CHIMNEY CURRENTS. 253 



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 tend- 

 ency, in consequence of the velocity of the earth being 

 the greatest at the equator, and which, outstripping the 

 momentum which the winds have acquired in other lati- 

 tudes, tends to throw them behind, or in a westerly di- 

 rection. 



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 the 

 column of rarefied air tending upward ; and, as a conse- 

 quence, the stronger will be the draught. In kindling a 

 fire in a cold chimney, there is very little current till this 

 column becomes heated. The upward motion of heated 

 currents is governed by laws similar to the downward mo- 

 tion of water in tubes, where the velocity is increased 

 with the height of the head. But as air is more than 

 eight hundred times lighter than water, slight causes will 

 afifect its currents, which would have no sensible influence 

 on the motion of liquids. For instance, a strong wind 

 striking the top of a chimney may send the smoke down- 

 ward into the room ; and a current can not be induced 

 through a horizontal pipe without connecting with it an 

 upright pipe of considerable height. 



CONSTRUCTION OF CHIMNEYS. 



In constructing chimneys to produce a strong draught, 

 the throat immediately above the fire, which should have 

 a breadth equal to that of the fireplace, should be con- 

 tracted to a width of about four inches, so that the column 



