CAUSE OF WINDS. 785 



ing from the vicinity of the fire, flowing along the 

 ceiling to the walls and windows, becoming colder and 

 descending, and finally flowing along the floor towards 

 the fire to complete 'the circle and to ascend again. 

 The draught in the chimneys of fireplaces and lamps 

 is also a consequence of the fact of hot air being lighter 

 than cold air. 



The circulation of air in a room may be demonstrated by sprink- 

 ling a few drops of perfume upon the floor very near to the fire- 

 place. Of several persons in the room, those near the window will 

 perceive the odour first, those in the middle of the room next, and 

 those near the fireplace last. The experiment will not succeed com- 

 pletely unless all draught from windows or side- doors is carefully 

 excluded. 



When the air in each of two adjoining rooms has a 

 different temperature, and communication is allowed 

 between them, the colder and heavier air flows below, 

 along the floor, from the colder room into the warmer, 

 while the warmer and lighter air flows above, near the 

 ceiling, from the hotter room into the colder. By a 

 similar circulation the whole atmosphere of the earth 

 is kept in continual motion. Winds are caused by 

 currents of air resulting from a difference in tempera- 

 ture between adjacent regions of the earth. Thus if 

 the temperature of a certain extent of country rises, 

 the air in contact with it becomes heated, it expands 

 and ascends towards the higher regions of the atmo- 

 sphere, while, in consequence of the difference in the 

 specific gravity of the hot and the neighbouring cold 

 air, a current of cold air will be produced in an oppo- 

 site direction ; that is, two distinct winds will be pro- 

 duced, an upper one setting from the heated region, 

 and a lower one setting towards it. 



3E 



