304 THE NATURALIST IX NICARAGUA. [Ch. XVI. 



during the heat of the day, there were often two at once 

 in action in different parts of it. They were only a few 

 yards in diameter, but reached to a height of over one 

 hundred feet, and were often, in their higher part, bent 

 out of their perpendicular by upper aerial currents. The 

 dust and leaves they carried up rendered their upward 

 spiral movement very conspicuous. No one who studied 

 these whirlwinds could for a moment believe that they, 

 were caused by conflicting currents of air. They occurred 

 most frequently when there was least wind ; and this 

 particular plain seemed to be peculiarly suitable for their 

 formation, because it was nearly surrounded by trees, 

 and currents of air were prevented. They lasted several 

 minutes, slowly moving across the plain, like great pillars 

 of smoke. 



When attentively watched from a short distance, it 

 was seen that as soon as one was formed, the air 

 immediately next the heated soil, which, was before 



V 



motionless or quivering, like over a furnace, was moving 

 in all directions towards the apex of the dust- column. 

 As these currents approached the whirlwind, they 

 quickened and carried with them loose dust and leaves 

 into the spiral whirl. The movement was similar to that 

 which occurs when a small opening is made at the 

 bottom of a wide shallow vessel of water : all the liquid 

 moves towards it, and assumes a spiral movement as it 

 is drawn off. 



The conclusion I arrived at, and which has since been 

 confirmed by further study of the question, was, that the 

 particles of air next the surface did not always rise 

 immediately they were heated, but that they often 

 remained and formed a stratum of rarified air next .the 



