122 



RELATION OF HEAT TO COMFORT 



Is water power connected in any way with heat en- 

 ergy? Explain. 



Can you show how wind power and animal muscle 

 power are derived from heat energy? 



Scientists believe that all energy available on the 

 earth comes either directly or indirectly from the 

 sun. Can you trace all of the energy sources shown 

 in the graph back to the sun? On the basis of the 

 data shown in the graph, what conclusions can you 

 reach concerning the sources of energy in the United 

 States? 



How are the common fuels used to supply heat 

 energy on the earth obtained? Wood was formerly 

 one of the chief sources of heat energy in America. 

 In the pioneer days of this country wood supplied 

 at least eighty per cent of the heat energy used. Even 

 though other natural resources such as coal, gas, 

 and oil are now used more extensively, wood still 

 is an important source of heat energy. This is espe- 

 cially true in rural districts where the farm woodlot 

 is still used to supply fuel for cooking and heating. 

 In and near cities the chief use of firewood is for 

 burning in fireplaces. Nearly one hundred million 

 cords of firewood are still used annually in the United 

 States. 



As wood became scarcer, its cost increased, and 

 other sources of heat energy were sought and de- 

 veloped. Another factor which has influenced the 

 decline of wood as a fuel is the fact that other fuels 

 give much more heat, pound for pound, than it does. 

 For example, it requires nearly three pounds of wood 

 to produce as much heat as can be obtained from 

 one pound of anthracite coal. 



Coal is our most common source of heat energy 

 in this country. As you have learned from the study 

 of the graph in Figure 187, coal supplies more than 

 fifty-eight per cent of the heat energy used in this 

 country. There are several reasons for this. Coal is 

 relatively cheap as compared with other fuels, is 

 easily accessible to most of the country, and requires 

 a minimum of equipment to burn it. Oil and gas 

 must have special burners attached to the furnace 

 when they are used for house heating. 



There are two principal types of coal used in this 

 country, the soft or bituminous, and the hard or 

 anthracite. Soft coal contains about fifty-four per 

 cent carbon, forty per cent gaseous material, and five 

 per cent moisture, while hard coal contains ninety- 

 five per cent carbon, two per cent gaseous material, 

 and three per cent moisture. The differences in the 

 composition of the two types of coal is due to the 

 changes which they have undergone in their forma- 

 tion. 



Coal was formed millions of years ago from plants. 

 Geologists, scientists who study rock formations, in- 



form us that ages ago the climate in those parts of 

 the earth where coal is now found was very warm 

 and tropical. Also they believe that there was more 

 carbon dioxide in the atmosphere than there is at 

 present. Certain parts of the earth were low and 

 swampy. As these conditions were ideal for plant 

 life, giant ferns, mosses, grasses, and trees grew in 

 abundance and flourished. As the plants in the 

 swamps and low lying regions died, they fell into the 

 water and were prevented from decaying. Do you 

 know why? This process continued for centuries, 

 producing thick layers of plant remains. It is believed 

 that a sinking of the land followed and that every- 

 thing was covered over with water and heavy layers 

 of sand and clay that later hardened into rock. The 

 pressure of the rocks, combined with heat and chemi- 

 cal action, changed the layers of plant remains into 

 coal by driving out the 

 gas and tar. As you have 

 seen, soft coal contains 

 a larger percentage of 

 gases than hard coal, in- 

 dicating that it was not 

 subjected to extreme 

 heat and pressure. Hard 

 coal was probably 

 formed from soft coal 

 through the action of 

 heat and pressure which 

 forced out most of the 

 gases and tar. Figure 188 

 shows the artist's con- 

 ception, based on scien- 

 tific evidence, of how a 

 forest of the Carbonifer- 

 ous Period (the geologi- 

 cal period when much 

 of our coal was formed) 



might have appeared. Do the plants and animals 

 shown resemble any of our modern forms? 



Coal occurs in nearly horizontal layers in the earth. 

 In some places the deposits are so near the surface 

 that the coal can be mined with steam shovels by a 

 method known as "strip" mining. In other places 

 shafts are sunk into the earth and galleries dug into 

 the coal veins when they are reached. In still other 

 places horizontal "drifts" are opened into the side 

 of a mountain or hill to reach the coal vein. Figure 

 189 shows a man at work in a coal mine. 



Figure 190 is a map of the principal coal areas of 

 the United States. 



Hard coal is used chiefly for house heating because 

 it is a cleaner fuel than soft coal. However, most of 

 our available heat energy in this country comes from 

 soft coal because it is much more abundant and there- 



Amcrican Museum of Nat. History 



FIG. 188. A SCENE FROM THE 



COAL AGE 



