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 man no storm had occurred fraught.with so disastrous results.

471. On the 28th July 1818 a hailstorm passed over Orkney, its course being marked for twenty miles, the breadth being only a mile and a half. 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 the greatest rapidity, traversing the distance in less than nine hours. The length of the one 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.

472. 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.

473. 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 the 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 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.

Whirlwinds and Waterspouts or Trombes.

Juli

474. 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 be either way according to the direction of the stronger of the two winds which give rise to it. Thus, suppose a

Fig. 46.

whirlwind produced by the brusling of a north with 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.

475. Whirlwinds from a different cause 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. 46 and 47 represent two of these remarkable phenomena. The large arrows in fig. 46 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. 47 shows the general appear

ance 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 direction, and the barometer is not perceptibly affected by it. A low bank of dark cloud is seen in the horizon, which rapidly increases, and before the spectator is aware the storm bursts upon him, wrapping everything in midnight dark

An enormous quantity of dust is whirled aloft, which is sometimes broken into distinct columns, each whirling on its axis.

ness.

Violent gusts or squalls succeed each other at intervals, which gradually become weaker, and at the close of the storm a fall of rain generally takes place. The air is often highly electrical from the friction of the dust-loaded currents against each other. The simoom may be regarded as in part a whirlwind or a succession of whirlwinds of this description.

476. Extensive fires, such as the burning of the prairies in America, and volcanic eruptions, also cause whirlwinds by the upward current produced by the heated air; and these, as well as the other whirlwinds already mentioned, are occasionally accompanied with rain and electrical displays.

477. Waterspouts.—Waterspouts are whirlwinds occurring over the sea or over sheets of fresh water. When fully formed they appear as tall pillars stretching from the sea upward to the clouds, and exhibiting the same whirling motion round their axis, and the same progressive movement of the mass, as the dust-whirlwinds. As they consist of vortices of wind in rapid motion, the sea is tossed into violent agitation round their bases as they career onwards, the waves being broken up so as to resemble the surface of a glacier or water in rapid ebullition. The danger arising from them consists in the enormous velocity of the wind, and the sudden changes in its direction experienced by ships in passing through then. It is a popular fallacy that the water of the sea is sucked up by them, it being only the spray from the broken waves that is carried up the whirling vortex ; this is shown by the fact that the water poured down on the decks of vessels from waterspouts is either wholly fresh, or only slightly brackish.

Aurora Borealis and Terrestrial Magnetism.

478. The aurora borealis is the luminous appearance in the northern sky, which forms, in most vivid displays, spectacles of surpassing beauty. The aurora is observed also in the neighbourhood of the south pole, and is there called aurora australis. From their lively tremulous motion they have been called “the merry dancers.” When fully developed, the aurora consists of a dark segment, surmounted by an arch of light, at right angles to the magnetic meridian, from which luminous streamers quiver and dart toward the zenith. Several auroral arches are sometimes seen at once. Thus, on one occasion at Bossekop, in Finland, nine arches were visible, separated by dark intervals, and resembling in their arrangement magnificent curtains of light hung behind and below each other, their brilliant folds stretch

ing completely across the sky. Sometimes the streamers appear to unite near the zenith, forming what is called the corona of the aurora, towards which the dipping needle at the time points. The convergence of the rays in this case is only apparent, being merely the effect of perspective.

479. The aurora is of great extent, having been sometimes observed simultaneously in Europe and America ; and their height has been estimated variously up to 300 miles above the earth. From observations made on one which appeared in England during March 1826, Dalton calculated its height at 100 miles. Sir John Herschel determined the height of one seen on the 9th March 1861 by himself in Kent, and at the same time by Mr Lowe at Nottingham, to be eighty-three miles. Of auroras which have been seen near the earth, the one thus described by Captain Parry is the most remarkable :-“While Lieutenants Sherer, Ross, and myself were admiring the extreme beauty of the northern lights, we all sirnultaneously uttered an exclamation of surprise, at seeing a bright ray of aurora shoot suddenly downward from the general mass of light, and between us and the land, which was there distant only three thousand yards. I have no doubt that the ray of light actually passed within that distance of us.”

480. The aurora is not often seen in summer, partly, no doubt, on account of the short nights and clear skies. There is, however, a double maximum and minimum occurrence in the year, which the following table from Kaemtz, giving the number of auroras seen in each month, clearly establishes :

In addition to this annual period, there would appear to be a secular period comprising a number of years. One of these periods extended from 1707 to 1790, attaining its maximum about 1752. After this a period of twenty years intervened, during which they became rare ; but from the year 1820 they have become

more numerous.

481. The culminating point of the auroral arch being in the magnetic meridian, and the centre of the corona in the line of the dipping-needle produced, point out an evident connection between the aurora and terrestrial magnetism. The magnetic needle is also much agitated during aurora. When the arch is motionless, so is the needle; but as soon as streamers are shot out, its declination changes every moment, and this happens though the