SECT. xv.J HUEKICANES. 141 



The breadth of the whirlwind is greatly augmented when 

 the path of the storm changes on crossing the tropic. The 

 vortex of a storm has covered an extent of the surface of the 

 globe 500 miles in diameter. 



The revolving motion accounts for the sudden and violent 

 changes observed during hurricanes. In consequence of the 

 rotation of the air, the wind blows in opposite directions on 

 each side of the axis of the storm, and the violence of the 

 blast increases from the circumference towards the centre of 

 gyration, but in the centre itself the air is in repose : hence, 

 when the body of the storm passes over a place, the wind 

 begins to blow moderately, and increases to a hurricane as the 

 centre of the whirlwind approaches ; then, in a moment, a 

 dead and awful calm succeeds, suddenly followed by a re- 

 newal of the storm in all its violence, but now blowing in a 

 direction diametrically opposite to its former course. This 

 happened at the Island of St. Thomas, on the 2nd of August, 

 1837, where the hurricane increased in violence till half-past 

 seven in the morning, when perfect stillness took place for 

 forty minutes, after which the storm recommenced in a con- 

 trary direction. 



The sudden fall of the mercury in the barometer in the 

 regions habitually visited by hurricanes is a certain indication 

 of a coming tempest. In consequence of the centrifugal force 

 of these rotatory storms the air becomes rarefied, and, as the 

 atmosphere is disturbed to some distance beyond the actual 

 circle of gyration or limits of the storm, the barometer often 

 sinks some hours before its arrival, from the original cause of 

 the rotatory disturbance. It continues sinking under the first 

 half of the hurricane, and again rises during the passage of 

 the latter half, though it does not attain its greatest height 

 till the storm is over. The diminution of atmospheric pressure 

 is greater and extends over a wider area in the temperate 

 zones than in the torrid, on account of the sudden expansion 

 of the circle of rotation when the gale crosses the tropic. 



As the fall of the barometer gives warning of the ap- 



