THE REASON WHY. 155 



1 And, lo, the smoke of the country went up as the smoke of a furnace." 

 GEN. Xix. 



air of the short chimney and the external air ; it therefore has not 

 so great an ascensive power. 



682. Why does smoke issue in folds and curls ? 



Because it is pressed upon by the cold air which always rushes 

 towards a rarer atmosphere. It thus illustrates the development 

 of storms. 



683. Why do some chimneys smoke when the doors and 

 windoivs are closed? 



Because the draught of air is not sufficient to supply the wants 

 of the fire, and enable it to create an upward current. 



684. Wliat is the best method of conveying air to fires ? 



Tubes builc in the walls, communicating with the outer air, and 

 terminating underneath the grates. 



685. Why is this the lest method of ventilation ? 



Because doors and windows may then be made air-tight, and 

 draughts across rooms be prevented. 



686. Why do chimneys that stand under elevated objects, 

 such as hills, trees, and high buildings, smoke ? 



Because the wind, striking against the elevated object, flies 

 back, and a part of it rushes downward. 



687. Why do sooty chimneys smoke ? 



Because the accumulation of the soot diminishes the size of the 

 flue, and lessens the ascensive power of the draught, by reducing 

 the quantity vtwarm. air. It also obstructs the motion of the air, 

 by the roughness of its surface. 



688. Why do chimneys smoke in damp and gusty weather ? 



Because the ascending air is suddenly chilled by gusts of damp 

 and cold air, and driven down the chimney. 



