144 On Dust Whirlwinds and Cyclones. [No. 2. 



spout and the dust whirlwind ; which are familiar instances of their 

 effects when passing over water or a dry sandy soil : but during a 

 storm, when the whole atmosphere is filled with dust, or aqueous 

 vapour, no such marked indication of their presence, is perceptible. 



On such occasions however, the peculiar motions of a vane, oscillat- 

 ing as it constantly does from 3 to 4 points, or more, during the pass- 

 ing gusts, marks plainly enough the action of these spirals. 



This peculiar motion of a vane during a storm, may perhaps be 

 accounted for, by supposing that the electrical whirls or the eddies 

 caused by them in passing, strike it on one side, and twist it round to 

 a certain distance ; when it is immediately brought back to its original 

 position in the direction of the storm's course, by winds that closely 

 follow after, excited by the passage of the electrical whirl through the 

 air, setting it in motion, and causing winds, blowing with more or less 

 obliquity to a certain distance on either side of the track of the spiral, 

 just as we observe still water is affected by a solid body drawn through 

 it. 



This phenomenon I have invariably found to accompany the passage 

 of dust whirlwinds over a vane, and as it is presumed the active por- 

 tion of rotatory storms, (and probably of all storms,) is composed of 

 a mass of swiftly moving spirals of a similar nature, the same effect 

 on the surrounding air, observable in the small whirls, will likewise be 

 produced on a much larger scale, in every variety of Cyclone or 

 Tornado. 



The combined action of both forces, viz. the spiral motion of the 

 body of the storm, or electrical zone, gyrating onwards and from above 

 downwards, and its local effect upon the air through which it passes 

 will produce a curved progressive motion in the winds, taken as a 

 whole, as described by Colonel Reid and Mr. Piddington — and ships 

 caught in its vortex, may be impelled round and round with the body 

 of the storm, as was proved long ago by the latter gentleman to have 

 happened to the brig Charles Heddle in the Mauritius Hurricane of 

 1845. 



Reflecting on the spiral working of the storm throughout, it is easy 

 to conceive why the central portion of it, should be so much more 

 violent, than at the outer margin, and why the incurving winds and 

 powerful vortices, so marked thereabouts, render the condition of a 



