WINDS, AND WATER-SPOUTS.] 



METEOROLOGY. 



1137 



only in the day-time, and for the most part only lasts a 

 few hums. In Lower Egypt, the direction of the 

 simoom is from the south-west ; in Mecca, it comes from 

 the east ; in Surat, from the north ; in Bassoria, from 

 the north-west ; in Bagdad, from the west ; and in 

 Syria, from the south-east ; in every case, proceeding 

 from the neighbouring desert, where the air has suffered 

 rarefaction. The simoom, far from being poisonous, is 

 in some localities beneficial to health, by drying up 

 aqueous exhalations, which, if not removed, would give 

 rise to foyers and other diseases. This is particularly 

 the case on the western coast of Africa. It is, neverthe- 

 less, always injurious to vegetation. 



The Italian sirocco and the solano of Spain may be 

 regarded as European continuations of the harmattan 

 or simoom of Western Africa. The sirocco, although 

 usually restricted to Malta, Sicily, and Southern Italy, 

 sometimes extends into Germany and Switzerland : in 

 the Litter locality it is denominated the fuhn. The fdhn, 

 although prejudicial to trees, develops, to a surprising 

 degree, the vegetation of young plants, and can hardly 

 be regarded as a calamity. It is most prevalent in 

 Switzerland, near the Lake of the Four Cantons. Its 

 period of duration does not usually exceed a few hours, 

 though sometimes this period is exceeded ; and it rarely 

 occurs in winter. 



The southern part of Australia is subject to a hot 



north wind, presenting such a marked resemblance to 



the sirocco, that geographers are led to the natural in- 



iioe, that the unknown interior of the Australian 



1 1 lent U a desert of sand and rock, like the Sahara 



aii'l the wilds of Arabia Petnua. 



Cold Wtmb. These winds are less noticeable and 

 fewer in number than those already mentioned. Their 

 low temperature is usually referable to the circumstance 

 of their passing over mountain ranges covered with 

 snow. The most considerable winds of this kind exist 

 in Mongolia, Beloochistan, and the Russian steppes. 

 To this class also belongs the mi-ifnil, a north-east wind 

 prevalent in southern France, and which is exceedingly 

 prejudicial to vegetable life. 



Whirlwlnli. When two violent winds meet, the re- 

 sult is a whirlwind, HO called from its rotatory character. 

 If a whirlwind occur at sea, or over water, it elevates a 

 large column of water aloft, sometimes to the height of 

 in uiy hundred feet, thus giving rise to the meteor.'c 



Fi. 87. 



phenomenon termed a watcr-spnnt. If a whirlwind 



<> -cur mi laud, it lifr.s up dust, boughs, the roofs of houses, 

 nd Dtliur solid matters, producing a column of well- 

 defined shape. These whirlwind columns, whether they 

 VOL. 1 



consist of water or solids, present the same general for- 

 mation and contour. They consist 6f a hollow coae, 

 sometimes straight, but more frequently curved or 

 horn-shaped, its upper portion proceeding from a cloud ; 

 its lower part consisting of an aggregation of water or of 

 sand and dust, according to the locality. The upper 

 and lower portions of these columns are so much denser 

 than the remainder, that they are generally opaque, 

 whereas the middle portion is generally transparent. The 

 tint of these colours is various sometimes grey, some- 

 times brown or nearly black, and occasionally fiery red. 



Independent of the circular or axial motion of these 

 whirlwind columns, they pursue an onward course, some- 

 times straight, at other times curved. The velocity of 

 this course differs within wide limits. Sometimes a man 

 on foot can readily keep pace with it, whilst at other 

 times they proceed at the rate of nine or ten miles an 

 hour ; sometimes more. 



Whirlwind columns, whether they eventually become 

 water-spouts or not, always originate on laud, or in the 

 vicinity of land where the winds and temperature are 

 mutable. They are usually attended with thunder, 

 lightning, and other electrical phenomena ; and they con- 

 stitute the centre of an aerial commotion, all around the 

 focus of which, a profound calm prevails. Bodies which 

 they have taken up are not readily deposited, but carried 

 along in their onward course. Sometimes they are quite 

 in the clouds, at other times on the surface of the earth 

 or water ; and their formation may be prevented. Even 

 when already formed, they may frequently be destroyed 

 by some violent aerial commotion, such as that produced 

 by the discharge of a piece of ordnance a fact well 

 known to seafaring men. The size and height of these 

 whirlwind currents are various ; occasionally they present 

 a diameter of no more than two feet ; while the diameter 

 of some has been estimated at two hundred, or even 

 more. Again, the height of some is no more than 

 thirty feet ; whereas others have been known, the height 

 of which was no less than three thousand feet. 



WATER-SPOUTS. Of these columnar whirls, the water- 

 spout is l<-s-i damaging than the dry-whirl, probably be- 

 cause the weight of tlui I which it carries diminishes the 

 violence of its rotatory motion. Not the least oxtraor- 



Fig. 8. 



dinary amongst the many curious circumstances relative 

 to water-spouts, is the well-attested fact that, although 

 occurring at sea, they have been occasionally known to 

 brr.-ik, and deluxe a ship with a torrent of fresh water. 



Influence of Wind on the Baromiter. Although, the 

 barometer has hitherto boon considered in ruforenoe only 

 to the pressure of a tranquil column of air, its variations 



