Mr. T. Belt on the Origin of Whirlwinds. 51 



Atlantic is within that great bight formed by the coasts of North 

 and South America, having for its apex the Gulf of Mexico ; and 

 that of the Indian Ocean is bounded by an arc of laud having 

 at one end the continent of Australia, at the other the island of 

 Madagascar and the southern termination of the continent of 

 Africa. 



In these and suchlike regions of the immense ocean the 

 materials for the hurricane are piled up. Here, from day to 

 day, the lower atmosphere is gradually heated by the direct rays 

 of the sun during the day, by irradiation from the sea during 

 the night. As in Australia the quivering of the air over the 

 hot dried ground precedes the eddy, and in Africa the mirage 

 foreshadows the simoom, so the close stifling atmosphere in 

 the West Indies foretells the hurricane. The hurricane is the 

 breaking up of a continuance of warm weather, which at the 

 latter end has been exceedingly sultry. This fact of itself is 

 sufficient proof that the air next the surface does not gradually 

 rise as it is heated ; if so, the temperature would be compara- 

 tively equable, and no such accumulation of heat could take 

 place. Whilst on this part of the subject, I may mention that 

 the temperature of the atmosphere has not been sufficiently con- 

 sidered in treating of hui-ricanes. In every account of a cyclone 

 we find minute readings of the barometer, whilst the ther- 

 mometer is almost neglected. Now, although the barometer is 

 invaluble to the mariner for indicating the approach of a hurri- 

 cane, yet the latter is the effect of elements whose quiescent 

 existence is shown by the thermometer; and it is for the in- 

 terest of science that greater attention should be paid to it in 

 those regions where cyclones originate, to do away with our 

 present defective information on the subject. 



It is a well known feature of cyclones, that they rotate in 

 opposite directions in the two hemispheres. In the northern 

 this direction is E., N., W., S., whilst in the southern it is 

 E., S., W., N., or contrary to the apparent course of the sun. 

 It is this constancy in the method of their rotation that enables 

 the skilful mariner to calculate his safest course when he en- 

 counters a whirlstonn. I have seen no attempt to account for 

 this constant element in the action of cyclones, excepting on 

 electrical grounds ; but as we know that the Australian eddies 

 rotate iudiff'erently in either direction, we must find some solu- 

 tion applying only to the larger meteors. 



At the commencement of a cyclone, when an opening is 

 forced through the overlying atmosphere, the heated strata, ex- 

 tending over a large area, rush towards the focus from all sides, 

 and those currentu of air are turned out of their direct course 

 by the action of the earth's rotation in the same manner as the 



E2 



