or four feet, thus proving that radiation is proceeding with extreme rapidity in the one case, from the comparatively solid vegetable matter and the soil, but not from the ambient air.

Accordingly, the portions of air in immediate contact

with the grass, become much colder than those far above, and are compelled to deposit dew; and if the air over a given locality remain tranquil for some hours, this phenomenon will solely ensue from the lower and colder portions.

Small valleys, and hollow-ways, permit the air to repose undisturbed; and although they are apparently situations sheltered from cold, yet they are frequently more subject to reduction of temperature than higher situations; and accordingly, much to our surprise, we find delicate plants chilled, or even frost-bitten, in hollows, whilst others suffer no injury upon the adjacent slopes.

"In the close and sheltered streets of cities, the deposition of dew is very rarely observed, because there the objects are necessarily exposed to each other's radiation, and an interchange of heat takes place, which maintains them at a temperature uniform with that of the air; a deposition of dew in this case can only take place when the natural temperature of the air falls below its point of saturation."

At sunrise during Summer or Autumn, we may frequently see a dense white mist, only a few feet in height, directly incumbent upon a grass or corn-field, whilst the air above is clear and bright with the rays of the sun; and we may even walk along the foot-path across the field, with the lower part of our body enveloped in this mist, whilst the upper is perfectly free from its humidity.

This phenomenon of mist is consequent upon the calm of the previous night having admitted rapid radiation to ensue from the earth, which then became so much colder than the great volume of the atmosphere above, that the portion in immediate contact with the earth was suddenly