426 PHYSICAL GEOGnAPHY OF THE SEA, AND ITS METEOROLOGY. 



farther promoted. What sets the whirlwind a-brewing is anotlier 

 question ; but its elements being put in motion, there is a dimi- 

 nished barometric pressure, first, on account of centrifugal ten- 

 dency ; next, on account of the ascending column of air, which 

 ex])ands and ascends, — ascends and expands on account of such 

 diminished pressure; — and next, though not least, on account of 

 tlie 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 (lie 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 difi'erent 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.* 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 ob- 

 servers 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 liave had the privilege of examiuinc: f^t the 

 Meteoroh)gical Departuietit of the Board of Trade, Admiral Fitzroy's admirable 

 diagrams, in IMS., of the "Royal Charter" storm of October, 1859. It was a 

 true cyclone— the best type of one, and the most perfectly developei on a 

 large scale that has ever fallen under my observation. Its largest diameter did 

 not mcasm-c less than 300 miles. 



