374 Report on the Dust Whirlwinds, [No. 4, 



gusts of wind from various directions prevailed, and at times with 

 rain. In the evening, the horizon seemed encircled by sheet light- 

 ning, and heavy rain fell. The 13th was cool, and the early part of 

 the day clear. About 4 p.m. the whole circumference of the horizon 

 became dark, wind came on from W. by N. but no thunder was 

 audible. 



The 14th was characterized by irregular gusts of wind, at times 

 carrying along with them masses of dust, but the evening was clear. 



The above notes are entered here as showing the description 

 of weather that generally succeeds for a short time the occurrence 

 of storms. 



17. — 20th May 1 853. Since the occurrence of the last storm, the 

 western horizon has continued to present a cloudy appearance every 

 afternoon at sunset ; cirri and strati appearing near the earth in 

 the afternoon, but clearing away again during the early night. At 

 4, 30 p. M. of 20th a heavy cloud of dust of unequal density was 

 seen approaching from S. W., the dust as it were in pillars, with 

 intermediate spaces of comparative clearness. They had not how- 

 ever that appearance of violent agitations that characterizes most 

 cyclones on land, and the summits of the pillars appeared lost in 

 cumulus-like clouds heavily surcharged with dust. The force of 

 the wind was not very violent, nor did the direction of it vary 

 during the hour the storm lasted. The diameter of the cyclone 

 extended from S. E. to W. a few flashes of forked lightning were 

 visible, and a few peals of thunder were heard. As the storm of 

 wind passed over the station in a N. E. direction rain began to fall in 

 torrents and afterwards continued so during the night. 



In the notes taken on the spot during the prevalence of the above 

 storm it is stated that " it would seem as if the above cyclone being 

 about to break up, had lost its circular motion* before reaching the 



* This is of course merely theoretical, yet, it is quite easy to imagine how such 

 a circumstance might take place. If, for instance, the individual spiral currents, 

 whose existence may be said to be definitely ascertained, were, from a disturbance 

 in the balance of the particular agencies on which they depend, to be for a time 

 more under the influence of those that give such meteors their onward course, 

 than those in obedience to which they are made to revolve on their own axes, the 

 latter motion would be retarded by the former, and would soon altogether cease. 



