90 LIGHT SCIENCE FOR LEISURE HOURS. 



times her present annual consumption, or at 350 

 millions of tons. Few will regard this as an unduly 

 low estimate when they remember that it is exceedingly 

 unlikely that the present population of Britain will ever 

 be tripled, and that a triple population could be com- 

 mercially far more active (in relation to its numbers) 

 than the present population, with no greater consumption 

 of coal per head. Now, to begin with, if this enormous 

 annual consumption began immediately, we should yet 

 (with Mr. Jevons's assumption as to the quantity of 

 available coal) have 570 years' lease of power instead 

 of 110. But, as a matter of fact, so soon as we have 

 recognised the principle that there is a limit to the 

 increase of annual consumption, we are compelled to- 

 believe that that limit will be approached by a much 

 gentler gradient, so to speak, than the same consump- 

 tion as attained on Mr. Jevons's assumption. According 

 to his view, in fact, an annual consumption of 350 

 millions of tons per annum will be attained early in the- 

 twentieth century ; but according to the theory which 

 sets such a consumption as the highest ever to be 

 attained, we should place its attainment several hun- 

 dreds of years later. This is a vague statement, I 

 admit, but the very fact on which I am mainly insisting 

 is this, that the evidence at present in our hands is 

 insufficient as a basis of exact calculation. Now, if we- 

 set 500 years hence as the time when the annual con- 

 sumption of coal will have reached the above enormous 

 amount, we should set the total consumption during 

 those centuries at about one-half that due to an annual 



