392 WILLIAM FERREL. 



general facts concerning the prevailing winds of the world had been 

 accumulated and were presented with much force by Maury ; and at 

 the same time, various theories had grown up to account for the facts. 

 These theories all had two general principles in common ; first, that 

 there must be a convectional circulation between the equator and the 

 poles ; and second, that the motions thus excited must be deflected by 

 the earth's rotation. As commonly stated, it was understood that there 

 must be high pressure at the poles, where the air is cold, just as there 

 is low pressure around the equator where the air is warm ; and as 

 stated by Dove, who at the time of Ferrel's entrance into the science 

 was its leading authority, the currents from the equator to the north 

 pole must flow from the southwest, while the return currents from the 

 north pole to the equator must flow from the northeast. Ferrel per- 

 ceived the essential incompleteness of this view of the subject. He 

 first showed that the prevailing explanation of the effect of the earth's 

 rotation was incomplete, and then, applying this important element in 

 its proper measure, he introduced the idea of a rearrangement of at- 

 mospheric pressures in consequence of convectional motions. This 

 great principle may be followed all through Ferrel's theories of cyclones 

 and tornadoes, as well as through his theory of the circulation of the 

 atmosphere as a whole. Its quantitative introduction into meteorology 

 seems to me to be Ferrel's greatest achievement. 



Ferrel showed that the convectional interchange between the equa- 

 tor and the poles must resolve itself into two great circumpolar whirls, 

 one in either hemisphere ; that in the greater part of the whirls the 

 currents must move eastward, the trade-wind belts being the only con- 

 siderable regions of westward motion ; that, in consequence of the cir- 

 cumpolar whirls, the expected high polar pressures must be reduced to 

 relatively low pressures, especially in the southern hemisphere, where 

 the disturbing effects of continental interruptions are least; and that 

 the air thus held away from the poles must be found in the tropical 

 belts of high pressure, then coming to be recognized as great atmos- 

 pheric features. It is not too much to say that the introduction of 

 this theory has made a new science of meteorology. Ferrel's mark 

 is permanently imprinted upon it. 



We are apt, in reviewing a step of advance like this, to imagine that 

 it was made at a single stride ; but such was certainly not the case here. 

 The theory of atmospheric circulation grew slowly in Ferrel's mind, 

 and several years passed before it was fully developed. During its 

 progress, Ferrel's efforts were constantly directed towards quantitative 

 estimates of forces and results. This feature of his work is strikingly 



