48 Mr. T. Belt on the Origin of Whirlwinds. 



but I believe I shall not incur the charge of going beyond the 

 limits of legitimate speculation. 



Every resident in Australia must have observed, during the 

 hot season of the year, eddies of air carrying up dust, leaves, and 

 other light substances to a great height, appearing at a distance 

 like moving columns of dust. Though only a few yards in dia- 

 meter, they are of great violence, often unroofing or overturning 

 the shght tents of the gold-seekers. The dust and leaves they 

 carry up render their upward spiral motion very conspicuous. 

 The columns sometimes remain stationary, but generally they 

 have a regular horizontal movement. Clouds of dust envelope 

 their base, out of which they rise to a considerable height, often 

 bent out of their perpendicular by upper aerial currents. They 

 are especially frequent on the level plains, where, from the 

 absence of trees, the rays of the sun exert great power. 



Small whirlwinds or eddies of air are not pecuhar to Australia. 

 Humboldt speaks of some observed by him on the -llanos of South 

 America, and ascribes them to the meeting of opposing gusts of 

 wind. The vertical columns of sand seen by Clarke on the 

 steppes of Russia, and by Bruce over the deserts of Africa, are 

 similar phsenomena. They sometimes, but rarely, occur in En- 

 gland, carrying up loose hay and other light substances, and 

 scattering them over the surrounding country. Franklin de- 

 scribes a whirlwind that he witnessed in Maryland, which began 

 by taking up the dust that lay on the road in the form of an 

 inverted sugar-loaf, and soon after grew to the height of forty or 

 fifty feet, being twenty or thirty in diameter. It advanced in a 

 direction contrary to the wind ; and although the rotatory motion 

 of the column was surprisingly rapid, its onward progress was so 

 slow as to allow a man to keep pace with it on foot. Franklin 

 followed it on horseback, and saw it enter a wood, where it 

 twisted and turned round large trees ; boughs and leaves were 

 carried up so high, that from their height they were reduced to 

 the apparent size of flies. This last, though a much more violent 

 whirlwind than those experienced in Australia, is strictly analo- 

 gous to them. 



If those eddies of air are attentively observed, it will be per- 

 ceived that currents of air are moving along from all sides 

 towards the lower apex of the column. The temperature of 

 the air next the surface is sensibly diminished by their action. 

 Often when travelling over the parched plains, I have seen the 

 air quivering over the hot ground as from a furnace ; suddenly 

 (within a few paces perhaps) a miniature storm has arisen ; and 

 when after a few minutes' violence it has as suddenly ceased, 

 the quivering of the air has been no longer perceptible, and the 

 atmosphere has felt less oppressive : again and again the same 



