A HISTORY OF SCIENCE 



But these local whirls, it must be understood, are 

 local only in a very general sense of the word, inasmuch 

 as a single one may be more than a thousand miles in 

 diameter, and a small one is two or three hundred miles 

 across. But quite without regard to the size of the 

 whirl, the air composing it conducts itself always in one 

 of two ways. It never whirls in concentric circles; it 

 always either rushes in towards the centre in a de- 

 scending spiral, in which case it is called a cyclone, or it 

 spreads out from the centre in a widening spiral, in 

 which case it is called an an ti - cyclone. The word 

 cyclone is associated in popular phraseology with a 

 terrific storm, but it has no such restriction in techni- 

 cal usage. A gentle zephyr flowing towards a " storm- 

 centre" is just as much a cyclone to the meteorologist 

 as is the whirl constituting a West-Indian hurricane. 

 Indeed, it is not properly the wind itself that is called 

 the cyclone in either case, but the entire system of 

 whirls including the storm-centre itself, where there 

 may be no wind at all. 



What, then, is this storm-centre? Merely an area 

 of low barometric pressure an area where the air has 

 become lighter than the air of surrounding regions. 

 Under influence of gravitation the air seeks its level 

 just as water does; so the heavy air comes flowing in 

 from all sides towards the low-pressure area, which thus 

 becomes a " storm-centre." But the inrushing currents 

 never come straight to their mark. In accordance with 

 Ferrel's law, they are deflected to the right, and the 

 result, as will readily be seen, must be a vortex current, 

 which whirls always in one direction namely, from 

 left to right, or in the direction opposite to that of the 



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