:! 



PNEUMATICS. 173 



else the air rushing in, would occasion a wind con- 

 verging to a central space, to supply the lighter 

 ascending stream. It is, therefore, evident, that any 

 agent that alters the density of a part of the air 

 will produce a wind. 



The density of the air is changed by compression 

 and by heat. Its elasticity is increased by the 

 addition of moisture; and electricity may have 

 some effect of the same kind. 



In considering the causes of winds, the principal 

 agent to be attended to must be heat. 



The different winds may be reduced to three 

 classes, viz. general, periodical, and variable 

 winds. 



The general winds are usually called trade-winds. 

 They always blow nearly in the same direction. In 

 the open seas, that is, in the Atlantic and Pacific 

 Oceans, under the equator, the wind is found to 

 blow almost constantly from the eastward: this 

 wind prevails on both sides of the equator, to the 

 latitude of 28°. To the northward of the equator, 

 the wind is between the north and east; and the 

 more northerly, the nearer the northern limit. To 

 the southward of the equator, the wind is between 

 the south and east; and the more southerly, the 

 nearer the southern limit. 



Periodical winds are such as blow in a certain 

 direction for a time ; and at stated seasons change, 

 and blow for an equal space of time from the oppo- 

 site point of the compass. These may be divided 

 into two classes, viz. monsoons, or winds that 

 change annually ; and land and sea breezes, or winds 

 that change diurnally. 



In the sea between Madagascar and New Holland, 

 the S. E. wind extends no farther to the northward 

 than about the tenth degree of south latitude, the 



