The Earth as Modified by Human Action

by George P. Marsh

Geographical Effect of Physical Improvements in the Netherlands.

The changes which human action has produced within twenty centuries in the Netherlands and the neighboring provinces, are, certainly of no small geographical importance, considered simply as a direct question of loss and gain of territory. They have also, as we shall see hereafter, undoubtedly been attended with some climatic consequences, they have exercised a great influence on the spontaneous animal and vegetable life of this region, and they cannot have failed to produce effects upon tidal and other oceanic currents, the range of which may be very extensive. The force of the tidal wave, the height to which it rises, the direction of its currents, and, in fact, all the phenomena which characterize it, as well as all the effects it produces, depend as much upon the configuration of the coast it washes, and the depth of water, and form of bottom near the shore, as upon the attraction which occasions it. Every one of the terrestrial conditions which affect the character of tidal and other marine currents has been very sensibly modified by the operations I have described, and on this coast, at least, man has acted almost as powerfully on the physical geography of the sea as on that of the land. [Footnote: See, on the influence of the artificial modification of the coast-line on tides and other marine currents, Staring, De Bodem van Nederland, i., p. 279.]

Ancient Hydraulic Works.

The hydraulic works of the Netherlands and of the neighboring states are of such magnitude that—with the exception of the dikes of the Mississippi—they quite throw into the shade all other known artificial arrangements for defending the land against the encroachments of the rivers and the sea, and for reclaiming to the domain of agriculture and civilization soil long covered by the waters. But although the recovery and protection of lands flooded by the sea seems to be an art wholly of Netherlandish origin, we have abundant evidence that, in ancient as well as in comparatively modern times, great enterprises more or less analogous in character have been successfully undertaken, both in inland Europe and in the less familiar countries of the East.

In many cases no historical record remains to inform us when or by whom such works were constructed. The Greeks and Romans, the latter especially, were more inclined to undertake and carry out stupendous material enterprises than to boast of them; and many of the grandest and most important constructions of those nations are absolutely unnoticed by contemporary annalists, and are first mentioned by writers living after all knowledge of the epochs of the projectors of these works had perished. Thus the aqueduct known as the Pont du Gard, near Nimes, which, though not surpassing in volume or in probable cost other analogous constructions of ancient and of modern ages, is yet among the most majestic and imposing remains of ancient civil architecture, is not so much as spoken of by any Roman author, [Footnote: One reason for the silence of Roman writers in respect to great material improvements which had no immediate relation to military or political objects, is doubtless the contempt in which mechanical operations and mechanical contrivances were held by that nation of spoilers. Even the engineer, upon whose skill the attack or defence of a great city depended, was only praefectus fabrum, the master-artisan, and had no military rank or command. This prejudice continued to a late period in the Middle Ages, and the chiefs of artillery were equally without grade or title as soldiers.

"The occupations of all artisans," says Cicero, "are base, and the shop can have nothing of the respectable." De Officiis, 1, i., 42. The position of the surgeon relatively to the physician, in England, is a remnant of the same prejudice, which still survives in full vigor in Italy, with regard to both trade and industry. See p. 6, ante.] and we are in absolute ignorance of the age or the construction of the remarkable tunnel cut to drain Lake Copais in Boeotia. This lake, now reduced by sedimentary deposit and the growth of aquatic and semi-aquatic vegetation to the condition of a marsh, was originally partially drained by natural subterranean outlets in the underlying limestone rock, many of which still exist. But these emissaries, or katavothra, as they are called in both ancient and modern Greek, were insufficient for the discharge of the water, and besides, they were constantly liable to be choked by earth and vegetables, and in such cases the lake rose to a height which produced much injury. To remedy this evil and secure a great accession of fertile soil, at some period anterior to the existence of a written literature in Greece and ages before the time of any prose author whose works have come down to us, two tunnels, one of them four miles long, and of course not inferior to the Torlonian emissary in length, were cut through the solid rock, and may still be followed throughout their whole extent. They were repaired in the time of Alexander the Great, in the fourth century before Christ, and their date was at that time traditionally referred to the reign of rulers who lived as early as the period of the Trojan war.

One of the best known hydraulic works of the Romans is the tunnel which serves to discharge the surplus waters of the Lake of Albano, about fourteen miles from Rome. This lake, about six miles in circuit, occupies one of the craters of an extinct volcanic range, and the surface of its waters is about nine hundred feet above the sea. It is fed by rivulets and subterranean springs originating in the Alban Mount, or Monte Cavo, the most elevated peak of the volcanic group just mentioned, which rises to the height of about three thousand feet. At present the lake has no discoverable natural outlet, and it is not known that the water ever stood at such a height as to flow regularly over the lip of the crater. It seems that at the earliest period of which we have any authentic memorials, its level was usually kept by evaporation, or by discharge through subterranean channels, considerably below the rim of the basin which encompassed it, but in the year 397 B.C., the water, either from the obstruction of such channels, or in consequence of increased supplies from unknown sources, rose to such a height as to flow over the edge of the crater, and threaten inundation to the country below by bursting through its walls. To obviate this danger, a tunnel for carrying off the water was pierced at a level much below the height to which it had risen. This gallery, cut entirely with the chisel through the rock for a distance of six thousand feet, or nearly a mile and one-seventh, is still in so good condition as to serve its original purpose. The fact that this work was contemporaneous with the siege of Veii, has given to ancient annalists occasion to connect the two events, but modern critics are inclined to reject Livy's account of the matter, as one of the many improbable fables which disfigure the pages of that historian. It is, however, repeated by Cicero and by Dionysius of Halicarnassus, and it is by no means impossible that, in an age when priests and soothsayers monopolized both the arts of natural magic and the little which yet existed of physical science, the Government of Rome, by their aid, availed itself at once of the superstition and of the military ardor of its citizens to obtain their sanction to an enterprise which sounder arguments might not have induced them to approve.

Still more remarkable is the tunnel cut by the Emperor Claudius to drain the Lake Fucinus, now Lago di Celano, in the former Neapolitan territory, about fifty miles eastward of Rome. This lake, as far as its history is known, has varied very considerably in its dimensions at different periods, according to the character of the seasons. It lies 2,200 feet above the sea, and has no visible outlet, but was originally either drained by natural subterranean conduits, or kept within certain extreme limits by evaporation. In years of uncommon moisture it spread over the adjacent soil and destroyed the crops; in dry seasons it retreated, and produced epidemic disease by poisonous exhalations from the decay of vegetable and animal matter upon its exposed bed. Julius Caesar had proposed the construction of a tunnel to lower the bed of the lake and provide a regular discharge for its waters, but the enterprise was not actually undertaken until the reign of Claudius, when—after a temporary failure, from errors in levelling by the engineers, as was pretended at the time, or, as now appears certain, in consequence of frauds by the contractors in the execution of the work—it was at least partially completed. From this imperfect construction, it soon got out of repair, but was restored by Hadrian, and is said to have answered its design for some centuries. [Footnote: The fact alluded to in a note on p. 97, ante, that since the opening of a communication between Lake Celano and the Garigliano by the works noticed in the text, fish, of species common in the lake, but not previously found in the river, have become naturalized in the Garigliano, is a circumstance of some weight as evidence that the emissary was not actually open in ancient times; for if the waters had been really connected, the fish of the lake would naturally have followed the descending current and established themselves in the river as they have done now.] In the barbarism which followed the downfall of the empire, it again fell into decay, and though numerous attempts were made to repair it during the Middle Ages, no tolerable success seems to have attended any of these efforts until the present generation.

Draining of Lake Celano by Prince Torlonia.

Works have been some years in progress and are now substantially completed, at a cost of about six millions of dollars, for restoring, or rather enlarging and rebuilding, this ancient tunnel, upon a scale of grandeur which does infinite honor to the liberality and public spirit of the projectors, and with an ingenuity of design and a constructive skill which reflect the highest credit upon the professional ability of the engineers who have planned the works and directed their execution. The length of the Roman tunnel was 18,634 feet, or rather more than three miles and a half, but as the new emissary is designed to drain the lake to the bottom, it must be continued to the lowest part of the basin. It will consequently have a length of not less than 21,000 feet, and, of course, is among the longest subterranean galleries in Europe. Many curious particulars in the design and execution of the original work have been observed in the course of the restoration, but these cannot here be noticed. The difference between the lowest and highest known levels of the surface of the lake is rather more than forty feet and the difference between the areas covered by water at these levels is not less than nine thousand acres. The complete drainage of the lake, including the ground occasionally flooded, will recover, for agricultural occupation, and permanently secure from inundation, about forty-two thousand acres of as fertile soil as any in Italy. [Footnote: Springs rising in the bottom of the lake have materially impeded the process of drainage, and some engineers believe that they will render the complete discharge of the waters impossible. It appears that the earthy and rocky strata underlying the lake are extremely porous, and that the ground already laid dry on the surface absorbs an abnormally large proportion of the precipitation upon it. These strata, therefore, constitute a reservoir which contributes to maintain the spring fed chiefly, no doubt, by underground channels from the neighboring mountains. But it is highly probable that, after a certain time, the process of natural desiccation noticed in note to p. 20, ante, will drain this reservoir, and the entire removal of the surface-water will then become practicable.] The ground already dry enough for cultivation furnishes occupation and a livelihood for a population of 16,000 persons, and it is thought that this number will be augmented to 40,000 when the drainage shall be completely effected.