THE STORY OF NINETEENTH-CENTURY SCIENCE 



part ; and though exceptionally its course may be to the 

 southeast instead, it almost never departs so widely 

 from the main channel as to progress to the westward. 

 Thus it is that storms sweeping over the United States 

 can be announced, as a rule, at the seaboard in advance 

 of their coming by telegraphic communication from the 

 interior, while similar storms come to Europe off the 

 ocean unannounced. Hence the more practical availa- 

 bility of the forecasts of weather bureaus in the former 

 country. 



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 diam- 

 eter, 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 anti-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 be- 

 come lighter than the air of surrounding regions. Under 



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