6o 



ILLINOIS ACADEMY OF SCIENCE. 



Duration of Supply. 



Estimates on the duration of supply are of course subject to 

 errors of great magnitude. Figures for original coal are ex- 

 tremely uncertain ; those for waste are doubtful ; and the future 

 holds large modifying factors. It is estimated that if the present 

 marvelous growth continues, and the output doubles each ten- 



Plate IV. Average Yearly Production of COAL in decades for 

 the United States. 



year period, that the easily accessible coal of the United States 

 will be gone by 2015 A. D., or in 103 years. It has been shown 

 that totals by twenty-year periods avoid variables due to business 

 depression and prosperity, and ofifer a curve based on the decrease 

 in rate of increase of production. The figures for the curve are 

 as follows, based on production in millions of tons for twenty-year 

 periods ending with 1907: 



TABLE I. 



Production, Per cent of 

 Millions of Tons. Increase. 



1828-47 37-3 



1848-67 306.0 720 



1868-87 1451-0 374 



1888-1907 5068.0 249 



The curve indicates that under present conditions of waste our 

 entire supply of coal will last till 2040 A. D. 



Of course, the assumptions for such estimates are faulty ; and 

 the errors already mentioned with reference to the amount of 



