CAUSES OF SAND PILLARS. 143 



the rapidity of the whirl, and the strength of the 

 downward rush, produce a loud sucking noise, caused 

 by the air forcing its way down, through the middle 

 of the \vater, which then forms, as we can easily see, 

 a species of hollow cylinder of fluid. 



We should therefore be inclined to hold, that in all 

 probability these moving and gyrating columns of dust 

 are, in the same way, not hollow, but motionless, and 

 perhaps also free from dust, at their centres; while 

 the dust particles revolve with enormous velocity 

 around this still centre. We have no actual proof at 

 hand to show that this is so, but such is our personal 

 belief because, in the case of rotatory cyclones affect- 

 ing a large area, this still portion is almost invariably 

 present at their centres, and may have a diameter of 

 several miles. 



Then, as to the next, or horizontal form of these 

 air-jets, those are merely another modification of the 

 above phenomenon; only the axis of the propelling 

 force is approximately parallel to the terrestrial surface ; 

 but in this case, as we believe, the whirl is to a 

 great extent neutralized by the attraction and obstruc- 

 tion of the immovable surface of the solid earth. 

 Nevertheless, that it exists in a modified form is 

 clear from the violent way in which masses of sand, 

 etc., are scooped up and whirled aloft. We remember 

 on one occasion witnessing a storm of this kind from 

 the deck of a vessel in the Red Sea, near its north 

 western end. An enormous cloud appeared sweeping 

 along the coast in a southerly direction, evidently 

 consisting of dust and sand, for the sky was at the 

 time elsewhere cloudless, and the atmosphere at sea 

 a steady pleasant breeze; whereas this dust storm 



