428 PHYSICAL GEOGRAPHY OF THE SEA, AND ITS METEOROLOGY. 



800. Tlic three forces. — There are in the various parts of the 

 storm at least three forces at work in effecting a change of wind, 

 as observed on board ship at sea. (1.) One is diurnal rotation : 

 it alone can never work a change of direction exceeding 90° ; 

 (2.) another is the varying position or travelling motion of the 

 place of barometric depression ; the change effected by it cannot 

 exceed 180°; (3.) and the third is the whirling motion imparted 

 by the rush to a common centre — as the whirl of water at the 

 flood-gate of the mill, the whirlwind in the street, for example. 



801. TJie effect of each. — Hence it appears that in a storm the 

 wind may shift from any one of three causes, and we are not 

 entitled to call it a cyclone unless the wind shift more than 180°. 

 If the change of direction be less than 90°, the shifting may 

 be due to diurnal rotation alone; if it be less than 180°, the 

 shifting may be, and is probably, due to (1) and (2). The 

 sailor has therefore no proof to show that he has been in a 

 cyclone unless the wind during the storm changed its directions 

 more than 180°. Cyclones, there is reason to believe, are often 

 whirlwinds in a storm. This may be illustrated by referring 

 again to our miniature whirlwinds on the land ; there we often 

 see a number of them at one time and about the same piace ; aud 

 they often appear to skip, raging here, then disappearing for a 

 moment, then touching the ground again, and pursuing the 

 former direction. 



802. A storm within a storm. — Observations have proved that 

 this is the case on land, and observations have not established 

 that this is not the case at sea ; observations are wanting upon 

 this subject. Tornadoes on the land often divide themselves, 

 sending out branches, as it were. It remains to be seen whether 

 cyclones do not do the same at sea ; and whether, in those wide- 

 spread and devastating storms that now and then sweep over the 

 ocean, there be only one vortex or several ; and if only one, 

 whether the whole storm partake of the cyclone character. In 

 other words, may there not be a storm within a storm — that is, a 

 cyclone travelling with the storm and revolving in it ? I ask 

 the question because the theory, as at present expounded, does 

 not satisfy all the facts observed ; and because the existence of 

 storms or whirlwinds within a storm would. 



803. Tlie Black Sea storm of 1854. — The celebrated Black Sea 

 storm of 1854, which did so much damage to the allied fleet, 

 is still maintained by some to be a true cyclone ; and by the 



