228 



PNEUMATICS. 



the column of rarefied air tending upward, and, as a 

 consequence, the stronger will be the draught. In kin- 

 dhng a fire in a cold chimney, there is very little cur- 

 rent till this column becomes heated. The upward 

 motion of heated currents is governed by laws similar 

 to the downward motion of water in tubes, where the 

 velocity is increased with the height of the head. But 

 as air is more than eight hmidred times lighter than 

 water, shght causes wiU affect its currents, which would 

 have no sensible influence on the motion of liquids. 

 For instance, a strong wind striking the top of a chim- 

 ney may send the smoke downward into the room ; 

 and a current can not be induced through a horizontal 

 pipe without connecting with it an upright pipe of con- 

 siderable 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 fire-place, should be 

 contracted to a width of about four inches, 

 so that the column of rising air above may 

 draw the air up through the throat with 

 increased velocity, as shown in Fig. 186. 

 This arrangement also allows the fire to be 

 built so as to throw the heat more fully out 

 into the room. By leaving the shoulder 

 at b square or flat, it will tend to arrest 

 any reversed or downward current in a bet- 

 ter manner than if built sloping, as shown 

 by the dotted line at a, which would act 



