230 The Naturalist in Nicaragua 
minute a spiral column was formed, reaching, perhaps, to the 
height of fifty feet, consisting of dust and dry dead leaves, all 
whirling round with the greatest rapidity. The column was 
only a few yards in diameter. It moved slowly along, nearly 
parallel with our course, but only lasting a few minutes. 
Before I could point it out to Velasquez, who had ridden on 
ahead, it had dissolved away. I had been very familiar 
with these air eddies in Australia, and had hoped to carry on 
some investigations concerning them, begun there, in Central 
America; but, though common on the plains of Mexico and 
of South America, this was the only one I witnessed in Central 
America. 
The interest with which I regarded these miniature storms 
was due to the assistance that their study was likely to give 
in the discussion of the cause of all circular movements of 
the atmosphere, including the dreaded typhoon and cyclone. 
The chief meteorologists who have discussed this difficult 
question have approached it from the side of the larger hurri- 
canes. There is a complete gradation from the little dust 
eddies up through larger whirlwinds and tornadoes to the 
awful typhoons and cyclones of China and the West Indies; 
and it has long been my opinion that if meteorologists 
devoted their attention to the smaller eddies that can be 
looked at from the outside, and their commencement, con- 
tinuance, and completion watched and chronicled, they 
could not fail to obtain a large amount of information to 
guide them in the study of cyclonic movements of the 
atmosphere. 
Unless the smaller whirlwinds are quite distinct from the 
larger ones in their origin, the theories advanced by meteor- 
ologists to account for the latter are certainly untenable. 
According to the celebrated M. Dove, cyclones owe their 
origin to the intrusion of the upper counter trade-wind into 
the lower trade-wind current. More lately, Prof. T. B. 
Maury has stated that “ the origin of cyclones is found in the 
tendency of the south-east trade-winds to invade the terri- 
tory of the north-east trades by sweeping over the equator 
into our hemisphere, the lateral conflict of the currents 
giving an initial impulse to bodies of air by which they begin 
to rotate.” Cyclones having thus originated, Prof. Maury 
2 Law of Storms, p. 246. 
