286 THE ATMOSPHEEE AND METEOEOLOGY. 



the normal speed of tlie wind of the tempest, whicli blows in the 

 same direction over the periphery of the cyclone, will be augmented 

 by the speed of the storm itself. In return, the wind which will 

 blow towards the east, will be partially neutralized, and along all the 

 outline of the circle, the direction and the speed will be modified 

 according to proportions rigorously established by calculation. These 

 are the modifications to which the successive winds along the outline 

 of the tempest are subject, and which often render the cyclones difficult 

 to recognize in the regions of the temperate zone, where the speed of 

 rotation of the storms is considerably diminished. Under the tropics, 

 where the whirlwind, being still restricted, is in its primitive force, 

 we remark the less this inequality of these partial winds of the 

 hurricane. It is, however, important enough to be recognized by 

 mariners. One half of the disk of the tempest is called by them 

 "dangerous semi- circle,'^ and the other ** manageable semi-circle." 

 Now, this part of the hurricane, which the great violence of the winds 

 renders dangerous, is always found on the side of the cyclone where 

 the wind proceeds in the same direction as the storm. That half 

 of the disk where the wind adds its own speed to that of the move- 

 ment of translation is in the northern hemisphere, to the right of 

 the trajectory of the revolving circle ; in the southern hemisphere, it 

 is to the left.* The accompanying figure gives an idea of the con- 

 trast which occurs between the two sides of the hurricane on the 

 path that it traverses in the Indian Ocean. 



* Marie Davy, Mouvemcnts de V Atmosphere et des Mers. 



