THE EARTH. 337 



'? As a foundation for his hypothesis, the Doctor 

 assumes, that the lower region of the air is often 

 more heated, and so more rarefied, than the 

 upper, and consequently specifically lighter. This 

 being granted, he presupposes a tract of land or 

 sea, of about 60 miles in extent, unsheltered by 

 clouds, and unrefreshed by the wind, for several 

 days without intermission, till it becomes violently 

 heated, together with the lower region of the air 

 in contact with it ; so that the latter becomes spe- 

 cifically lighter than the superincumbent higher 

 region of the atmosphere, w r herein the clouds are 

 usually floated : he supposes also, that the air sur- 

 rounding this tract has not been so much heated 

 during those days, and therefore remains heavier. 

 The consequence of this, he conceives, should be, 

 that the heated lighter air should ascend, and the 

 heavier descend ; and as this rising cannot ope- 

 rate throughout the whole tract at once, because 

 that would leave too extensive a vacuum, the 

 rising will begin precisely in that column which 

 happens to be lightest or most rarefied ; and the 

 warm air will flow horizontally from all parts of 

 this column, where the several currents meeting, 

 and joining to rise, a whirl is naturally formed, 

 in the same manner as a whirl is formed in a tub 

 of water, by the descending fluid receding from 

 all sides of the tub towards the hole in the centre. 



" As the several currents arrive at this central 

 rising column with a considerable degree of hori- 

 zontal motion, they cannot suddenly change it to 

 a vertical motion j therefore as they gradually, in 

 approaching the whirl, decline from right to 



VOL. i. Y. 



