SPIEAL OP THE HUEEICANE. 287 



CHAPTEH XI. 



SPICAL OP THE HUERICANES IN THE TWO HEMISPHERES. — THEORY OF CYCLONES. 

 — NAUTICAL INSTRUCTIONS TO AVOID HURRICANES. 



At their departure from the tropical regions, where they fell into 

 collision with the trade-winds, or the monsoons, the greater part 

 of the cyclones of the new world proceed first towards the north- 

 west, parallel to the line of the Antilles, or else along the shores 

 of Columbia, and Central America ; then, turning back, like a billiard- 

 ball that rebounds in the opposite direction from the impulse re- 

 ceived, they follow the coast-line of the United States, describing in 

 the air an orbit correspondiD g with the course of the Gulf-stream. 



In the southern hemisphere the phenomenon is inverted ; 

 the cyclones of the Indian Ocean take their origin to the south of 

 Ilindostan, and move to the south-west towards Eeunion, Mauritius, 

 and Madagascar, then turn abruptly in a south-westerly direction 

 towards the Antarctic seas. The spiral movement of the wind in this 

 great tourbillon is effected from west to east by the north, that is to 

 say, in the same direction as the hand of a watch. The movement is 

 opposite to that which is taken by the hurricanes of the northern 

 hemisphere. 



What is the cause of the cj-clone itself, and w^hence comes this 

 sudden change, which occurs in its direction towards the exterior 

 limit of the trade-winds ? According to Dove, this is the explanation 

 of these phenomena : — 



When enormous quantities of warm air ascend over the deserts of 

 Asia and Africa, these expanded aerial masses must spread laterally. 

 Those which are carried over the North Atlantic in a westerly di- 

 rection, contrary to that of the earth's movement, meet the returning 

 current, which flows from the south-west to the north-east, in the 

 opposite direction of the trade- winds. From this results a conflict 

 between the two atmospheric currents ; a whirlwind of air is propa- 

 gated in spirals in the north-westerly direction, which is the result 

 of the two forces at issue. At the same time the revolving mass 



