SAND WHIRLWINDS. 139 



residents that they often see them quite 1000 feet 

 high. On the Indian plains, and in Australia 

 also, large examples may; often be seen; so that this 

 phenomenon is by no means peculiar to the sandy 

 desert. 



It is not easy to determine the exact cause of their 

 formation. The traveller may himself be standing 

 at a spot where the sultry air is unruffled by a breeze ; 

 and yet he may behold several of these travelling 

 pillars of sand, moving slowly across the plain around 

 him; but if we might venture to hazard an opinion, 

 we should say that they are caused by ascending 

 columns of heated air, which gradually suck in ground 

 currents of the surrounding air to fill the vacuum created 

 there; as this movement progresses its action will tend 

 to increase in intensity until the inrush becomes ex- 

 ceedingly powerful. The gyrating movement, we 

 should judge, would then result from its meeting 

 opposing currents of air which would thus create an 

 eddy, exactly as a sharp curve, or other obstruction, 

 in a rapidly flowing river, will produce a backwater with 

 a species of miniature whirlpool. After a certain 

 time, when the forces which created these dust pillars 

 have spent their strength, their action becomes ex- 

 hausted, and the column of sand or dust collapses, 

 and falls in heavy showers to the ground. This, in 

 fact, is exactly what may be seen to occur, for the 

 collapse of these dust columns often takes place quite 

 suddenly, and without any apparent cause. 



The fall of the sand from these whirlwinds in former 

 days was much dreaded by travellers who encountered 

 them in the great deserts, where they occur in their 

 most serious form, for they were under the impression 



