Dove on the Law of Storms. 329 



rotatory tendency would not be produced at all, if there were no 

 resisting mass in the space d b h ; it will therefore be proportion- 

 ate to the resistance thus opposed to the westerly deflection of 

 the storm, and the force of the rotatory movement of the storm 

 will be the greater the more the original direction of its path is 

 preserved. In the zone of the northern trade winds, the space 

 d b h is filled with air which is flowing from N. E. to S. W. ; 

 the resistance will here be the greatest, so that the air in b may 

 be so checked in its westward tendency, that it may preserve its 

 direction towards d almost unaltered, whilst a tends towards g ; 

 the rotatory motion of the storm will therefore be the most vio- 

 lent, whilst at the same time its course will be rectilinear, and its 

 breadth unaltered. But when it reaches the temperate zone, 

 there will be in the space d b h air which is already moving from 

 S. W. to N. E. ; the resistance hitherto encountered at b will 

 therefore be suddenly greatly diminished, or altogether removed ; 

 and hence the direction b d is suddenly changed into the direc- 

 tion b h, so that the storm is suddenly deflected almost at right 

 angles, and at the same time its breadth increases rapidly from 

 the cessation of the difference between the movements of the 

 points in a and the points in b. The phenomena in the southern 

 hemisphere may be derived in the same manner ; the rotation is 

 in the opposite sense, and the change of direction on passing the 

 boundary of the tropics is analogous.* 



As the West India hurricanes originate at the inner boundary 

 of the trade winds, where at the limit of the so-called region of 

 calms the air ascends and flows over the trade in an opposite 

 direction, it is probable that portions of this upper current pene- 

 trating through the lower one, give the first occasion of those 

 storms. The high mountains of several of the islands, by offer- 

 ing a mechanical impediment, may be one cause of this effect, 



* The above derivation of the rotatory movement only applies when great mass- 

 es of air, of a considerable extension in breadth, are set in motion ; lesser whirls 

 of wind or water, as water-spouts, &c, are produced by other causes, and there- 

 fore probably do not show either regular direction or definite opposition in refer- 

 ence to the two hemispheres. Mr. Redfield observed a small tornado rotating like 

 the greater storms; but Colonel Reid saw from the government-house, Bermuda, 

 a water-spout rotating in the opposite sense. The observations of Akin at Green- 

 bush near Albany, of Dwight at Stockbridge in Massachusetts, and of Dr. Cowles 

 at Amherst, of whirlwinds of great force taking place in forest fires in calm weath- 

 er, show that a strong ascending current may also produce a rotatory motion. 



Vol. xliV, No. 2.— Jan.-March, 1843. 42 



