A more useful picture than that of the isogeothermal lines would be one of all the arable land covering the continents of the Old World and the New,—indicating the composition of this nutritive earth, the nature of the soil on which it reposes, as well as the various kinds of cultivation appropriate in different climates. Here is a work to be achieved,—a work which would benefit the whole human race,—a work differing vastly from the conquests and achievements of too many of those "heroes" the world delights to honour.
"Peace hath her victories
More renowned than war."
In this immense task, of which, as yet, not even the outlines have been sketched, particular attention would require to be paid to the subsoil; for upon this the success of all cultivation literally depends.
Fig. 69.—a, Humus, or stratum of arable earth,—the horizontal line shows the depth reached by the labourer; b, subsoil; c, subsoil; d, arable earth; e, humus in an inclined stratum; f, humus mixed with subsoil.
Arable land is the most superficial stratum of the cultivable terrestrial crust; it is this which the plough turns up and subdues; it is this which, properly manured, and enriched by the decomposition of organic matter, furnishes to vegetables their principal nourishment. As it varies in thickness, it necessarily presents one or other of the following circumstances:—1st, The labourer, penetrating the entire stratum of arable earth (Fig. 69, a), will strike down to the subsoil (Fig. 69, b); or, 2d, he will not traverse the entire stratum (Fig. 69, c); or, 3d, after having traversed the entire depth of the humus, he will reach a portion of the subsoil (Fig. 69, d); or, 4th, after having gone through both humus and subsoil, he will discover another layer of arable earth, which may be either pure humus, in a thick inclined stratum (Fig. 69, e), or humus mixed with the débris of the subsoil.
As for the subsoil, it may, by its composition, completely modify, stimulate, or delay the action of the vegetable mould, however rich this may be in assimilating principles. Thus, where the subsoil is argillaceous, the pluvial waters are arrested by it as by a bed of impervious cement, and render the ground too damp and cold to yield abundant harvests. In such a case subsoil-drainage is the best remedy. But if the earth be porous, the moisture gradually percolates through its various layers, fertilising and warming, communicating to the plants the needful humidity, and assisting in the production of that most glorious of all the scenes of cultivated nature—a corn-field thickly ripe with golden grain. In the poet's "Palace of Art" no finer picture can be seen than this:
"The reapers at their sultry toil.
In front they bind the sheaves. Behind