DUST and Dust-storms.
Poussiere, Poudra, • Fs. I Gird, Staub, Ohrfeige, . GER. Polvere, . . . Ir. Khak, Pesbash, . TURK.
Dust is earried along with winds to great dis tances. Sirocco or African dust has been found by the microscope to consist of infusoria and organ isms whose habitat is, not Africa but S. America, and carried in the track of the S.E. trado wind of S. America. In tho dust of the Cape Verdes, Malta, Genoa, Lyons, and the Tyrol, Ehrenberg discovered separate forms. Dust is blown from Arabia and Africa far to seaward, causing a great haze. During four months of the year, a largo quantity of dust is blown from the N.IV. shores of Africa, and falls on the Atlantic over a space of 1600 miles in latitude, and for a distance of from 300 to GOO miles from the coast. But dust has been seen to fall at a distance of 1030 miles from the shores of Africa. Darwin mentions that in some of the dust 330 and 380 miles from Africa, falling in the sea near the Capo de Verde Islands, particles of stone occur 1000th of an inch square. Dust-storms aro very fre quent in India, and usually have et north to south course. One commenced at Allababad about seven A.M., and continued till one P.M., retaining the same fury as when it began. On the evening of the 17th, Secunderabad had been visited with an unusually severe dust-storm. It came from the N.W. and was accompanied by lightning and thunder. The air to a consider able height was rendered almost opaque by dense clouds of red dust. The wind raged with great fury for upwards of half an hour, and on its abating was followed by a heavy shower of rain. A dust-storm passed over !Madras on Sunday the 19th, beginning at one P.M. It had passed over Kristnapatam, seventeen miles S.E. of Nellore, at half-past ten o'clock in the forenoon of that day, accompanied by a slight fall of rain. In the north of the district between Ongole and Ramapatarn, there was a heavy fall of rain in the forenoon of Sunday, averaging from two to four inches. At Chingleput, thirty-six miles south of Madras, the storm was experienced in full force at that station at two P.M. the same day. It came from the N.W., and the wind was laden with vast quantities of reddish dust ; no refreshing shower succeeded the storm.
A dust-storm occurred over 3800 square miles, from Ningpo to Shang-hai, on the 15th March 1846. It consisted of a congeries of light downy fibres or hairs, with silex adhering to them, and an admixture of an alkaline salt. In China, ac cording to Richthofen, beds appearing like fine sediment, several hundred feet in thickness, and extending over an enormous area, owe their origin to dust blown from the high lands of Central Asia.
1Vhirling dust-storms aro caused by spiral columns of tho electric fluid passing from the atmosphere to the earth ; they have an onward motion, a revolving motion, like revolving storms at sea, and peculiar spiral motion from above dovntwards, like a corkscrew. It seems probable that in an extensive dust-storm there are many of these columns moving on together in the same direction, and during the continuance of tho storm many sudden gusts tako place at intervals, during which tiine the electric tension is at its mammum. These storms, in the Panjab, mostly commence from N.W. or IV., and in the course of an hour, more or less, they have nearly completed the circle, and have passed onwards. Precisely tho samo phenomena, in kind, are observable in all casoi of duat-storms ; from the ono of a few Inches in diameter, to those that extend for fifty milei and upwards, tho phenomena are identical. It is a
curious fact that some of the smaller duat-storms occasionalIT seen in extensive and arid plaina, both in the Panjab and in Afghanistan above the Bolan pass, called in familiar language devils, are either stationary for a long time, that is, upwards of an hour, or nearly so, and during the whole of this time the dust and minute bodies on the ground aro kept whirling above into the air; in other cases, these tunall dust-storms are seen slowly advancing, and when numerous, usually proceed in the same direction. Birds, kites, and vultures are usually seen soaring high up just above thesis spots, apparently following the direction of tho column. They may be looking for prey, or in volved in and unable to fly out of, the invisible part of the electrified tfdrial column, of which the lower part only is visible to ns by tho dust raised. Tho phenomena connected with dust-storms seem to be identical with those present in waterspouts and white squalls at sea, and revolving storms and tornadoes of all kinds; and they apparently origi nate from the same cause viz. movinn. columns of electricity. In 1847, at Lahore, an ot:erver, being desirous of ascertaining tho nature of dust-storms, projected into the air an insulated copper wire on a bamboo on the top of his house, and brought the wire into his room, and connected it with a gold-leaf electrometer and a detached wire com municating with the earth. A day or two after during the passage of a small dust-storm, he had the pleasure of observing the electric fluid passing iu vivid sparks from one wire to another, and of course strongly affecting the electrometer. After wards, by tho same means, he observed at least sixty dust-storrns of various sizes, all present ing the same phenotuena in kind. Commonly, towards the close of a storm of this kind, a fall of rain suddenly takes place, and instantly the stream of electricity ceases, or is much diminished; and whetf it continues, it seems only on occasions when the storm is severe and continues for some time after. The barometer steadily rises throughout. In the 'Punjab plains, the fluctuation of the barometric coluum Is very slight, seldom more than two or three tenths of an inch at a time. The average height at Lahore is 1180, corrected for temperature, indicating, it is supposed, above 1150 feet above the level of the sea, taking 30 inches as the standard. A large dust-storm is usually preceded by certain pecu liarities in the dew-point, and the manner in which the particles of dew are deposited on the bulb of a thermometer. The mode of taking the dew point is to plunge a common thermorueter in a little ice, and let it run down 20° or 30°. The manner in which tho electricity acts upon the dust and light bodies it meets with in its passage, is siuiple enough. The particks are aimilarly electri fied aud mutually repulsive, and then, togethe-r with the whirling motion communicated to them, are whisked into the air. The earn° takes place when the electricity moves over water. The sur face of tho water becomes exposed to the electric agency, and its particles, mndered mutually repulsive, are in the aline way whirled into the air. At sea the waterspout is thus fortned. First of all is seen the cloud descending, and beneath may be observed the water in a cone.—Bengal Asiatic Soc. Journal, No. v. of 1850, p. 790 ; Darwin, p. 239.