Whirlwinds and Cyclones 233 
surrounded by trees, and currents of air were prevented. 
They lasted several minutes, slowly moving across the plain, 
like great pillars of smoke.! 
When attentively watched from a short distance, it was 
seen that as soon as one was formed, the air immediately 
next the heated soil, which was before motionless, or quiver- 
ing as over a furnace, was moving in all directions towards 
the apex of the dust-column. As these currents approached 
the whirlwind, they quickened and carried with them loose 
dust and leaves into the spiral whirl. The movement was 
similar to that which occurs when a small opening is made 
at the bottom of a wide shallow vessel of water: all the 
liquid moves towards it, and assumes a spiral movement as 
it is drawn off. 
The conclusion I arrived at, and which has since been 
confirmed by further study of the question, was, that the 
particles of air next the surface did not always rise immedi- 
ately they were heated, but that they often remained and 
formed a stratum of rarefied air next the surface, which was 
in a state of unstable equilibrium. This continued until the 
heated stratum was able, at some point where the ground 
favoured a comparatively greater accumulation of heat, to 
break through the overlying strata of air, and force its way 
upwards. An opening once made, the whole of the heated 
air moved towards it and was drained off, the heavier layers 
sinking down and pressing it out. Sir George Airey has 
suggested to me that the reason of the particles of air not 
rising as they are heated, when there is no wind blowing, 
may be due to their viscosity: and this suggestion is correct. 
That air does not always rise when heated, appears from the 
hot winds of Australia, which blow from the heated interior 
towards the cooler south, instead of rising directly upwards. 
Sultry, close weather, that sometimes lasts for several days, 
would also be impossible on the assumption that air rises as 
soon as it is heated. 
This explanation supplies us with the force that is necessary 
1A friend of mine tells me that he saw a similar whirlwind rise at 
noon one still summer day, and traverse the dusty road on the Chesil 
Bank between Portland and Weymouth. It travelled fully half a 
mile, about as fast as he could walk; and the point where it met the 
ground was not thicker than his walking stick. By and by it swept 
out to sea, where the dust gradually fell. 
