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HEIGHT OF THE SNOW-LINE ABOVE THE SEA.

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It will be observed from this table, that, speaking generally, the snow-line from lat. 0° to 20° sinks only a very little; from 20° to 70° it continues to fall equally; but from 70° to 78° it falls with great rapidity.

428. To this general rule there are, however, several noteworthy exceptions. Thus, it is about 4000 feet higher on the north than on the south side of the Himalayas, owing (1) to the greater depth of snow which falls on the south side; (2) to the greater dryness of the climate of Tibet, which increases the evaporation from the surface of the snow and the heating power of the sun to melt snow at these great heights; and (3) to the rocks and soil of the north being in a great measure destitute of vegetation, and therefore capable of absorbing more heat than the regions south of the Himalayas, which are covered with vegetation.

429. It is higher in the centre of continents than near the coasts, because the rainfall is less, and the heat greater ; thus,

while in the Caucasus it is 11,063 feet, it is only 8950 in the Pyrenees, both places being nearly in the same latitude. Similarly, it is higher on the east than on the west side of continents, as is strikingly shown by Kamtchatka, 5249 feet, and Unalaschta, 3510 feet, situated respectively on the west and east coasts of the North Pacific.

430. South of the equator it rises very considerably from lat. 0° to 18°, and more so on the west than on the east slopes of the Cordilleras, owing to the small quantity of rain and snow which fall on the west side of these mountains. It is as high in S. lat. 33° as in N. lat. 19°; but south of this parallel it rapidly sinks, owing to the heavy rains precipitated by the north-west winds which there prevail ; so that in the south of Chili it is 6000 feet lower than at the same distance from the equator among the Rocky Mountains of North America, and 3000 feet lower than in Western Europe.

431. The mean temperature of the snow-line varies greatly from the equator to the poles, being at some places 35°, and at others as low as 20°. In the Swiss Alps it is about 25°, and in Norway about 23°.

HAIL

432. The hard pieces of ice which fall in showers are called hail. Hail is very different from snow, both in its formation and in the circumstances attending its precipitation.

433. Hailstones are generally of a conical or of a round shape, and when cut across are found to be composed of alternate layers of clear and opaque ice, enveloping a white snowy nucleus. Less frequently they are composed of crystals radiating from the centre outwards (figs. 43 and 44). The interior occasionally contains several nuclei, in which cases the hailstones appear to be a conglomeration of several hailstones, as in fig. 45, which represents one that fell at Bonn on 7th May 1822; the surface is rough, and in the case of the larger hailstones often bristling with small icicles.

434. Hailstones vary much in size, some being as small as

the smallest shot, while others are several inches in diameter. In August 1813, hailstones the size of eggs fell upon the

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British army in the Pass of Maga in the Pyrenees ; the storm lasted twenty minutes, and was not accompanied with thunder or lightning. On 4th June 1814, hail, from 13 to 15 inches in diameter, fell in Ohio. In the Orkney Islands, on the 24th July 1818, during thunder, a very remarkable shower of hail took place; the stones were as large as a goose's egg, and mixed with large masses of ice. In June 1835, hail fully three inches in circumference fell near Edinburgh from a dense cloud during a thunderstorm. On 8th May 1832, an immense mass of aggregated hailstones fell in Hungary, measuring about a yard in length, and nearly two feet in depth. A

mass nearly twenty feet in circumference, of an angular shape, and composed of lozenge-shaped pieces congealed together, is said to have fallen in Ross-shire, in August 1849. Masses of this sort are probably formed by regelation after the hailstones have fallen, by which their surfaces

are made to adhere together when Fig. 46.

rolled over each other by the

wind. 435. A hailstone (fig. 46) described by Captain Delcrosse as having fallen at Baconnière in July 1819, was 15 inches in circumference, and had a beautifully radiated structure, showing it to be a single hailstone. On the 8th August 1857 Professor Tyndall saw hail fall among the Alps in the form of perfect spheres of ice, just as if the rain-drops had solidified in their descent. Further particulars regarding these and other meteors will be found in Thomson's 'Introduction to Meteorology,'—a work replete with exact, learned, and curious information of every sort in the different departments of the science.

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436. No satisfactory theory of the origin of hail has yet been proposed which fully explains all the phenomena connected with hailstorms. Hail appears to be formed by a cold current of air forcing its way into a mass of air much warmer and nearly saturated, the temperature of the united mass being below the freezing-point. The warm moist air is easily accounted for, since hail generally falls in summer and during the day. The difficulty is to account for the intensely cold current which is sufficient to reduce the warm saturated mass below 32°.

437. In mountainous regions, cold currents from the fields of snow rushing down the sides of the mountains and mixing with the heated air of the valleys are no doubt frequent causes of hail; and we have seen that such places are peculiarly subject to hailstorms.

438. The sudden ascent of moist warm air into the upper regions of the atmosphere, where a cold current prevails at the time, is, in all probability, a common cause of hail. This is confirmed by the circumstances generally attendant on hailstorms—viz., the sultry, close weather which precedes them, the slight but sudden barometric depression, the whirlwinds and ascending currents which accompany them, and the fall in the temperature which follows after the storm has CHAPTER XI.

passed.

WINDS.

439. Wind is air in motion. The speed of the wind varies, from the lightest breath that scarcely stirs the leaf on its branch, to the hurricane which, sweeping on in its fury, lashes the ocean into a tempest, strikes down the stateliest trees of the forest, and levels even substantially-built houses with the ground.

440. The force of the wind is measured by anemometers, of which there are different sorts—some measuring the velocity, others the pressure. Of the anemometers which measure the velocity of the wind, the simplest and best is the Hemispherical-Cup Anemometer, generally called Robinson's Anemometer, fig. 47. It consists of four hollow hemispheres or cups screwed on to the ends of two horizontal rods of iron crossing each other at right angles, and supported on a vertical axis which turns freely. When placed in the wind, the cups revolve ; and the arms are of such a length that when a mile of wind has passed the anemometer, 500 revolutions are registered by the instrument. The accuracy of its construction may be tested by conveying it rapidly through the air on a perfectly calm day the distance of a mile and back again the same distance, and noting the number of revolutions made. The number of revolutions is registered by a system of indexwheels set in motion by an endless screw on the upright axis, which are read off in the same way as a gas-meter. The number of miles travelled by the wind during a day, an hour,

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