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be to a large extent determined by the configuration and vegetable covering of the earth's surface. They seldom occur during the night or during winter; but most frequently in summer, and during the hottest part of the day. They are most frequent within the tropics, less so in temperate climates, unless in the vicinity of mountains, and still less in the arctic regions. They are more common in the neighbourhood of mountains than in plains. Thus the south of France, lying between the Alps and the Pyrenees, suffers much from hailstorms, the vines being often broken and destroyed by their violence. The annual loss from this cause has been estimated at above two millions sterling. So important is a proper knowledge of these and similar storms considered in France, that the services of upwards of 1200 observers have been secured in that country to note the chief features of storms (orages). The observations are transmitted to the Imperial Observatory at Paris, and the results published in a magnificent atlas of storm-charts. The following storm is one of the most remarkable.
597. On the 7th May 1865 the north-east of France was visited by a hailstorm of unwonted violence, which has since been called the Storm of Câtelet, the place where it was most severe. It was preceded by six weeks of unusually dry and warm weather; after which, and for a few days before the storm, the temperature became quite scorching, and several times storms appeared to be forming, but they all passed off without hail or rain. At seven in the morning of the 7th the air freshened and the barometer rose. A wind from N.E. chased light clouds before it with astonishing speed; but higher up a S.W. wind prevailed, bearing slowly before it woolly-looking clouds, which grew denser towards noon. Still the air was calm at the surface of the earth, and continued so till three P.m., when dense heavy clouds, piled on each other, rose out of the south-west, and thunder began to be heard. Below, the mass of cloud was of a pale livid colour, out of which lightning darted continually; above, were many layers or banks of sombre-tinted clouds, forming a broad base to the lower electrically-charged cloud, which resembled an inverted pyramid. The storm ascended the valley of the Somme, but little damage was done till it had crossed the heights and descended into the valley of the Escaut, where it fell with terrible violence on Vend’huile, le Câtelet, and Beaurevoir. At Câtelet the hailstones were as large as pigeon's eggs, and some even appeared as large as hen's eggs; but these latter, on examination, were found to be composed of several hailstones rolled together. Next morning, at one place the hail had piled together a mass of ice which was upwards of a quarter of a mile in length, 22 yards in breadth, and at certain points fully 16 feet in height; and at another place there was a similar mass of hailstones one mile and a quarter in length, and a furlong in breadth, which was not fully melted till after the 13th of the month. The damage done by this storm was immensethe tiles on the roofs of houses, the glass, and even the sashes of windows, were smashed to pieces ; a mill was levelled with the ground; large trees were torn up by the roots ; and crops of rye, barley, and wheat were beaten to the ground and destroyed. In the memory of the inhabitants of that part of France no storm had occurred fraught with such disastrous results.
598. This class of storms are found in France to be invariably bound up, or associated, with barometric depressions, and their general direction is influenced as is that of the wind in the same circumstances. The general direction in which they advanced in 1865 was from S.W. to N.E.; and otherwise on this point they resembled the storms of Europe already described. But when they were confined to the lower parts of the atmosphere they were diverted from their course on coming up against high table-lands and mountain-ranges much in the same way apparently as rivers are when high banks oppose their course. The rate of their progressive motion varied from 12 to 45 miles an hour, thus showing in this respect also a close correspondence with the larger storms of Europe.
599. On the 28th July 1818, a hailstorm of unusual violence passed over Orkney, its course being marked over a district twenty miles in length by one and a half in breadth. It did not last longer at any place than nine minutes, during which 9 inches of ice fell. On the 13th July 1788, a hailstorm passed directly from the south-west of France to Utrecht. It moved in two parallel columns with very great rapidity, traversing the distance in less than nine hours. The length of the one column was 435 miles, and of the other 497 miles, and the breadth respectively 5 and 10 miles. Between the two tracts there was a space of twelve miles, where no hail, but heavy rain, fell. At each place the storm lasted only a few minutes; and along its course property valued at above a million sterling was destroyed.
600. In rainy weather, such as frequently occurs in March and September, when, between the intervals of sunshine, a cloud appears in the west, overspreads the sky, and as it passes pours down a considerable quantity of rain, if the barometer be watched from the time of the cloud's appearance in the west to its disappearance in the east, it will be observed to fall a little and then rise, the fall being of a strictly local character. This may be considered typical of the class of storms under discussion. Though no barometric observations are recorded during the thunderstorms of France in 1865, it is highly probable that a local barometric depression accompanied each storm in its course. In all cases before the storm the air is close and sultry and highly charged with moisture. These storms may thus be regarded as secondary storms or sub-storms within the area of the more general storm or atmospheric disturbance passing over Europe at the time, with the wind in all probability circling round them as they are carried forward in the larger storm. Barometric and wind observations made almost every minute during thunderstorms would go far to explain their true character. For registering these small barometric fluctuations so as to arrive at some explanation of gusts of wind, heavy showers, and
some other weather-changes, King's self-recording barometer, in use at the Liverpool Observatory, is the best.
WHIRLWINDS OR WATERSPOUTS AND TROMBES.
601. Whirlwinds are in several respects very different from the storms already described. They seldom last longer than a minute, sometimes only a few seconds; their breadth varies from 20 to a few hundred yards; their course seldom exceeds 25 miles in length; and while they last the changes of the wind are sudden and violent. The direction of the eddy of the whirlwind, especially when of small diameter, differs from the rotation of the winds in a storm, in that it may take place either way according to the direction of the
Fig. 51. stronger of the two winds which give rise to it. Thus, suppose a whirlwind produced by the brushing of a north
against a south wind, then if the north wind be the stronger, and on the west, the whirl will be in the direction of the hands of a watch, but if the south wind be the stronger the eddy will turn in the opposite direction.
602. Whirlwinds are often originated in the tropics during the hot season ; especially in flat sandy deserts, which, becoming unequally heated by the sun, give rise to numerous ascending columns of air. In their contact with each other, these ascending currents give rise to eddies, thus producing whirlwinds which carry up with them clouds of dust. Of this description are the dust-whirlwinds of India, which have been described and profusely illustrated by P. F. B. Baddeley. Figs. 49 and 50 represent two of these remarkable phenomena. The large arrows in fig. 49 show the rotation of the whole whirlwind round its axis, while the small arrows show the rotation of each column round its own axis. Fig. 50
shows the general appearance of these dust-whirlwinds viewed at a distance. A dust-storm is occasioned by a number of whirlwind columns moving together over the earth's surface. The storm generally comes on without warning from any direc