Whirlwinds and Cyclones 231 
considers that they are continued and intensified by the 
vapour condensed in their vortex forming a vacuum. 
Humboldt had long ago ascribed whirlwinds to the meet- 
ing of opposing currents of air.? There is this dynamical 
objection to the theory. The movements of the air in whirl- 
winds are much more rapid than in any known straight 
current, such as the trade winds; and it is impossible that 
two opposing currents should generate between them one of 
much greater force and rapidity than either. If force A joins 
with force B, surely force C, the product, must have the 
power of both A and B. But even if this fundamental 
objection to the theory could be set aside, the small whirl- 
winds could not thus arise, as they are most frequent when 
the air is nearly or quite motionless. 
Then, again, when we turn to Prof. Maury’s theory that 
the cyclones, having been initiated by the conflict of con- 
trary currents, are continued and intensified by the con- 
densation of vapour in their vortex forming a vacuum, we 
find it negatived by the fact that in the smaller whirlwinds 
the air is dry, and there is consequently no condensation of 
vapour; yet, in comparison with their size, they are of as 
great violence as the fiercest typhoon. Tylor describes the 
numerous dust whirlwinds he saw on the plains of Mexico,? 
Clarke those on the steppes of Russia, and Bruce those on the 
deserts of Africa, and nowhere is there mention made of any 
condensation of vapour. I have seen scores of whirlwinds 
in Australia, many rising to a height of over one hundred 
feet; yet there was never any perceptible condensation of 
vapour, though some of them were of sufficient force to tear 
off limbs of trees, and carry up the tents of gold-diggers into 
the air. Franklin describes a whirlwind of greater violence 
than any of these. It commenced in Maryland by taking 
up the dust over a road in the form of an inverted sugar-loaf, 
and soon increased greatly in size and violence. Franklin 
followed it on horseback, and saw it enter a wood, where it 
twisted and turned round large trees: leaves and boughs 
were carried up so high that they appeared to the eye like 
flies. Again there was no condensation of vapour. 
1 Quarterly Journal of Science, 1872, p. 418. 
2 Aspects of Nature, vol. i. p. 17. 
3 Anahuac, by E. B. Tylor, p. 21. 
