HUBBICANES. 



motion of the piston and that of the current of air which 

 follows it. 



What is here supposed to take place in the tube is exhibited on 

 a larger scale in the atmosphere. Any physical cause which 

 produces a compression of the atmosphere from north to south will 

 produce a north wind ; and any cause which produces a rarefac- 

 tion from north to south will produce a south wind. 



56. Of all the causes by which winds are produced, the most 

 frequent is the sudden condensation of vapour suspended in tho 

 atmosphere. In general the atmosphere above us consists of a 

 mixture of air properly so called, and water, either in the state of 

 vapour, or in a vesicular state, the nature and origin of which has 

 not yet been clearly ascertained. In either case its sudden con- 

 version into the liquid state, and its consequent precipitation to 

 the earth, leaves the space it occupied in the atmosphere a vacuum, 

 and a corresponding rarefaction of the air previously mixed with 

 the vapour ensues. The adjacent strata immediately rush in to 

 re-establish the equilibrium of pneumatic pressure, and winds are 

 consequently produced. 



The propagation of winds by rarefaction manifested in directions 

 contrary to that of the winds themselves, is common in the North 

 of Europe. Wargentin gives various examples of this. When a 

 west wind springs up, it is felt, he observes, at Moscow before it 

 reaches Abo, although the latter city is four hundred leagues west 

 of Moscow, and it does not reach Sweden until after it has passed 

 over Finland. 



57. The intertropical regions are the theatre of hurricanes. It 

 is there only that these atmospheric commotions are displayed in 

 all their terrors. In the temperate zones tempests are not only 

 more rare in their occurrence but much less violent in their force. 

 In the circumpolar zone the winds seldom acquire the force which 

 would justify the title of a storm. 



The hurricanes of the warm climates spread over a considerable 

 width, and extend through a still more considerable length. Some 

 are recorded which have swept over a distance of four or five 

 hundred leagues with a nearly uniform violence. 



It is only by recounting the effects produced by these vast com- 

 motions of the atmospheric ocean, that any estimate can be formed 

 of the force which air, attenuated and light as that fluid is, may 

 acquire when a great velocity is given to it. In hurricanes such as 

 that which took place at Guadaloupe on the 25th July, 1825, houses 

 the most solidly constructed were overthrown. A new building 

 erected in the most durable manner by the government was razed 

 to the ground. Tiles carried from the roof were projected against 

 thick doors with such force as to pass through them like a cannon 



87 



