1872.] The Origin of the Great Cyclones. 431 
as in motion—now, with slow, but steady and unfaltering 
tread, marching upon meridian lines to the northern tropic ; 
and now, having faced about at our summer solstice, with 
unchanging step countermarching toward the southern 
tropic. The day’s march of this imaginary but real host, 
if measured in a line running due north and south, is the 
invariable distance of about 15 geographical miles. Before 
its forward and gleaming movement, either north or south 
of the Equator, and in front of its ever-outstretched lines, 
all opposing climatic forces waver, and finally give way. 
The tropical belts of calms, the zones of the perennial 
trade-winds, the bands of the fierce anti-trades, the monsoon 
influences, barometric areas, the cyclone-breeding districts, 
ocean currents, gulf streams, nay, the very ice barriers that 
have intrenched themselves in strong cordon within the 
mysterious periphery of the polar circles, swaying back and 
forth, up and down on the earth’s surface, in ceaseless 
vibration, ever and everywhere follow the lordly sun in his 
declination, and strictly adjust their movements to his. 
In nothing that has been said has it been at all my design 
to assert that cyclonic storms originate nowhere but in the 
intra-tropical regions and under the causal influence of the 
conflicting trade-winds. I have discussed only the great 
storms of the parent-zone of aérial disturbance. The 
celebrated Royal Charter Storm, charted by Admiral Fitzroy, 
and so memorable in meteorologic annals, is a type of a 
large number that originate in high latitudes. No practical 
meteorologist, accustomed to study every day the atmo- 
spheric movements, and to chart the undulations of the 
aérial ocean— 
**Varium et mutabile semper ’— 
can have failed to notice that the nuclei of storms are 
formed without regard to latitude. The interference of 
strong air currents of different temperatures, the upward 
deflection of surface currents by mountain barriers, pro- 
ducing condensation of aqueous vapour, the flames of a 
burning city, as Chicago, the widespread conflagration of 
a prairie, will often produce the physical circumstances 
which give birth to depressions. In the present state of 
meteorologic science it may be rash to go further. But, 
whatever theories meteorologists may devise capable of 
explaining how rotative or retrograde storms may be origi- 
nated, I have not much hesitation in venturing the opinion 
that future researches will show, as a matter of fact, they 
are formed chiefly, if not exclusively, along the edges of the 
