live a great n imber of animalcules. The cells are generally red, which is supposed to be their original color, the green tint being probably acquired by exposure to the air and light.

These extraordinary hues may, therefore, be regarded as originating in the same plant, in different stages of development.

290. USES OF SNOW. Snow subserves many important purposes. Gathered in exhaustless stores upon the high mountains of the globe, it feeds, as it gradually melts beneath the heat of summer, thousands of rivers, which, flowing on from clime to clime, enrich the soil and crown the land with plenty.

The snow-capped mountains are the natural refriger ators of the glowing regions that lie within the tropics; cooling the winds that pass over them, and mitigating the fierce temperature of the atmosphere.

In the higher latitudes, where the winters are severe, the snow forms a warm covering for the soil, and defends vegetation from the rigors of the frost. It is well known, that grain, during an open winter, is frequently destroyed by the cold; and, in the mild climate of England, Alpine plants have perished, in consequence of being deprived of their natural covering of snow.

During the long night of the polar climes, the intensity of the darkness is diminished by the presence of the snow; inasmuch as it reflects, instead of absorbing, like the bare ground, the faint light that there proceeds from the sky. Nor is it to be forgotten, that, in these inclement regions, the wretched natives would be unsheltered during the winter, were it not for the snow; since this, when cut into blocks, supplies the Esquimaux with the means of constructing their huts."66

What is said in regard to the uses of snow? 6

CHAPTER VI.

OF HAIL.

291. HAIL. The ice that descends in showers, and usually in summer, is called hail. It is different from sleet, which is nothing more than frozen rain, and occurs only in cold weather.

292. STRUCTURE. Hailstones are generally pearshaped, and if they are divided through the centre, they are found to be composed of alternate layers of ice and snow, around a white, snowy nucleus, resembling the coats of an onion. The surface is rough, and is sometimes studded with icicles.

ence.

293. SIZE. Hail varies in size, from stones as small as a pea to those which are several inches in circumferBenvenuto Cellini relates in his memoirs, that during his journey from Italy to France, he was overtaken by a terrible storm in the vicinity of Lyons; hailstones falling of the size of lemons, and with sufficient force to kill even men and cattle.

At Roncesvalles, in August, 1813, there fell upon a division of the British army a storm of hail, in which the stones ranged in size from a bean to a hen's egg. The tin camp-kettles of the soldiers were indented by the masses of ice, some of which were round, and armed with icicles three inches in length.

In May, 1847, hailstones of immense size descended near the town of McDonough, in Georgia; one of them was measured an hour after it fell and found to be ten inches in circumference. During a terrific storm, that occurred at Cazorta, in Spain, on the 13th of June, 1829, the roofs of the houses were broken in by the hail. Some of the stones are stated to have weighed nearly four pounds and a half. It is probable that such extra

Define hail.

What is the form and structure of the hailstone?

What is said of its size?

Narrate the facts stated.

ordinary masses as those which have been mentioned, are formed by the union of several hailstones frozen together.

GEOGRAPHICAL DISTRIBUTION.

294. Hailstorms are most frequent in the temperate climes, and rarely occur within the tropics, except in the vicinity of mountains whose summits tower above the limit of perpetual frost. Although by no means common, they are not unknown in the high northern latitudes. Simpson, on the 12th of August, 1839, was exposed in the straits of Boothia, in 68° 32′ N. Lat., to a tremendous thunder-storm, accompanied with torrents of rain and heavy showers of hail.

It is mostly in summer, and usually at the hottest part of the day, that hail is observed to fall. Scarcely any occurs in the night.

ORIGIN.

295. The structure of the hailstone shows that it is not formed at once; for the concentric layers around the snowy nucleus, consist of different accessions of moisture, successively condensed and congealed upon the surface of the stone.

The light, porous texture of the snowy centre, seems to indicate, that the place of origin must be some region in the atmosphere where the air is rare, and the cold intense; since the appearance of the centre is similar to that presented by a drop of water, when frozen under the exhausted receiver of an air-pump.

296. It is necessary then for the production of hail, that a warm, humid body of air should mingle with another so extremely cold, that their temperature, after uniting, shall be below the freezing point. This combination must also take place during the warmest period

Where do hailstorms frequently occur?

Where rarely?

When do they usually prevail?

What indicates that the hailstone is not formed at once?
Where must it originate?

What conditions are necessary for the production of hail}

of the year and the day. In accounting for an intense degree of cold under such circumstances, consists the great difficulty of explaining the phenomena of hail

storms.

297. Until within а few years, almost every meteorologist attributed the cold of hailstorms to the agency of electricity. It is well known that air, when electrified, is expanded, and that expansion produces cold. It was therefore imagined, that the electrified state of the atmosphere before a storm, caused such a reduction of temperature as to freeze the falling moisture and produce hail.

Volta, a distinguished philosopher of France, supposed the cold to be the result of evaporation, but employed electricity in a singular manner, as explained below.

298. VOLTA'S THEORY. According to this theory, two clouds, differently electrified, are supposed to extend through the sky, one directly above the other. The cold, caused by evaporation from the upper surface of the lower cloud, is considered to be so intense, that the vapor is frozen, and the nucleus of the hailstone then formed. Its size is afterwards increased by the humidity it gathers in vibrating backwards and forwards between the two clouds, like the dancing figures upon electrical plates. (C. 969.) At last it becomes so large, as to break through the lower cloud, and fall to the earth.

299. The sanction of a great name gave weight to this fanciful view, and in 1821, throughout the southern districts of France, which are peculiarly liable to hail storms, hail-rods were erected, in order to draw the electricity from the clouds, and thus protect the vineyards. Their efficacy, however, is exceedingly questionable.

The electric hypothesis is, moreover, at variance with facts. The forests, which constitute a vast assemblage of hail-rods, are often ravaged by hail; and in the tor

What effect has been attributed to electricity ?
Explain Volta's theory.

rid zone, where the development of atınospherical electricity is greatest, hailstorms are almost unknown.

300. OLMSTED'S THEORY. Prof. Olmsted, of Yale College, considers electricity as an effect, and not the cause of hailstorms. According to his theory, which has been very extensively received, the cold body of air derives its low temperature, not from electricity, but from some known source of cold; and the combination, which occasions the hail, may arise in various ways, the principal of which appear to be the following.

301. First. An exceedingly cold wind, coming from a region far above the limit of perpetual frost, may meet with a current of warm air, blowing from a point many thousand feet below this limit.

Secondly. By the force of whirlwinds, large volumes of warm air from the surface of the earth may be suddenly transported into the higher and colder regions of the atmosphere.

Thirdly. In the vicinity of lofty mountains, cold blasts are frequently known to sweep down their sides from the snowy peaks and glaciers, and mingle with the warm atmosphere of the vales.

Each of these methods we will discuss separately.

302. CURVE OF PERPETUAL CONGELATION. In Art. 53, we have seen that a point can be reached in every latitude, where moisture, once frozen, always remains so. An imaginary line passing through these points, and extending from pole to pole, forms what is termed the curve of perpetual congelation, which possesses the peculiar figure shown in the annexed cut.

Does the electric theory agree with facts?

What are Professor Olmsted's views in regard to electricity?
Whence comes, according to his theory, the cold of the hailstorm
In what three ways may hailstorms arise?

What is the curve of congelation?