HEATING BUILDINGS 99 



The coloring matter will be uniformly distributed in a 

 very short time. These currents distribute the heat so 

 thoroughly that the water in all parts of the flask is 

 kept at about the same temperature. 



The water nearest the flame becomes heated and ex- 

 pands. It is thus made less dense than the surrounding 

 water, which forces the less dense water to the surface. 

 While on the surface it cools a fraction of a degree and 

 becomes more dense and so descends at the edge of the 

 flask and forces the slightly warmer water up from the 

 bottom. It can easily be seen how this method differs 

 from conduction. In convection the whole mass of mole- 

 cules moves and warms others by contact, while in conduc- 

 tion the molecules heat their neighbors by a vibratory motion. 



This method of heat transfer is illustrated by the 

 heating of buildings with hot air, steam, and hot water, 

 and even to some extent by stoves or open fires. It is 

 also illustrated by the ocean currents. The water in 

 and near the Gulf of Mexico is heated by the sun, and 

 this warm water, the Gulf Stream, moves across the 

 Atlantic Ocean and warms the whole of Western Europe. 

 The Japan Current coming across the Pacific Ocean 

 warms our Western coast. These are two great natural 

 heating plants. The convection currents of the ocean 

 are partly caused by the unequal heating of the water 

 by the sun. 



Winds are also partially caused by unequal heating of 

 the earth's surface, and therefore they are convection 

 currents which distribute heat. When any part of the 

 earth's surface is heated, the air over that part also 

 becomes heated and so expands and becomes less dense, 

 and hence it is not so heavy as the cooler air surrounding 

 this heated area. This cooler air is soon flowing toward 



