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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
tion, and the barometer is said not to be 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 darkness. An enormous quantity of dust is whirled aloft, which is sometimes broken into distinct columns, each whirling on its axis. 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, arising probably 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. Sir S. W. Baker thus graphically describes the behaviour of the dust-whirlwinds which occur in Nubia in April, May, and June:-“I have frequently seen many such columns at the same time in the boundless desert, all travelling or waltzing in various directions, at the fitful choice of each whirlwind; this vagrancy of character is an undoubted proof to the Arab mind of their independent and diabolical character."
603. 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.
604. 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 axes, 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 of 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 which encounter them. 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 conclusively proved 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.
605. 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 motions they have been called “the merry dancers.” When fully developed, the aurora consists of a dark segment of a hazy or slaty appearance surmounted by an arch of light, at right angles to the magnetic meridian, from which luminous streamers quiver and dart towards the magnetic zenith. Careful measurements of several arches show that, except near the horizon, they may be regarded as portions of small circles parallel to the earth's surface. Near the horizon the extremities of the arches sometimes appear to be bent inwards; and Hansteen mentions having twice seen at Christiania the arch in the form of an ellipse almost entire. 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 stretching 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.
606. Auroras are very unequally distributed over the earth's surface. In the Smithsonian Report for 1865, Professor Loomis gives the results of a comprehensive examination of auroral observations, to which we refer for fuller information respecting this interesting meteor. In reference to the geographical distribution, he remarks that “At Havanna but six have been recorded within a hundred years. As we travel northwards from Cuba, auroras increase in frequency and brilliancy; they rise higher in the heavens, and oftener attain the zenith. If we travel northwards along the meridian of Washington (U.S.), we find on an average near the parallel of 40° only ten auroras annually. Near the parallel of 42°, the average number is twenty annually; near 45°, it is forty; and near 50°, it is eighty. Between this point and the parallel of 62°, auroras are seen almost in every night, high in the heavens, and as often to the south as the north. Farther north they are seldom seen except in the south, and from this point they diminish in frequency and brilliancy as we advance towards the pole. Beyond lat. 62°, the number of auroras is reduced to forty annually ; beyond lat. 67°, it is reduced to twenty; and near lat. 78°, to ten annually. If we make a like comparison for the meridian of St Petersburg, we shall find a similar result, except that the auroral region is situated farther northward than it is in America—the region of eighty auroras annually being found between the parallels of 66° and 75°. The region of greatest auroral action averaging at least eighty annually, is a zone of an oval form surrounding the north pole, and whose central line crosses the meridian of Washington in lat. 56°, and that of St Petersburg in lat. 71°. Accordingly, auroras are more frequent in the United States than they are in the same latitudes of Europe. On the parallel of 45° we find in North America an average of forty auroras annually, but in Europe less than ten. The form of this auroral zone does not bear any resemblance to the lines of equal magnetic intensity, but it bears some resemblance to the line of equal magnetic dip. It bears also considerable resemblance to a magnetic parallel, or line everywhere perpendicular to a magnetic meridian;