110 



THE PRODUCTION AND USE OF HEAT 



Pennsylvania more than one-half of the coal is thus left in the 

 mine only about 40 per cent, is removed. The remaining 

 60 per cent, of the coal is left in such shape that it can prob- 

 ably never be recovered. This means that for every ton of 

 anthracite coal which has been mined, about 1J^ tons have 

 been forever lost to the use of mankind. In the bituminous, 

 fields, there has been less waste. For every ton of bituminous 

 coal mined about % ton has been left in the mine. 



TONS 



300,000,000- 



s ; * 



FIG. 84, Annual production of coal in the tJnited States, 1820-1910. 



125. How Long Will Our Coal Supply Last? Government 

 officials have made careful estimates of the number of tons 

 of coal in all known coal deposits of the United States. Ac- 

 cepting this estimate and supposing that the consumption of 

 coal will continue to increase at the same rate in the future as 

 it has in the past, it has been shown that our available coal 

 supply will be exhausted in about 120 years, or about the year 

 2030. We therefore see how necessary it is that we avoid as 

 far as possible all waste of coal. 



At the present time a much larger portion of our coal is 



