412 PHYSICAL GEOGRAPHY OF THE SEA, AND ITS METEOROLOGY. 



of him, as in the direction of d, but the other would place it to 

 the southward of him, as in the direction c . By the rule, ship 

 d would be led towards the real track of the storm, i. e., into 

 danger, and ship c" away from it.* From all this it would appear 

 that the cyclone theory is defective in this : when the wind 

 hauls in the storm, the sailor who is contending with it, lacks a 

 rule by which he may know the influence which causes it to 

 haul. The gyrating disc of a cyclone can never, I apprehend, 

 exceed a few miles in diameter. On shore we seldom find it 

 exceeding in breadth as many rods, in most cases not of as many 

 fathoms, as its advocates give it miles at sea. I think the dust- 

 whirl in the street is a true type of the tornado (cyclone) at sea. 



800. There are in the various parts of the storm at least 

 The three forces, throo forcos at work iu 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.) und 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. Hence it appears that in a storm the wind may shift from 

 The effect of each, any ouo of throo causos, 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°. Cy- 

 clones, 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 touch- 

 ing the ground again, and pursuing the former direction. 



802. Observations have proved that this is the case on land, 

 A storm within a storm, and obsorvations have not established that this is 

 not the case at sea ; observations are wantmg upon this subject. 

 Tornadoes on the land often divide themselves, sending out 



* See letter to Commodore Wiillerstorf, p. 457, vol. ii., 8th eJ., Maury's Sailing 

 Directions. 



