But our enumeration of the uses of trees is not yet complete. Besides the influence of the forest, in mountain ranges, as a means of preventing the scooping out of ravines and the accumulations of water which fill them, trees subserve a valuable purpose, in lower positions, as barriers against the spread of floods and of the material they transport with them; but this will be more appropriately considered in the chapter on the waters; and another very important use of trees, that of fixing movable sand-dunes, and reclaiming them to profitable cultivation, will be pointed out in the chapter on the sands. The vast extension of railroads, of manufactures and the mechanical arts, of military armaments, and especially of the commercial fleets and navies of Christendom within the present century, has greatly augmented the demand for wood,[272] and, but for improvements in metallurgy which have facilitated the substitution of iron for that material, the last twenty-five years would almost have stripped Europe of her only remaining trees fit for such uses.[273] The walnut trees alone felled in Europe within two years to furnish the armies of America with gunstocks, would form a forest of no inconsiderable extent.[274] _The Forests of Europe._ Mirabeau estimated the forests of France in 1750 at seventeen millions of hectares [42,000,000 acres]; in 1860 they were reduced to eight millions [19,769,000 acres]. This would be at the rate of 82,000 hectares [202,600 acres] per year. Troy, from whose valuable pamphlet, _Etude sur le Reboisement des Montagnes_, I take these statistical details, supposes that Mirabeau's statement may have been an extravagant one, but it still remains certain that the waste has been enormous; for it is known that, in some departments, that of Ariege, for instance, clearing has gone on during the last half century at the rate of three thousand acres a year,[275] and in all parts of the empire trees have been felled faster than they have grown. The total area of France, excluding Savoy, is about one hundred and thirty-one millions of acres. The extent of forest supposed by Mirabeau would be about thirty-two per cent. of the whole territory.[276] In a country and a climate where the conservative influences of the forest are so necessary as in France, trees must cover a large surface and be grouped in large masses, in order to discharge to the best advantage the various functions assigned to them by nature. The consumption of wood is rapidly increasing in that empire, and a large part of its territory is mountainous, sterile, and otherwise such in character or situation that it can be more profitably devoted to the growth of wood than to any agricultural use. Hence it is evident that the proportion of forest in 1750, taking even Mirabeau's large estimate, was not very much too great for permanent maintenance, though doubtless the distribution was so unequal that it would have been