1854.] Report on the Dust Whirhvinds. 377 



the extremities of which, it will be observed approximate much more 

 closely than those of fig. 1). 



My private notes go on to say, — " It appeared therefore that the 

 storm was only commencing its course at this station, that the 

 minor currents whose motion was circular, as shown in the chart 

 have not yet extended so far along the circuit line as to form a com- 

 plete circle, but were, when observed, in progress to do so. 



It also explained the phenomenon of occasional dense columns of 

 dust separated by comparatively clear spaces that are seen in almost 

 every storm, and which is endeavoured to be represented in the sub- 

 joined sketch in which the figures 1, 2, 3, 4 and 5, represent the 

 smaller circular currents the general circuit line of which is repre- 

 sented by large arrows, the figures 6, 7, 8 and 9, indicating the spaces 

 intervening between these currents. 



It appears self-evident that as the circumference of two or more 

 adjoining spirals only tonch each other at a comparatively small 

 part, such particular point of union must be less obscured by the 

 floating dust and hence, so much more transparent than those parts 

 where large quantities of impalpable sand, &c. are revolving as repre- 

 sented by the small arrows. 



The onward course of this storm is represented on the chart by 

 the large arrow. The cyclone was attended by some thunder, but 

 no fall of rain took place, nor was the violence of the wind consi- 

 derable ; and by 8| p. m. it had passed completely over the station, 

 rendering the temperature very agreeable. 



The above storm is one of a very interesting character, presenting 

 various points of dissimilarity from any hitherto observed. Some of 

 the points we gather from it are 



a. The minor spirals of a storm may arise together, and attain 

 their onward progress before the whole circumference of such cy- 

 clone has been completed by their lateral extension. 



b. The circular movement of the atmosphere was from E. to L. 



c. The appearance of the storm being much like what is repre- 

 sented in the sketch shows that such are in reality formed of spirals 

 as already adduced. 



21. — 20th June, 1853. Cumuli and strati had partially covered 

 the sky during the day. At 6 p. m. thunder in the W. was audible, 



