The Earth as Modified by Human Action

by George P. Marsh

The new tunnel follows the line of the Claudian emissary—which though badly executed was admirably engineered—but its axis is at a somewhat lower level than that of the old gallery, and its cross-section is about two hundred and fifteen square feet, allowing a discharge of about 2,400 cubic feet to the second, while the Roman work had a cross-section of only one hundred and two square feet, with a possible delivery of 424 cubic feet to the second.

In consequence of the nature of the rock and of the soil, which had been loosened and shattered by the falling in of much of the crown and walls of the old tunnel—every stone of which it was necessary to remove in the progress of the work—and the great head of water in the lake from unusually wet seasons, the technical difficulties to be surmounted were most baffling and discouraging in character, and of such extreme gravity that it may well be doubted whether the art of engineering has anywhere triumphed over more serious obstacles. This great "victory of peace"—probably the grandest work of physical improvement ever effected by the means, the energy, and the munificence of a single individual—is of no small geographical and economical, as well as sanitary, importance, but it has a still higher moral value as an almost unique example of the exercise of public spirit, courage, and perseverance in the accomplishment of a noble and beneficent enterprise by a private citizen. [Footnote: The draining of Lake Celano was undertaken by a company, but Prince Alessandro Torlonia of Rome bought up the interest of all the shareholders and has executed the entire work at his own private expense. Montricher, the celebrated constructor of the great aqueduct of Marseilles, was the engineer who designed and partly carried out the plans, and after his lamentable death the work has been directed with equal ability by Bermont and Brisse.—See Leon De Rothou, Prosciugamento del Lago Fucino, 8vo. Firenza, 1871.]

The crater-lake of Nemi, in the same volcanic region as that of Albano, is also drained by a subterranean tunnel probably of very ancient construction, and the Valle-Riccia appears to have once been the basin of a lake long since laid dry, but whether by the bursting of its banks or by human art we are unable to say.

The success of the Lake Celano tunnel has suggested other like improvements in Italy. A gallery has been cut, under circumstances of great difficulty, to drain Lake Agnano near Naples, and a project for the execution of a similar operation on the Lake of Perugia, the ancient Trasimenus, which covers more than 40,000 acres, is under discussion.

Many similar enterprises have been conceived and executed in modern times, both for the purpose of reclaiming land covered by water and for sanitary reasons. [Footnote: A considerable work of this character is mentioned by Captain Gilliss as having been executed in Chili, a country to which we should hardly have looked for an improvement of such a nature. The Lake Taguataga was partially drained by cutting through a narrow ridge of land, not at the natural outlet, but upon one side of the lake, and eight thousand acres of land covered by it were gained for cultivation.—U. S. Naval Astronomical Expedition to the Southern Hemisphere, i., pp. 16, 17.

Lake Balaton and the Neusiedler Sea in Hungary have lately been, at least partially, drained.

The lakes of Neuchatel, Bienne, and Morat, in Switzerland, have been connected and the common level of all of them lowered about four feet. The works now in operation will produce, in the course of the year 1874, a further depression of four feet, and recover for agricultural use more than twelve thousand acres of fertile soil.] They are sometimes attended with wholly unexpected evils, as, for example, in the case of Barton Pond, in Vermont, and in that of a lake near Ragunda in Sweden, already mentioned on a former page. Another still less obvious consequence of the withdrawal of the waters has occasionally been observed in these operations. The hydrostatic force with which the water, in virtue of its specific gravity, presses against the banks that confine it, has a tendency to sustain them whenever their composition and texture are not such as to expose them to softening and dissolution by the infiltration of the water. If, then, the slope of the banks is considerable, or if the earth of which they are composed rests on a smooth and slippery stratum inclining towards the bed of the lake, they are liable to fall or slide forward when the mechanical support of the water is removed, and this sometimes happens on a considerable scale. A few years ago the surface of the Lake of Lungern, in the Canton of Unterwalden, in Switzerland, was lowered by driving a tunnel about a quarter of a mile long through the narrow ridge, called the Kaiserstuhl, which forms a barrier at the north end of the basin. When the water was drawn off, the banks, which are steep, cracked and burst, several acres of ground slid down as low as the water receded, and even the whole village of Lungern was thought to be in no small danger. [Footnote: In the course of the year 1864 there were slides of the banks of the Lake of Como, and in one case the grounds of a villa near the water suffered a considerable displacement. More important slips occurred at Fesiolo on the shore of Lago Maggiore in 1867 and 1869, and on the Lake of Orta in 1868. These occurrences excited some apprehensions in regard to the possible effects of projects then under discussion for lowering the level of some of the Italian lakes, to obtain an increased supply of water for irrigation and as a mechanical power, but as it was not proposed to depress the surface below the lowest natural low-water level, there seems to have been little ground for the fears expressed.

See, for important observations on the character and probable results of these projects, Tagliasecchi, Nostizie etc. del Canali dell' Alta Lombardia, Milano, 1871.

Jacini says: "A large proportion of the water of the lakes, instead of discharging itself by the Ticino, the Adda, the Oglio, the Mincio, filters through the silicious strata which underlie the hills, and follows subterranean channels to the plain, where it collects in the fontanili, and being thence conducted into the canals of irrigation, becomes a source of great fertility."—La Proprieta Fondiaria, etc., p.144. The quantity of water escaping from the lakes by infiltration depends much on the hydrostatic pressure on the bottom and the walls of the lake-basins, and consequently the depression of the lake surface, diminishing this pressure, would diminish the infiltration. Hence it is possible that the lowering of the level of these lakes would manifest itself in a decreased supply of water for the springs, fontanili, and wells of Lombardy.]

Mountain Lakes.