302 THE NATURALIST IN NICARAGUA. [Ch. XVI. 



But even if this fundamental objection to the theory 

 could be set aside, the small whirlwinds 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 

 contrary currents, are continued and intensified by the 

 condensation of vapour in their vortex forming a vacuum, 

 we find it negatived by the fact that in the smaller 

 whirlwinds the air is diy, and there is consequently no 

 condensation of vapour ; and 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 con- 

 densation of vapour. I myself have seen scores of 

 whirlwinds in Australia, some of them rising to a height 

 of over one hundred feet ; yet there was never any per- 

 ceptible 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. 



* " Analiuac," by E. B. Tylor, p. 21. 



