THE STORY OF NINETEENTH-CENTURY SCIENCE 



American meteorologist, who had been led to take up 

 the subject by a perusal of Maury's discourse on ocean 

 winds, formulated a general mathematical law, to the 

 effect that any body moving in a right line along the 

 surface of the earth in any direction tends to have its 

 course deflected, owing to the earth's rotation, to the 

 right hand in the northern and to the left hand in the 

 southern hemispheres. This law had indeed been stated 

 as early as 1835 by the French physicist Poisson, but no 

 one then thought of it as other than a mathematical 

 curiosity ; its true significance was only understood after 

 Professor Ferrel had independently rediscovered it (just as 

 Dalton rediscovered Hadley's forgotten law of the trade- 

 winds) and applied it to the motion of wind currents. 



Then it became clear that here is a key to the phe- 

 nomena of atmospheric circulation, from the great polar- 

 equatorial maelstrom which manifests itself in the trade- 

 winds, to the most circumscribed riffle which is an- 

 nounced as a local storm. And the more the phenom- 

 ena were studied, the more striking seemed the parallel 

 between the greater maelstrom and these lesser eddies. 

 Just as the entire atmospheric mass of each hemisphere 

 is seen, when viewed as a whole, to be carried in a great 

 whirl about the pole of that hemisphere, so the local dis- 

 turbances within this great tide are found always to 

 take the form of whirls about a local storm-centre 

 which storm-centre, meantime, is carried along in the 

 major current, as one often sees a little whirlpool in the 

 water swept along with the main current of the stream. 

 Sometimes, indeed, the local eddy, caught as it were in 

 an ancillary current of the great polar stream, is de- 

 flected from its normal course and may seem to travel 

 against the stream ; but such deviations are departures 



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