SECT. XV. HURRICANES. 125 



They are owing to currents of air running upon each other in 

 horizontal strata, differing in their electric state, in temperature 

 and moisture, as well as in velocity and direction. 



When north and south winds blow alternately, the wind at 

 any place will veer in one uniform direction through every point 

 of the compass, provided the one begins before the other has 

 ceased. In the northern hemisphere a north wind sets out with 

 a smaller degree of rotatory motion than the places have at which 

 it successively arrives, consequently it passes through all the 

 points of the compass from N. to N.E. and. E. A current from 

 the south, on the contrary, sets out with a greater rotatory 

 velocity than the places have at which it successively arrives, so 

 by the rotation of the earth it is deflected from S. to S.W. and W. 

 Now, if the vane at any place should have veered from the N. 

 through N.E. to E., and a south wind should spring up, it would 

 combine its motion with the former and cause the vane to turn 

 successively from the E. to S.E. and S. But by the earth's rota- 

 tion this south wind will veer to the S.W. and W., and, if a 

 north wind should now arise, it would combine its motion with 

 that of the west, and cause it to veer to the N.W. and N. Thus 

 two alternations of north and south wind will cause the vane at 

 any place to go completely round the compass, from N. to E., 

 S., W., and N. again. At the Eoyal Observatory at Greenwich 

 the wind accomplishes five circuits in that direction in the course 

 of a year. When circumstances combine to produce alternate 

 north and south winds in the southern hemisphere, the gyration 

 is in the contrary direction. Although the general tendency of 

 the wind may be rotatory, and is so in many instances, at least 

 for part of the year, yet it is so often counteracted by local cir- 

 cumstances, that the winds are in general very irregular, every 

 disturbance in atmospheric equilibrium from heat or any other 

 cause producing a corresponding wind. The most prevalent winds 

 in Europe are the N.E. and S.W. ; the former arises from the 

 north polar current, and the latter from causes already mentioned. 

 The law of the wind's rotation was first described by Dr. Dalton, 

 but has been developed by Professor Dove, of Berlin. 



Hurricanes are those storms of wind in which the portion of 

 the atmosphere that forms them revolves in a horizontal circuit 

 round a vertical or somewhat inclined axis of rotation, while the 

 axis itself, and consequently the whole storm, is carried forward 



