SECT. XV. HURRICANES. 127 



warmer strata of air with those that are higher and colder, pro- 

 ducing torrents of rain and violent electric explosions. 



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 renewal 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 contrary direction. 



The sudden fall of the mercury in the barometer in the regions 

 habitually visited by hurricanes is a certain indication of a com- 

 ing 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 dis- 

 turbance. It continues sinking under the first half of the hur- 

 ricane, is at a maximum sometimes of two inches in the centre of 

 gyration, 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 approach of 

 a hurricane, so the laws of the storm's motion afford the sea- 

 man knowledge to guide him in avoiding it. In the northern 



