306 THE NATURALIST IX NICARAGUA. [Ch. XVI. 



I shall only now make some observations on the largest 

 form of whirlstorm the dreaded cyclone. 



Just as over the little plain at Maryborough, protected 

 by the surrounding forest from the action of the wind, 

 the heated air accumulates over the surface until carried 

 off in dust eddies ; so, though on a vastly larger scale, in 

 that great bight formed by the coasts of north and south 

 America, having for its apex the Gulf of Mexico, there 

 is an immense area in the northern tropics, nearly sur- 

 rounded by land, forming a vast oceanic plain, shut off 

 from the regular action of the trade winds by the great 

 islands of Cuba and Hayti, where the elements of the 

 hurricane accumulate, and at last break forth. In this 

 and such like areas, the lower atmosphere is gradually 

 heated from week to week by the direct rays of the sun 

 during the day, by radiation from the sea during the 

 night ; and, as in Australia, the quivering of the air over 

 the hot ground foreshadows the whirlwind, and in Africa 

 the mirage threatens the simoom, so in the West Indies 



O ' 



a continuance of close, sultry weather, an oppressive 

 calm, precedes the hurricane. When at last the huge 

 vortex is formed, the heated atmosphere rushes towards 

 it from all sides, and is drained upwards in a spiral 

 column, just as in the dust eddy, on a gigantic scale. 

 Unlike the air of the dust-eddy, that of the hurricane 

 coming from the warm surface of the ocean is nearly 

 saturated with vapour, and this, as it is carried up and 

 brought into contact with the colder air on the outside 

 of the ascending column, is condensed and falls in tor- 

 rents of rain, accompanied by thunder and lightning. 



I advanced this theory to account for the origin of 

 whirlwinds in a paper read before the Philosophical 



