8cr. XV. HURRICANES. 121 



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 conse- 

 quence of the rotation of the air, the wind blows in op- 

 posite directions on each side of the axis of the storm, 

 and the violence of the blast increases from the circum- 

 ference toward the center of gyration, but hi the center 

 itself the ah- is in repose : hence, when the body of the 

 storm passes over a place, the wind begins to blow mod- 

 erately, and increases to a hurricane as the center 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 2d 

 of August, 1837, where the hurricane increased in vio- 

 lence 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 fell 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 be- 

 comes 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 i 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 rota- 

 tion when the gale crosses the tropic. 



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

 proach of a hurricane, so the laws of the storm's mo- 

 tion afford to the seaman the knowledge to guide him in 

 avoiding it. In the northern temperate zone, if the gale 

 begins from the S. E. and veers by S. to W., the ship 

 L 



