THE CENTURY'S PROGRESS IN METEOROLOGY 



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 Fer- 

 rers 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 hands 

 of a watch held with its face upward. The velocity of 

 the cyclonic currents will depend largely upon the dif- 

 ference in barometric pressure between the storm-centre 

 and the confines of the C3 7 clone system. And the veloc- 

 ity of the currents will determine to some extent the 

 degree of deflection, and hence the exact path of the 

 descending spiral in which the wind approaches the 

 centre. But in every case and in every part of the 

 cyclone system it is true, as Buys Ballot's famous rule 

 first pointed out, that a person standing with his back 

 to the wind has the storm-centre at his left. 



The primary cause of the low barometric pressure 

 which marks the storm - centre and establishes the 

 cyclone is expansion of the air through excess of tem- 

 perature. The heated air, rising into cold upper regions, 

 has a portion of its vapor condensed into clouds, and 

 now a new dynamic factor is added, for each particle of 

 vapor, in condensing, gives up its modicum of latent 

 heat. Each pound of vapor thus liberates, according to 

 Professor Tyndall's estimate, enough heat to melt five 

 pounds of cast iron ; so the amount given out where 

 large masses of cloud are forming must enormously add 

 to the convection currents of the air, and hence to the 

 storm -developing power of the forming cyclone. In- 



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