ioo GENERAL SCIENCE 



this heated area; the cool air moves upward as fast 

 as it is heated, thus carrying the heat away. The 

 movement of the air in this case is something similar to 

 the movement of the water in the flask in the illustra- 

 tion, but on a much larger scale. The winds and ocean 

 currents are the great natural convection currents which 

 distribute heat over the earth's surface. 



70. Primitive Methods of Heating. Man in his 

 first steps toward civilization lived much like some of 

 the savages to-day. He built his first fires out in the 

 open as camp fires and warmed himself by them. He 

 then built the same kind of a fire inside of the little hut 

 that he learned to make. Over this fire primitive man 

 also cooked his food. He gradually learned to build 

 stones around the fire or around the place where the 

 fire was to be built. Then to escape the evil of the 

 smoke, he built a tall top to his stone firebox. This top 

 was extended until it carried the smoke outside his little 

 house. This stone firebox with its top to carry away 

 the smoke finally developed into the open fireplace and 

 chimney, such as the early settlers used in America. 

 This type of open fire exists now in the form of the 

 modern grate in which wood, coal, or gas is burned. 

 Metal stoves and furnaces are modern inventions and very 

 convenient in comparison with the ancient methods. 



71. Modern Method of Heating. Modern man tries 

 to have the smoke and gases made by the fires carried 

 from his home without poisoning the air in his living 

 rooms. We to-day have several kinds of fuel which the 

 ancients did not have. What are they? 



There are four principal modern types of heating 

 which will be discussed in the following four sections. 



72. The Open Fire and Stove. An open fire of 



