immediately influenced by the warm beams of the sun. Indeed, at this time, more dew is deposited than at any other equal period in the twenty-four hours.

INFLUENCE OF THE SUBSTANCE BEDEWED.

262. CONSTITUTION. Since the production of dew requires that the body bedewed must be colder than the surrounding atmosphere, all substances, which rapidly lose their own heat and slowly acquire that of others, are susceptible of being copiously bedewed. On the contrary, substances possessing the opposite qualities contract but little dew.

Under the first class may be included glass, silk, down, wool, and, in general, all bodies of a porous texture; while metals and rocks belong to the second division.

263. If similar plates of polished glass and metal are exposed alike upon the soil during a favorable night, in the morning the glass will be drenched with dew, but the brightness of the metal will be scarcely dimmed. These different results arise from the fact, that, while the glass is deprived by radiation of ninety hundredths of its original heat, twelve hundredths is all that the metal loses. Besides, the glass, being a bad conductor, draws but little warmth from the earth to supply its loss; while the metal, being a good conductor, can easily restore any reduction of heat from the warm soil immediately below.

Large plates of metal, exposed throughout the night, never acquire a temperature more than three or four degrees below that of the atmosphere.

264. SURFACE AND FORM. A polished surface does not radiate so well as one that is rough and uneven; and the latter is always found, under a like exposure, to receive more dew. Points radiate heat most rapidly,

Account for its deposition at this time.

What substances are capable of being copiously bedewed?
What not? Give examples.

Account for the unequal deposition of dew on glass and metal.

What is said of polished and rough surfaces in this particular?

and are therefore the coldest portions of a radiating body; hence, of all the globules of dew that form upon blades of grass, the largest are found at the very extremities.

Grass is well known to be copiously bedewed; its form, as just mentioned, causes it to lose its own warmth with great rapidity, while its porous texture prevents it, at the same time, from replenishing its loss from the soil.

265. LOCATION. If a body is screened from the open sky, it contracts less dew than when fully exposed; for the screen arrests radiation in the manner of clouds; and the difference in temperature between the sheltered body and the contiguous air, is less than that which would exist between the same body and the surrounding atmosphere, were the substance bedewed entirely unsheltered. This has been proved by the experiments of Dr. Wells.

266. In four trials, made with two parcels of wool, in all respects alike, the first of which was placed upon the upper side of a board, four feet from the ground, and the second loosely attached to the under side, the gain, in dew, was as follows:

We hence perceive, why, beneath the shelter of trees, and on the under surfaces of leaves, but little dew is found.

267. Dew has never been found upon the surface of large bodies of water; for whenever the aqueous particles at the surface are cooled, they become heavier than those below them, and sink; while warmer and lighter particles rise to the top. These, in their turn, become

What of points?

Why are the largest beads of dew upon the end of the blades of grass? Why does an exposed body contract more dew than one which is sheltered? Give the results of Dr. Wells' experiments.

Why are the surfaces of large bodies of water free from dew?

heavier and descend; and the process continuing throughout the night, maintains the surface of the water and the air at nearly the same temperature.

Dr. Wells ascertained, by experiment, that even a small quantity of water gains no weight by exposure during a single night.

It appears, from the narrative of the U. S. Exploring Expedition, and from other sources, that on the ocean heavy deposits of dew sometimes occur upon the decks of vessels.

268. The exposed parts of the human body are never covered with dew; since the vital heat, varying from 96° to 98° Fah., effectually prevents such a loss of warmth as is necessary to its production.

269. COLOR. A few experiments were made by Dr. Wells, in order to ascertain the effect of color upon dew; but without any decisive results. In 1833, Dr, Stark, of Edinburg, made two experiments, from which he inferred, that under like exposures, more dew was deposited upon dark-colored bodies, than upon light-colored. But the author of this work, from an investigation prosecuted by himself during the summer of 1846, arrived at the conclusion, that color exerts no influence whatever upon the quantity of dew. This fact might also be inferred from the experiments of Dr. Bache on heat, which clearly show, that the amount of radiation is not affected by color.

270. OBSERVATIONS. The observations, which have been made in various regions of the globe, in regard to the occurrence of dew, strongly corroborate the theory of Dr Wells. In Bengal, during the month of November, the nights are beautifully serene, and accompanied with heavy dews; but in April and May, when high winds prevail, with a close and cloudy atmosphere, no

What experiment was made by Dr. Wells?

What is stated in the narrative of the Exploring Expedition?
Why is dew never found upon the human body?

What is said as to the influence of color?

What do the observations made in different regions attest?
Give instances.

dew is deposited. From September to March, the sun glows like an orb of fire over Southern Guinea; but the nights are cool, and the parched soil is refreshed with abundant dews. In Egypt, profuse dews, like rain, occur in the summer, when the nights are resplendent with stars; while at Thebes, where the sky is constantly serene, abundant dews are the only moisture that the earth receives from above, during the lapse of many years.

271. FACTS EXPLAINED. The explanation of several familiar facts, depends upon the foregoing principles. Thus, for instance, if, in a warm summer's day, a glass is filled with cold water, the exterior surface is seen covered with moisture; for the reason, that the glass, being colder than the air in contact, cools the latter below the dew-point. In summer, caves and cellars are damp; because the warm air that enters them from without is cooled down, and its humidity either floats in the atmosphere, or is condensed in beads of dew upon the stones.

272. BENEFICENT DISTRIBUTION. The mode in which the blessing of dew is dispensed to the earth, beautifully exemplifies the benevolence of our Creator.

In the temperate climes, where the frequent interchange of sun and shower preserves the earth from the extremes of heat and moisture, very little dew is needed, and but comparatively little is deposited. The regions however within the tropics are deprived of rain for months, and this destitution is partially supplied by the dews, which precisely at these seasons are most abundant.

273. The lake and the river are not visited by dew, for no form of vegetation exists within them that needs its presence. To the naked rock it comes but in scanty measure; for there is nothing here to nourish-save, perhaps, the thorny cactus, which, from its very form and

What facts are explained upon the foregoing principles ?
What does the distribution of dew exemplify?

Give the various illustrations.

nature, is adapted to its situation; and though springing from the cleft of a rock beneath a tropic sun, or striking its roots in the sands of the desert, is capable of deriving from the air an adequate supply of moisture. 274. Upon the foliage of the grove very little dew is deposited, in consequence of the inclined position of the leaves, their frequent motion, and the shelter they afford each other. Nor is it needed; for the natural supply of moisture rises deep from the soil through the parent trunk, diffusing itself throughout every branch to the remotest extremity of the slenderest bough.

275. The dew, however, blesses, in all its invigorating exuberance, the humble plant and tender herbage, a vast class of vegetable life, at once the most perishable and the most useful; it is the first of all to feel the effects of drought, and yet it is that which is necessary to the very existence of man. From the field, not from the forest, comes our support; and the failure of a single plant, the grass or the bladed grain, may bring upon a nation scarcity and famine.

CHAPTER V.

OF HOAR-FROST AND SNOW.

276. HOAR-FROST.

same manner as dew.

Hoar-frost is produced in the Late in the spring, and early in the fall, the surface of the earth, during serene nights, sinks in tempe ature below the freezing point, while the atmosphere, a few feet above, is higher by several degrees.

The moisture which is then deposited becomes congealed in sparkling crystals; and the stems of plants and the branches of low shrubs are often adorned with fringes, formed of the most beautiful and delicate crystallizations.

What is the subject of chapter fifth?
How is hoar-frost produced?

Describe its appearance.