CHIMNKV CURRENTS. 227 



CAUSES OF WIND. 



The motion of air in producing wind is explained by 

 the action of heat, although there are many irregular 

 currents whose cause is not well uuderstood. The 

 simplest illustration of the effect of heat in causing cur- 

 rents is furnished by the land and sea breezes in warm 

 latitudes. The rays of the sun during the day heat the 

 surface of the land, and the air in contact with it also 

 becoming heated, and thus rendered hghter, flows up- 

 ward ; the air from the sea rushes in to fill the vacancy 

 and causes the sea-breeze. During the night, the ra- 

 diation of heat from the land into the clear sky above 

 cools the surface to a lower temperature than that of 

 the sea ; consequently the air in contact with the sea 

 becomes heated the most, and rising, causes the wind 

 from the land to flow in and supply the place. Trade- 

 winds are caused in a similar way, but on a much 

 larger scale, by the greater heat of the earth at the 

 equator, which produces currents from colder latitudes. 

 These currents assume a westerly tendency, in conse- 

 quence of the velocity of the earth being the greatest 

 at the equator, and which, outstripping the momentum 

 which the winds have acquired in other latitudes, tends 

 to throw them behind, or in a westerly direction. 



SECTION II. 



CHIMNEY CURRENTS. 



Chimney Currents are produced by the heat of the 

 fire rarefying the air, which rises and carries the smoke 

 with it. The taller the chimney is, the longer will be 



