[ 6» ] 



the lower extremity of the fuperior fedion of the polar column. 

 What is here faid of the sequatorial and polar columns muft be 

 ■underftood alfo of the extratropical columns with refpedt to each 

 other, where great differences of heat prevail. 



Hence, in the higheft regions of the atmofphere, the denfer 

 •^equatorial air not being fupported by the collateral extratropical 

 -columns, gradually flows over, and rolls down to the north and 

 fouth. 



These fuperior tides confift chiefly of inflammable air, as it 

 is much lighter than any other, and is generated in great plenty 

 between the tropics ; it furnifhes the matter of the Aurora Borealis 

 and Aujiralis, by whofe combuftion it is deftroyed, elfe its quan- 

 tity would in time become too great, and the weight of the 

 atmofphere annually encreafed ; but its combuftion is the primary 

 fource of the greateft perturbations of the atmofphere, as will 

 prefently be fliewn. 



If the aflluence of the northern and fouthern air to the asqua- 

 tor by the trade winds kept pace with the effluence of the fupe- 

 rior air, an aequilibrium mi2;ht ftill in fome meafure be main- 

 tained. But the trade v/inds move only at the rate of twelve 

 feet per fecond, or about eight miles an hour * ; whereas, with- 

 out the tropics, or at leaft beyond latitude 30°, the currents of 

 the upper atmofphere are incomparably more rapid f. For as the 



• II. Bergm. Erdcklatet, p. ii6. 



f Saufl". Hygr. 300. I. Gentil. Voy. aux Indes, p. 48<S- 



mean 



