Page:Physical Geography of the Sea and its Meteorology.djvu/454

428 800. The 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. The 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 place; and 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 widespread 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. The 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