336 Notes on Dust Whirlwinds and Cyclones. [No. 4. 



sun-set, with the greatest regularity, when they are observed pro- 

 gressing Eastward, from the West, with considerable velocity, which 

 would lead to the supposition that, at this height, the prevailing cur- 

 rents, when not disturbed are always in that direction — the same thing 

 is often observed at sun-rise, only then, the clouds come up from the 

 West, and continue to increase for a time. 



21. — The peculiar manner in which the winds blow after a whirl- 

 wind of small diameter and swift progressive motion, is somewhat 

 remarkable, and deserves attention. Plant yourself in the direct line 

 of an advancing whirlwind, and allow it to blow over you. After it 

 has passed, there will generally be perceived more or less of a lull, or 

 the winds blowing in the direction of the track will be found light 

 and unsteady. This may continue for a minute or more, by which 

 time the whirlwind may have travelled onwards 150 or 200 yards. All 

 at once, the breeze freshens and blows steadily and with increased 

 force for a lengthened period ; after which, it may drop and then 

 again revive for a shorter period, becoming again unsteady, and by 

 degrees fainter, and then ceasing altogether. Though I have fre- 

 quently remarked the fact, I am not prepared to account for it, fur- 

 ther than to remark, that the light unsteady winds at first may be 

 occasioned by eddies caused by the rotatory action of the whirlwind 

 on the air more immediately adjacent, while the stronger winds may 

 be the oblique currents on either side of the track, meeting on the 

 line, and combining to produce increased power and velocity. 



More exact observations however are requisite to enable one acquaint- 

 ed with the theory of the motion of fluids to determine the real cause 

 of this striking phenomenon. 



22. — When whirlwinds are moving about, white patches of Cirro- 

 Cumuli are frequently seen on the clear blue sky, exactly resembling 

 flakes of teaseled cotton — having a rotatory motion throughout — 

 forming, and then rapidly dissolving, or ascending with whirling mo- 

 tions into the higher regions, becoming more and more faint as they 

 recede from the sight. 



