92 HUGH CHARLES KIDDLE. 
whirls may pass over clean grass without any visible vortex column, 
and that their aspect depends very much upon the nature of the 
loose material they pass over, for instance, if one cross a dusty 
road, instantly the vortex is outlined in dust swept up from the 
road ; if dry loose grass or leaves be in its path, the grass or leaves 
serve to make evident to the eye what is going on. Their appear- 
ance then, depends firstly, upon the matter they gather up, and 
secondly, upon the overhead wind current, as will appear farther 
on. I take it for granted that those whirls that are seen during 
calm weather may be taken as a type of the others, so I will describe 
the appearance of one as seen on a dusty road. The base is 
generally about two feet in diameter, and the column ascends to 
a height varying from forty to one hundred feet or more, as 
measured by the eye only. These whirls seldom remain stationary, 
however, as even in apparently calm weather, there is a slow pro- 
gressive motion. After the dust, or other loose material has been 
passed, and the supply from below thus cut off, the head of the 
column is still visible, revolving the particles of dust etc., till they 
are carried out of sight. 
When there is a light or moderate breeze blowing, these whirls 
-have a progressive motion, estimated at a rate as high as twenty 
miles an hour, during a moderate to fresh breeze. ‘The column is 
not so clearly defined as in calm weather, being shorter and broader 
in appearance, and, as a rule, not exceeding twenty or thirty feet 
in height. At this altitude the head of the whirl appears to be 
carried forward bodily and broken. This may be accounted for 
by assuming that at a certain height the directive force of the 
wind is greater than the rotatory force of whirl about its axis, | 
consequently the particles of dust etc. in the head of the whirl 
are carried forward, and present a broader appearance as they are 
(again) scattered by the wind. 
This widening out at the head is not characteristic of the “windy 
weather” whirl only, as I have observed some whirls, and large 
ones too, during calm weather, which presented the same appear- 
ance. These appeared to be about two feet in diameter atthe 
