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 raviation 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?

At Bossekop a fall of snow occurred when the thermometer stood at 10° Fah., and the diameter of the flakes then scarcely exceeded seven hundredths of an inch.

The snow-flake is composed of regular crystals, and its beautiful figures and rich diversity of forms have ever excited the admiration of observers. In solid ice, the crystals are so blended together that their symmetry is lost in the compact mass; but in snow, they are perfectly developed, when the flakes descend through a calm atmosphere. Any agitation of the air, or an increase of moisture or temperature, destroys their deli

cate structure.

If the crystals of snow were solid, they would be transparent, like other crystallized bodies; but they contain air, and to this circumstance is attributed their brilliant whiteness; for the air preventing the ready transmission of light through the snow-flake, the rays are copiously reflected from the assemblage of crystals The bulk of snow which has just fallen is ten or twelve times greater than that of the water obtained by melting it.

282. Though single crystals always unite at angles of 30°, 60°, or 120°, they nevertheless form, by their different modes of union, several hundred distinct varieties.

Scoresby, a celebrated Arctic navigator, has enumerated six hundred, and delineated ninety-six; and Kaemtz has observed twenty more, not figured by Scoresby.

283. SNOW-CRYSTALS. Although the varieties are so numerous, they are all comprised under five principal

classes.

1st. Crystals in the form of thin plates; they are generally very thin, transparent, and of a delicate

How small are they?

Of what is the snow-flake composed?
How is the whiteness of snow caused?

What is said of the bulk of snow?

State the number of varieties of snow-crystals.

In how many classes are they comprised? Describe them.

structure. This class includes many remarkable vari eties, which are represented by the first twenty-five figures of the annexed cuts, (15., 16.)

2d. Flakes either possessing a spherical nucleus, or a plane figure, studded with needle-shaped crystals, (figure 26.)

3d. Slender prismatic crystals; usually six-sided, but sometimes having only three sides.

4th. Pyramids with six sides; (figure 27.)

5th. Prismatic crystals, having, perpendicular to their length, both at the extremities and in the middle, thin, six-sided plates; (figures 28., 29. and 30.) The last

Fig. 16.

two classes are extremely rare, Scoresby having observed the fourth but once, and the fifth only twice, in all his voyages.

Flakes belonging to two consecutive falls of snow, possess different figures; but those which descend during the same storm, are usually alike in this par ticular.

284. NATURAL SNOW-BALLS. Balls of snow are sometimes formed by the action of a high wind upon light snow. Prof. Cleaveland, of Brunswick, in Maine,