farther promoted. What sets the whirlwind a-brewing is another question; but its elements being put in motion, there is a diminished barometric pressure, first, on account of centrifugal tendency; next, on account of the ascending column of air, which expands and ascends,—ascends and expands on account of such diminished pressure;—and next, though not least, on account of the heat which is set free by the condensation of the vapour which forms the clouds and makes the rain. This heat expands and pushes aside the upper air still more.
799. Objections to the theory.—After much study, I find some difficulties about the cyclone theory that I cannot overcome. They are of this sort : I cannot conceive it possible to have a cyclone with a revolving and travelling disc 1000, or 500 miles in diameter, as the expounders of the theory have it. Is it possible for a disc of such an attenuated fluid as common air, having 1000 miles of diameter with its less than wafer-like thickness in comparison, to go travelling over the earth's surface and revolving about a centre with tornado violence? With the log-books of several vessels before me that are supposed to have been in different parts of the same cyclone I have a number of times attempted to project its path, but I always failed to bring out exactly such a storm as the theory calls for.[1] I make a distinction between the hauling of the wind in consequence of diurnal rotation of the earth, and the rotation of the wind in the cyclone in consequence of its centripetal force. For the sake of illustrating my difficulties a little farther, let us suppose a low barometer with a revolving storm to occur at A in the southern hemisphere. Let the storm be travelling towards B. Let observers be at c'', d and e, and let c'' and d be each several hundred miles from A, and so far as to be clearly without the reach of the whirl. Now, then, will not the air at c'' and d blow north and east as directly for the place of low barometer as it would were that place an oblong, N A, instead of a disc, as per the arrows? And w4iy should it be a disc in preference to
- ↑ Since this was written, I have had the privilege of examining at the Meteorological Department of the Board of Trade, Admiral Fitzroy's admirable diagrams, in MS., of the "Royal Charter" storm of October, 1859. It was a true cyclone—the best type of one, and the most perfectly developed on a large scale that has ever fallen under my observation. Its largest diameter did not measure less than 300 miles.
