38 HEAT. 



upward current is not, it is to be remembered, because 

 of any tendency of heat to rise. Heat, on the con- 

 trary, travels in one direction as well as another. 

 But it is, -as before explained, because hot water is 

 lighter than cold. Dust of bituminous c.oal answers 

 the purpose in this experiment still better than " flowers 

 of sulphur." It is necessary to have something that 

 will neither sink or swim, but remain suspended in 

 the water. 



74. HEATING ROOMS. A room becomes 



How does a . 



room become heated by a stove in the same manner. 

 heated? rp ne ^ T ^ R i mme( ji a te contact with the 



hot surface becomes heated and rises. Cooler air comes 

 in from all sides to take its place, and be heated and 

 less in turn. A circulation is thus established pre- 

 cisely similar to that which occurs in the flask, as rep- 

 resented in the figure. Any light object, as a feather, 

 or a flock of cotton-wool, held over a stove or an open 

 flame, will prove by its ascent the existence of the up- 

 ward current. How rapidly heat thus passes upward by 

 convection, may be proved by holding the finger above 

 the flame of a lamp, and then at an equal distance at its 

 side, and comparing the effect. 



How is the at- ^' CONVECTION IN THE HEATING OF THE 



mosphere heat- ATMOSPHERE. Heat is distributed through 

 the earth's atmosphere in the same manner. 

 At the equator, where the surface is hottest, the air 

 heated by contact with it rises and flows off toward the 

 poles, while colder air from the polar regions flows in 

 to take its place, to be heated and rise in turn, contin- 

 uing the circulation. But for this arrangement, the 



