148 GREAT ATMOSPHERIC BILLOWS. 



a " depression " is passing overhead (like the trough 

 of the sea between the waves), it falls in a cor- 

 responding degree. 



Great cyclones, in point of fact, are simply gigantic 

 eddies of the atmospheric ocean, having a diameter it 

 may be of several hundreds of miles, which circle 

 round just like the eddies of a river, as they move along 

 the stream. These immense aerial whirlpools will of 

 course draw the clouds towards them, while the de- 

 scending eddies of cold air, will cause them to be 

 precipitated in rain ; but it does not necessarily follow 

 that these extensive cyclonic areas, are accompanied 

 by wind storms. When the wind circles round these 

 Vast arenas, it is apt to do so more slowly than in 

 the smaller cyclones, where the velocity of the whirl 

 is much greater: a small but rapid whirlwind will 

 therefore usually create a deep but transient depression, 

 which as it passes overhead, is immediately indicated 

 on earth by a sudden and correspondingly large fall in 

 the barometer, an instrument which, as we have said, 

 is merely a machine for measuring the height, or in 

 other words, the weight, of the atmosphere, as indeed 

 its name denotes. * 



A heavy fall in the glass therefore warns us that 

 one of these deep and probably violent cyclonic de- 

 pressions is approaching ; the greater and more sudden 

 that fall is, the more serious the disturbance of the 

 weather is likely to be : and yet it may after all pass 

 by, and nothing may happen ; this is because these 

 storms, in their furious and erratic course, are often 

 deflected from their original direction; and so, after 



* The word "Barometer" being formed of two Greek words, " CCQO$," 

 weight, and "jifrpov," a measure i.e. "a measure of weight." 



