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 a 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 question

able.

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 atmospherical 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 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.

So.

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 hailstor
In what three ways may hailstorms arise?

What is the curve of congelation?

90° 80° 70° 60° 50° 40° 30° 20° 10° 0° 10° 20° 30° 40° 50° 60° 70° 80° 90° CURVE OF PERPETUAL CONGELATION.

303. The heights of the curve from the surface of the globe vary but little from the equator to Lat. 30°; but from 30° to 60° the change is very great, and the line rapidly approaches the earth.

The difference in the height of the points of congelation, for every five degrees of latitude, is presented in the following table:

to imagine, for the sake of illustration, that a north

Describe its peculiarities.

wind, originating in 50° N. Lat., moves horizontally at the rate of sixty miles per hour, at an altitude of ten thousand feet; while a south wind blows simultaneously from 30° N. Lat. with the like velocity, and at the same height.

If they are upon the same meridian, they will meet in ten hours at 40° N. Lat., and since the first wind commences its course at M, three thousand feet above the limit of constant frost, it will be extremely cold; while the south wind will be comparatively warm, as it proceeds from a region, N, two thousand feet below the boundary of perpetual congelation.

By the union of air, thus widely differing in temperature, the inherent atmospheric vapor is both condensed and frozen, and the central portion of the hailstone formed, which, in its descent to the earth, is gradually enlarged by constant accretions of frozen moisture.

305. The prevalence of such opposite currents as have just been supposed, has already been shown (Art. 222); and it is by no means improbable that, in their ceaseless circuits, there are times in which they encounter each other. It may be asked, how can the different winds preserve their respective temperatures, in traversing a distance of ten degrees? To this it is answered, that a fluid in motion can pass through a fluid of the same kind in repose, and differing in respect to heat, without suddenly changing its own temperature. The waters of the Gulf-stream, flowing through the North Atlantic from the torrid zone, are warmer than the ocean, even in the latitude of Newfoundland.

306. The occurrence of hailstorms, under these circumstances, substantially agrees with facts. It will be seen, by referring to the figure, that the mingling of opposite winds, at a lofty elevation, in the tropics, C, C2, would occasion nothing but a combination of warm cur

Explain the action of opposite currents.

Why can the currents preserve their respective temperatures?

Show to what extent the occurrence of hailstorms, under these circumstances, accords with facts.

rents, and in the polar climes of cold currents, A, A2; in neither case could hail be the result of the union.

In the temperate regions, the admixture of warm and intensely cold currents can only be found, and precisely within these limits hailstorms are most prevalent.

Their frequency in summer is attributed to the cir cumstance, that the opposing winds are then most easily set in motion by the increased energy of the solar rays.

307. The space ravaged by hailstorms, often indicates the presence of aërial currents, the devastations being frequently confined to a long and narrow strip of country. Sometimes the storm proceeds in two parallel tracks, leaving the intervening region entirely uninjured.

Thus a hailstorm once commenced in the south of France in the morning, and in a few hours reached Holland. The places desolated formed two parallel paths from S. W. to N. E.; the length of one was 435 miles; and that of the other 497 miles. The average width of the eastern track was five miles, and that of the western ten; and upon the space comprised between them, which was twelve miles and a half in breadth, no hail fell, but only a heavy rain.

308. ACTION OF WHIRLWINDS. It has been stated, (Art. 132,) that whirlwinds are not always vertical, but frequently inclined towards the earth. In consequence of this position, the gyratory motion of the whirl (if its diameter is considerable) will, doubtless, often carry up hot and humid air from the surface of the earth into the higher regions of the atmosphere, bringing down in return large volumes of cold air from the upper strata ; thus causing such a combination as results in the production of hail. This action will be more extensive and energetic if, as some suppose, whirlwinds at times exist whose axes are parallel to the horizon.

309. It must also be remembered, that in the vortex of the whirlwind the air is rarefied, and into this partial

Explain the action of whirlwinds.