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. Hoar-frost is produced in the same manner as dew. Late in the spring, and early in the fall, the surface of the earth, during serene nights, sinks in temperature 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.

277. A species of hoar-frost occurs when a warm south wind succeeds a continuance of cold weather. Stone columns and buildings are then covered with a snowy incrustation, composed of an assemblage of minute crystals, caused by the influence of the low temperature of the stone upon the condensed vapor of the air.

The effect of a cold body upon moist air is well shown by the following facts related by Ballantyne, who resided for two years at York Factory, in the vicinity of Hudson's Bay. After narrating the adventures of a hunting expedition in the depth of winter, he thus describes an incident that occurred upon the return of himself and his companions to their dwelling. "It was curious to observe the change that took place in the appearance of our guns after we entered the warm room. The barrels and every bit of metal upon them instantly became white, like ground glass. This phenomenon was caused by the condensation and freezing of the moist atmosphere of the room upon the cold iron. Any piece of metal, when brought suddenly out of such intense cold into a warm room, will in this way become covered with a pure white coating of hoar-frost. It does not remain long in this state, however, as the warmth of the room soon heats the metal and melts the ice. Thus, in about ten minutes our guns assumed three different appearances. When we entered the house they were clean, polished, and dry; in five minutes they were as white as snow, and in five more were dripping wet."

278. Every thing that prevents the radiation of heat, arrests the formation of hoar-frost. During the chilly nights of spring, plants that are sheltered by trees are less liable to be frozen than those which are fully exposed; and a slight covering of straw, or even of paper, will often afford an effectual protection. Vineyards have frequently been saved from the effects of frost, by enveloping them during the night in a cloud of smoke.

What effect is caused by a warm south wind, after a period of cold weather? Relate the facts related by Ballantyne.

What arrests the formation of hoar-frost?

279. The effect of a screen in checking radiation, and thus preventing frost, has been finely illustrated by the experiments of David Scott, of India. Throughout the whole region of Upper India, ice is artificially procured by placing upon a layer of dry straw, in the bottom of small pits, and fully exposed to the clear sky, broad, shallow earthen pans, filled with water. Such

is the radiation during the night, that a thin crust of ice will sometimes form upon the water, when the temperature of the air on a level with the pits is as high as 41° Fah.

On one occasion, Mr. Scott extended a muslin turban across a pit, three feet above the pans. No ice was formed in the vessels immediately under it; but, in several that were partially covered, ice appeared upon the part of the water beyond the shelter of the muslin; while the surface beneath the turban remained in a fluid state. Two strings, crossing each other at a lower height above a pan, under favorable circumstances, divided the ice into four quarters, the water beneath the strings continuing unfrozen.

SNOW.

280. Snow is the frozen moisture that descends from the atmosphere when the temperature of the air at the surface of the earth is near or below the freezing point.

281. SNOW-FLAKE. At moderate heights, and in the temperate regions, snow commonly falls after several days of severe frost when the weather has moderated. The largest flakes occur when the air abounds with vapor and the temperature is about 32° Fah.; but as the moisture diminishes, and the cold increases, the snow becomes finer.

In the former case, it is not unusual to observe flakes an inch in diameter; and in the latter, they only measure a few hundredths of an inch.

Illustrate the influence of a screen, by the experiments of Scott.
Define snow. When does it usually fall?

Under what circumstances do the largest flakes occur?

Under what circumstances do the smallest? How large are they?