78 LIGHT SCIENCE FOR LEISURE HOURS. 



second horseshoe-nail in the well-known problem. 

 The effects, after a few years have passed, correspond 

 to the thousands of pounds by which the last shoe-nails 

 of that problem increase the cost of the horse. As Mr. 

 Leonard Lemoran points out in the paper mentioned 

 in the above note, if the assumed rate per cent, of 

 increase continue, 'we should draw in the year 1900 

 from our rocks more than 300 millions of tons, and 

 in 1950 more than 2,000 millions. 1 About 300,000 

 miners are now (1866) employed in raising rather more 

 than 92 millions of tons of coals ; therefore more than 

 eight million miners would be necessary to raise the 

 quantity estimated as the produce of 1950. One-third 

 of the present population of Great Britain would be 

 coal miners.' Or as Mr. Jevons himself sums up our 

 future, ' If our consumption of coal continue to multiply 

 for 110 years at the same rate as hitherto, the total 

 amount of coal consumed in the interval would be 

 100,000 millions of tons. Now as Mr. Hull estimated 

 the available coal in Great Britain, within a depth of 

 4,000 feet, at 83,000 millions of tons, it followed that, 

 adopting Mr. Jevons's mode of calculation, a century 

 would exhaust t all the coal in our present workings, as 

 well as all the coal seams which may be found at a depth 

 of 1,500 feet below the deepest working in the kingdom.' 



1 I have obtained a somewhat different result from a computation 

 I have just gone through. I make the consumption 201 millions in 

 1900, and 1,446 millions in 1950. Mr. Lemoran seems to have taken 

 the percentage at 3| instead of 3*. It is worth noticing how seriously 

 a small change in the percentage affects the result ; the consumption 

 in 1950 becoming 1,760 millions of tons, instead of 1,446 millions. 



