only 10,652 tons:—the 10,000 of the second year would be replaced by 10,326 tons in the third year, and the remaining 326 would be increased by 3¼ tons for each hundred, or by rather more than 10½ tons; so that in all there would be 10,662¼ tons, instead of 10,652. Now the difference in this third year seems small, though when it is applied to about nine thousand times 10,000 tons it is by no means small, amounting in fact to 95,000 tons; but when the principle is extended to sequent years its effects assume paramount importance. The small increase is as the small increase of a farthing for the 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.[7] 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 ‘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.’ It should be added, however, that Mr. Stanley Jevons mentioned 200,000 millions of tons as the probable limit of the coal supplies of Great Britain.