Page:Popular Science Monthly Volume 32.djvu/633

Rh farther down, without apparently having received any new supply. The inexhaustible abundance of this interior sheet also receives here an agricultural application which is, perhaps, to the present time, unique. The water which is drawn from it by means of shallow wells called fontanelle is, in consequence of its nearly constant temperature, which is higher in winter than that of the ambient air, eminently suitable for irrigation. By forcing it to flow constantly in a thin sheet over the ground, the peasants are able, in a cold country, to cut their grass in January as in the summer-time. There are more than a thousand of these artificial subsoil springs, occupying a zone about two hundred kilometres long, extending from Ticino to Verona.

All rocks which are penetrable to water by means of fissures are also capable of containing phreatic water. The water in these sheets is not stagnant, but is animated by a slow and continuous motion. Among the facts that prove this we may cite the transportation in the subsoil of impurities like coal-tar, in the same direction, over several hundred metres, in a series of wells, the alignment of which marks the direction of the current. This movement is due to the general incline of the sheet.

In volcanic masses, the scoriaceous dejections and the lava-flows, with their cavities of various dimensions, offer no less facility for infiltrations. Rain-waters penetrate them and reappear lower down. Among the flows of fifty volcanoes in Auvergne, that which issues from the Puy de Gravenoire, near Clermont, gives rise to three springs: first at Fontanat; then at Royat, where they issue from a cave opened in the scoria surmounted by prismatic lava; and at the lower end of the flow the water is discharged under similar conditions to the advantage of the city of Clermont. In the same way, after having formed at Murois those scoriaceous caves to which George Sand has lent an infernal aspect, the long flow of the Tartaret gives out in its course a series of springs, around which several villages have grouped themselves. Thus fire is found to prepare the way for water by creating subterranean conduits for it.

The natural action of the waters which we have studied in the superficial deposits is indicated with equal clearness at a greater depth, in the midst of the stratified rocks. In these last, in fact, certain beds, very penetrable to water, alternate with others which arrest its passage. Whether the beds be horizontal or inclined, the relief of the soil is frequently so gashed that the impermeable support of the filtering and water-bearing stratum crops out and determines a flow by virtue of hydrostatic laws. These natural reservoirs thus produce springs which are permanent, provided successive rains furnish a sufficient supply of water, while they also sometimes simply give place to irregular oozings. These effusions occur not on the continents only, but also in sea-basins.

The sedimentary rocks, in their great thickness, inclose a