234. The Naturalist in Nicaragua 
to drive the air with the great velocity with which it moves 
in whirlstorms. The upper, colder, and heavier air is press- 
ing upon the heated stratum, and the greater the area over 
which the latter extends, the greater will be the weight 
pressing upon it, and the greater the violence of the whirl- 
wind when an opening is formed for the ascent of the heated 
air. There is a gradual passage, from the small dust eddies, 
through larger whirlstorms such as that at Lough Neagh, to 
tornadoes and the largest cyclone; every step of the grada- 
tion might be verified by numerous examples; and if this 
book were a treatise on meteorology, it might be admissible 
to give them; but to do this would take up too much of my 
space, and I shall only now make some observations on the 
largest form of whirlstorm—the dreaded cyclone. 
Just as over the little plain at Maryborough, protected by 
the surrounding forest from the action of the wind, the 
heated air accumulates over the surface until carried off in 
eddies, so, though on a vastly larger scale, in that great 
bight formed by the coasts of North and South America, 
having for its apex the Gulf of Mexico, there is an immense 
area in the northern tropics, nearly surrounded by land, 
forming a vast oceanic plain, shut off from the regular action 
of the trade-winds by the great islands of Cuba and Hayti, 
where the elements of the hurricane accumulate, and at last 
break forth. In this and such like areas, the lower atmo- 
sphere is gradually heated from week to week, and, as in 
Australia the quivering of the air over the hot ground fore- 
shadows the whirlwind, and in Africa the mirage threatens 
the simoom, so in the West Indies a continuance of close, 
sultry weather, an oppressive calm, precedes the hurricane. 
When at last the huge vortex is formed, the heated atmosphere 
rushes towards it from all sides, and is drained upwards in a 
spiral column, just as in the dust-eddy, on a gigantic scale. 
Unlike the air of the dust-eddy, that of the hurricane coming 
from the warm surface of the ocean is nearly saturated with 
vapour, and this, as it is carried up and brought into contact 
with the colder air on the outside of the ascending column, 
is condensed and falls in torrents of rain, accompanied by 
thunder and lightning. 
I advanced this theory to account for the origin of whirl- 
winds in a paper read before the Philosophical Institute of 
