The Earth As Modified By Human Action Part 31

The Nile is larger than all the rivers of Lombardy together, [Footnote: From daily measurements during a period of fourteen years--1827 to 1840--the mean delivery of the Po at Ponte Lagoscuro, below the entrance of its last tributary, is found to be 1,720 cubic metres, or 60,745 cubic feet, per second. Its smallest delivery is 186 cubic metres, or 6,569 cubic feet, its greatest 5,156 cubic metres, or 152,094 cubic feet. The average delivery of the Nile being 101,000 cubic feet per second, it follows that the Po contributes to the Adriatic rather more than six-tenths as much water as the Nile to the Mediterranean--a result which will surprise most readers.

It is worth remembering that the mean delivery of the Rhone is almost identical with that of the Po, and that of the Rhine is very nearly the same. Though the Po receives four-tenths of its water from lakes, in which the streams that empty into them let fall the solid material they bring down from the mountains, its deposits in the Adriatic are at least sixty or seventy per cent. greater than those transported to the Mediterranean by the Rhone, which derives most of its supply from mountain and torrential tributaries. Those tributaries lodge much sediment in the Lake of Geneva and the Lac de Bourget, but the total erosion of the Po and its affluents must be considerably greater than that of the Rhone system. The Rhine conveys to the sea much less sediment than either of the other two rivers.--Lombardini, Cargiamenti nella condizione del Po, pp. 29, 39.

The mean discharge of the Mississippi is 675,000 cubic feet per second, and, accordingly, that river contributes to the sea about eleven times as much water as the Po, and more than six and a half times as much as the Nile. The discharge of the Mississippi is estimated at one-fourth of the precipitation in its basin--certainly a very large proportion, when we consider the rapidity of evaporation in many parts of the basin, and the probable loss by infiltration.--Humphreys and Abbott'S Report, p.

93.

The basin of the Mississippi has an area forty-six times as large as that of the Po, with a mean annual precipitation of thirty inches, while that of the Po, at least according to official statistics, has a precipitation of forty inches. Hence the down-fall in the former is one-fourth less than in the latter. Besides this, the Mississippi loses little or nothing by the diversion of its waters for irrigation.

Consequently the measured discharge of the Mississippi is proportionally much less than that of the Po, and we are authorized to conclude that the difference is partly due to the escape of water from the bed, or at least the basin of the Mississippi, by subterranean channels.

These comparisons are interesting in reference to the supply received by the sea directly from great rivers, but they fail to give a true idea of the real volume of the latter. To take the case of the Nile and the Po: we have reason to suppose that comparatively little water is diverted from the tributaries of the former for irrigation, but enormous quant.i.ties are drawn from its main trunk for that purpose, below the point where it receives its last affluent. This quant.i.ty is now increasing in so rapid a proportion, that Elisee Reclus foresees the day when the entire low-water current will be absorbed by new arrangements to meet the needs of extended and improved agriculture. On the other hand, while the affluents of the Po send off a great quant.i.ty of water into ca.n.a.ls of irrigation, the main trunk loses little or nothing in that way except at Chiva.s.so. Trustworthy data are wanting to enable us to estimate how far these different modes of utilizing the water balance each other in the case under consideration. Perhaps the Ca.n.a.l Cavour, and other irrigating ca.n.a.ls now proposed, may one day intercept as large a proportion of the supply of the lower Po as Egyptian dikes, ca.n.a.ls, shadoofs, and steam-pumps do of that of the Nile.

Another circ.u.mstance is important to be considered in comparing the character of these three rivers. The Po runs nearly east and west, and it and its tributaries are exposed to no other difference of meterological conditions than those which always subsist between the mountains and the plains. The course of the Nile and the Mississippi is mainly north and south. The sources of the Nile are in a very humid region, its lower course for many hundred miles in almost rainless lat.i.tudes with enormous evaporating power, while the precipitation is large throughout the Mississippi system, except in the basins of some of its western affluents.] it drains a basin fifty, possibly even a hundred, times as extensive, its banks have been occupied by man probably twice as long. But its geographical character has not been much changed in the whole period of recorded history, and, though its outlets have somewhat fluctuated in number and position, its historically known encroachments upon the sea are trifling compared with those of the Po and the neighboring streams. The deposits of the Nile are naturally greater in Upper than in Lower Egypt. They are found to have raised the soil at Thebes about seven feet within the last seventeen hundred years, and in the Delta the rise has been certainly more than half as great.

We shall, therefore, probably not exceed the truth if we suppose the annually inundated surface of Egypt to have been elevated, upon an average, ten feet, [Footnote: Fraas and Eyth maintain that we have no trustworthy data for calculating the annual or secular elevation of the soil of Egypt by the sediment of the Nile. The deposit, they say, is variable from irregularity of current, and especially from the interference of man with the operations of nature, to a degree which renders any probable computation of the amount quite impossible.--Fraas, Aus dem Orient, pp. 212, 213.

The sedimentary matter transported by the Nile might doubtless be estimated with approximate precision by careful observation of the proportion of suspended slime and water at different stations and seasons for a few successive years. Figari Bey states that at low stages the water of the Nile contains little or no sediment, and that the greatest proportion occurs about the end of July, and of course, while the river is still rising. Experiments at Khartum at that season showed solid matter in the proportion of one to a thousand by weight. The quant.i.ty is relatively greater at Cairo, a fact which shows that the river receives more earth from the erosion of its banks than it deposits at its own bottom, and it must consequently widen its channel unless we suppose a secular depression of the coast at the mouth of the Nile which produces an increased inclination of the bed of the river, and consequently an augmented velocity of flow sufficient to sweep out earth from the bottom and mix it with the current.

Hersch.e.l.l states the Nile sediment at 1 in 633 by weight, and computes the entire annual quant.i.ty at 140 millions of tons.--Physical Geography, p. 231.

The mean proportion of sedimentary material in the waters of the Mississippi is calculated at 1 to 1,500 by weight, and 1 to 2,900 in volume, and the total annual quant.i.ty at 812,500,000,000 pounds, which would cover one square mile to the depth of 214 feet.--Humphreys and Abbott, Report, p. 140.] within the last 5,000 years, or twice and a half the period during which the history of the Po is known to us.

[Footnote: We are quite safe in supposing that the valley of the Nile has been occupied by man at least 5,000 years. The dates of Egyptian chronology are uncertain, but I believe no inquirer estimates the age of the great pyramids at less than forty centuries, and the construction of such works implies an already ancient civilization.

It is an interesting fact that the old Egyptian system of embankments and ca.n.a.ls is probably more ancient than the geological changes which have converted the Mississippi from a limpid to a turbid stream, and occasioned the formation of the vast delta at the mouth of that river.

Humphreys and Abbot conclude that the delta of the Mississippi began its encroachments on the Gulf of Mexico not more than 4,400 years ago, before which period they suppose the Mississippi to have been "a comparatively clear stream," conveying very little sediment to the sea.

The present rate of advance of the delta is 262 feet a year, and there are reasons for thinking that the amount of deposit has long been approximately constant.--Report, pp. 435, 436.]

As I have observed, the area of cultivated soil is much less extensive now than under the dynasties of the Pharaohs and the Ptolemies; for--though, in consequence of the elevation of the river-bed, the inundations now have a wider NATURAL spread--the industry of the ancient Egyptians conducted the Nile water over a great surface which it does not now reach.

Had the Nile been banked in, like the Po, all this deposit, except that contained in the water diverted by ca.n.a.ls or otherwise drawn from the river for irrigation and other purposes, would have soon carried out to sea. This would have been a considerable quant.i.ty; for the Nile holds some earth in suspension at all seasons except at the very lowest water, a much larger proportion during the flood, and irrigation must have been carried on during the whole year. The precise amount of sediment which would have been thus distributed over the soil is matter of conjecture, but though large, it would have been much less than the inundations have deposited, and continuous longitudinal embankments would have compelled the Nile to transport to the Mediterranean an immense quant.i.ty over and above what it has actually deposited in that sea. The Mediterranean is shoal for some miles out to sea along the whole coast of the Delta, and the large bays or lagoons within the coast-line, which communicate both with the river and the sea, have little depth of water. These lagoons the river deposits would have filled up, and there would still have been surplus earth enough to extend the Delta far into the Mediterranean.

[Footnote: The present annual extension of the Delta is, if perceptible, at all events very small. According to some authorities, a few hectares are added every year at each Nile mouth. Others, among whom I may mention Fraas, deny that there is any extension at all, the deposit being balanced by a secular depression of the coast.

Elisee Reclus states that the Delta advances about 40 inches per year.--La Terre, i., p. 500.]

Obstruction of River Mouths.

The mouths of a large proportion of the streams known to ancient navigation are already blocked up by sand-bars or fluviatile deposits, and the maritime approaches to river harbors frequented by the s.h.i.+ps of Phenicia and Carthage and Greece and Rome are shoaled to a considerable distance out to sea. The inclination of the lower course of almost every known river bed has been considerably reduced within the historical period, and nothing but great volume of water, or exceptional rapidity of flow, now enables a few large streams like the Amazon, the La Plata, the Ganges, and, in a loss degree, the Mississippi, to carry their own deposits far enough out into deep water to prevent the formation of serious obstructions to navigation. But the degradation of their banks, and the transportation of earthy matter to the sea by their currents, are gradually filling up the estuaries even of those mighty floods, and unless the threatened evil shall be averted by the action of geological forces, or by artificial contrivances more efficient than dredging-machines, the destruction of every harbor in the world which receives a considerable river must inevitably take place at no very distant date.

This result would, perhaps, have followed in some incalculably distant future, if man had not come to inhabit the earth as soon as the natural forces which had formed its surface had arrived at such an approximate equilibrium that his existence on the globe was possible; but the general effect of his industrial operations has been to accelerate it immensely. Rivers, in countries planted by nature with forests and never inhabited by man, employ the little earth and gravel they transport chiefly to raise their own beds and to form plains in their basins. In their upper course, where the current is swiftest, they are most heavily charged with coa.r.s.e rolled or suspended matter, and this, in floods, they deposit on their sh.o.r.es in the mountain valleys where they rise; in their middle course, a lighter earth is spread over the bottom of their widening basins, and forms plains of moderate extent; the fine silt which floats farther is deposited over a still broader area, or, if carried out to sea, is in great part quickly swept far off by marine currents and dropped at last in deep water. Man's "improvement" of the soil increased the erosion from its surface; his arrangements for confining the lateral spread of the water in floods compel the rivers to transport to their mouths the earth derived from that erosion even in their upper course; and, consequently, the sediment they deposit at their outlets is not only much larger in quant.i.ty, but composed of heavier materials, which sink more readily to the bottom of the sea and are less easily removed by marine currents.

The tidal movement of the ocean, deep-sea currents, and the agitation of inland waters by the wind, lift up the sands strewn over the bottom by diluvial streams or sent down by mountain torrents, and throw them up on dry land, or deposit them in sheltered bays and nooks of the coast--for the flowing is stronger than the ebbing tide, the affluent than the refluent wave. This cause of injury to harbors it is not in man's power to resist by any means at present available; but, as we have seen, something can be done to prevent the degradation of high grounds, and to diminish the quant.i.ty of earth which is annually abstracted from the mountains, from table-lands, and from river-banks, to raise the bottom of the sea.

This latter cause of harbor obstruction, though an active agent, is, nevertheless, in many cases, the less powerful of the two. The earth suspended in the lower course of fluviatile currents is lighter than sea-sand, river water lighter than sea water, and hence, if a land stream enters the sea with a considerable volume, its water flows over that of the sea, and bears its slime with it until it lets it fall far from sh.o.r.e, or, as is more frequently the case, mingles with some marine current and transports its sediment to a remote point of deposit. The earth borne out of the mouths of the Nile is in part carried over the waves which throw up sea-sand on the beach, and deposited in deep water, in part drifted by the current, which sweeps east and north along the coasts of Egypt and Syria, and lodged in every nook along the sh.o.r.e--and among others, to the great detriment of the Suez Ca.n.a.l, in the artificial harbor at its northern terminus--and in part borne along until it finds a final resting-place in the north-eastern angle of the Mediterranean. [Footnote: "The stream carries this mud, etc., at first farther to the east, and only lets it fall where the force of the current becomes weakened. This explains the continual advance of the land seaward along the Syrian coast, in consequence of which Tyre and Sideon no longer lie on the sh.o.r.e, but some distance inland. That the Nile contributes to this deposit may easily be seen, even by the unscientific observer, from the stained and turbid character of the water for many miles from its mouths. s.h.i.+ps often encounter floating ma.s.ses of Nile mud, and Dr. Clarke thus describes a case of this sort:

"While we were at table, we heard the sailors who were throwing the lead suddenly cry out: 'Three and a half!' The s.h.i.+p slackened her way, and veered about. As she came round, the whole surface of the water was seen to be covered with thick, black mud, which extended so far that it appeared like an island. At the same time, actual land was nowhere to be seen--not even from the mast-head--nor was any notice of such a shoal to be found or any chart on board. The fact is, as we learned afterwards, that a stratum of mud, stretching from the mouths of the Nile for many miles out into the open sea, forms a movable deposit along the Egyptian coast. If this deposit is driven forwards by powerful currents, it sometimes rises to the surface, and disturbs the mariner by the sudden appearance of shoals where the charts lead him to expect a considerable depth of water. But these strata of mud are, in reality, not in the least dangerous. As soon as a s.h.i.+p strikes them they break up at once, and a frigate may hold her course in perfect safety where an inexperienced pilot, misled by his soundings, would every moment expect to be stranded."--Bottger, Das Mittchneer, pp. 188, 189.

This phenomenon is not peculiar to the locality in question, and it is frequently observed in the Gulf of Bengal, and other great marine estuaries.]

Thus the earth loosened by the rude Abyssinian ploughshare, and washed down by the rain from the hills of Ethiopia which man has stripped of their protecting forests, contributes to raise the plains of Egypt, to shoal the maritime channels which lead to the city built by Alexander near the mouth of the Nile, to obstruct the artificial communication between the Mediterranean and the Red Sea, and to fill up the harbors made famous by Phenician commerce.

Deposits of the Tuscan Rivers.

The Arno, and all the rivers rising on the western slopes and spurs of the Apennines, carry down immense quant.i.ties of mud to the Mediterranean. There can be no doubt that the volume of earth so transported is very much greater than it would have been had the soil about the headwaters of those rivers continued to be protected from wash by forests; and there is as little question that the quant.i.ty borne out to sea by the rivers of Western Italy is much increased by artificial embankments, because they are thereby prevented from spreading over the surface the sedimentary matter with which they are charged. The western coast of Tuscany has advanced some miles seawards within a very few centuries. The bed of the sea, for a long distance, has been raised, and of course the relative elevation of the land above it lessened; harbors have been filled up and destroyed; long lines of coast dunes have been formed, and the diminished inclination of the beds of the rivers near their outlets has caused their waters to overflow their banks and convert them into pestilential marshes. The territorial extent of Western Italy has thus been considerably increased, but the amount of soil habitable and cultivable by man has been, in a still higher proportion, diminished. The coast of ancient Etruria was filled with great commercial towns, and their rural environs were occupied by a large and prosperous population. But maritime Tuscany has long been one of the most unhealthy districts in Christendom; the famous Etruscan mart of Populonia has scarcely an inhabitant; the coast is almost absolutely depopulated, and the malarious fevers have extended their ravages far into the interior.

These results are certainly not to be ascribed wholly to human action.

They are, in a large proportion, due to geological causes over which man has no control. The soil of much of Tuscany becomes pasty, almost fluid even, as soon as it is moistened, and when thoroughly saturated with water, it flows like a river. Such a soil as this would not be completely protected by woods, and, indeed, it would now be difficult to confine it long enough to allow it to cover itself with forest vegetation. Nevertheless, it certainly was once chiefly wooded, and the rivers which flow through it must then have been much less charged with earthy matter than at present, and they must have carried into the sea a smaller proportion of their sediment when they were free to deposit it on their banks than since they have been confined by dikes.

It is, in general, true, that the intervention of man has. .h.i.therto seemed to insure the final exhaustion, ruin, and desolation of every province of nature which he has reduced to his dominion. Attila was only giving an energetic and picturesque expression to the tendencies of human action, as personified in himself, when he said that "no gra.s.s grew where his horse's hoofs had trod." The instances are few, where a second civilization has flourished upon the ruins of an ancient culture, and lands once rendered uninhabitable by human acts or neglect have generally been forever abandoned as hopelessly irreclaimable. It is, as I have before remarked, a question of vast importance, how far it is practicable to restore the garden we have wasted, and it is a problem on which experience throws little light, because few deliberate attempts have yet been made at the work of physical regeneration, on a scale large enough to warrant general conclusions in any one cla.s.s of cases.

The valleys and sh.o.r.es of Tuscany form, however, a striking exception to this remark. The succcess with which human guidance has made the operations of nature herself available for the restoration of her disturbed harmonies, in the Val di Chiana and the Tuscan Maremma, is among the n.o.blest, if not the most brilliant achievements of modern engineering, and, regarded in all its bearings on the great question of which I have just spoken, it is, as an example, of more importance to the general interests of humanity than the proudest work of internal improvement that mechanical means have yet constructed. The operations in the Val di Chiana have consisted chiefly in so regulating the flow of the surface-waters into and through it, as to compel them to deposit their sedimentary matter at the will of the engineers, and thereby to raise grounds rendered insalubrious and unfit for agricultural use by stagnating water; the improvements in the Maremma have embraced both this method of elevating the level of the soil, and the prevention of the mixture of salt-water with fresh in the coast marshes and shallow bays, which is regarded as a very active cause of the development of malarious influences. [Footnote: The fact that the mixing of salt and fresh water in coast marshes and lagoons is deleterious to the sanitary condition of the vicinity, has been generally admitted, though the precise reason why a mixture of both should be more injurious than either alone, is not altogether clear. It has been suggested that the admission of salt-water to the lagoons and rivers kills many fresh-water plants and animals, while the fresh water is equally fatal to many marine organisms, and that the decomposition of the remains originates poisonous minsmata. Other theories, however, have been proposed. The whole subject is fully and ably discussed by Dr. Salvagnoli Marchetti in the appendix to his valuable Rapporto aul Bonificamento delle Maremme Toscane. See also the Memorie Economico-Statistiche sulle Maremme Toscane, of the same author. A different view of this subject is taken by Raffanini and Orlandini in a.n.a.lisi, Storico-Fisico-Economica sulli insolubrita nelle Maremme Toscane, Firenze, 1869. See also the important memoir of D. Pantaleoni, Del miasma vegetale e delle Malattie Miasmatiche, in which the views of Salvagnoli on this point are combated.]

Improvements in the Tuscan Maremma.

In the improvements of the Tuscan Maremma, formidable difficulties have been encountered. The territory to be reclaimed was extensive; the salubrious places of retreat for laborers and inspectors were remote; the courses of the rivers to be controlled were long and their natural inclination not rapid; some of them, rising in wooded regions, transported comparatively little earthy matter, [Footnote: This difficulty has been remedied--though with doubtful general advantage--as to one important river of the Maremma, the Pecora, by clearings recently executed along its upper course. "The condition of this marsh and of its affluents are now, November, 1859, much changed, and it is advisable to prosecute its improvement by deposits. In consequence of the extensive felling of the woods upon the plains, hills, and mountains of the territory of Ma.s.sa and Scarlino, within the last ten years, the Pecora and other affluents of the marsh receive, during the rains, water abundantly charged with slime, so that the deposits within the first division of the marsh are already considerable, and we may now hope to see the whole marsh and pond filled up in a much shorter time than we had a right to expect before 1850. This circ.u.mstance totally changes the terms of the question, because the filling of the marsh and pond, which then seemed almost impossible on account of the small amount of sediment deposited by the Pecora, has now become practicable."--Salvagnoli, Rapporto sul Bonificamento delle Maremme Toscane, pp.li., lii.

Between 1830 and 1859 more than 36,000,000 cubic yards of sediment were deposited in the marsh and shoal-water lake of Castiglione alone.--Salvagnoli, Raccolta di Doc.u.menti, pp. 74, 75.] and above all, the coast, which is a recent deposit of the waters, is little elevated above the sea, and admits into its lagoons and the mouths of its rivers floods of salt-water with every western wind, every rising tide.

[Footnote: The tide rises ten inches on the coast of Tuscany. See Memoir by Fantoni, in the appendix to Salvagnoli, Rapporto, p. 189.

On the tides of the Mediterranean, see Bottger, Das Mittelmeer, p. 190.]

The western coast of Tuscany is not supposed to have been an unhealthy region before the conquest of Etruria by the Romans, but it certainly became so within a few centuries after that event. This was a natural consequence of the neglect or wanton destruction of the public improvements, and especially the hydraulic works in which the Etruscans were so skilful, and of the felling of the upland forests, to satisfy the demand for wood at Rome for domestic, industrial, and military purposes. After the downfall of the Roman empire, the incursions of the barbarians, and then feudalism, foreign domination, intestine wars, and temporal and spiritual tyrannies, aggravated still more cruelly the moral and physical evils which Tuscany and the other Italian States were doomed to suffer, and from which they have enjoyed but brief respites during the whole period of modern history. The Maremma was already proverbially unhealthy in the time of Dante, who refers to the fact in several familiar pa.s.sages, and the petty tyrants upon its borders often sent criminals to places of confinement in its territory, as a slow but certain mode of execution. Ignorance of the causes of the insalubrity, and often the interference of private rights, [Footnote: In Catholic countries, the discipline of the church requires a meagre diet at certain seasons, and as fish is not flesh, there is a great demand for that article of food at those periods. For the convenience of monasteries and their patrons, and as a source of pecuniary emolument to ecclesiastical establishments and sometimes to lay proprietors, great numbers of artificial fish-ponds were created during the Middle Ages.

They were generally shallow pools formed by damming up the outlet of marshes, and they were among the most fruitful sources of endemic disease, and of the peculiar malignity of the epidemics which so often ravaged Europe in those centuries. These ponds, in religious hands, were too sacred to be infringed upon for sanitary purposes, and when belonging to powerful lay lords they were almost an inviolable. The rights of fishery were a standing obstacle to every proposal of hydralic improvement, and to this day large and fertile districts in Southern Europe remain sickly and almost unimproved and uninhabited, because the draining of the ponds upon them would reduce the income of proprietors who derive large profits by supplying the faithful, in Lent, with fish, and with various species of waterfowl which, though very fat, are, ecclesiastically speaking, meagre.]prevented the adoption of measures to remove it, and the growing political and commercial importance of the large towns in more healthful localities absorbed the attention of Government, and deprived the Maremma of its just share in the systems of physical improvement which were successfully adopted in interior and Northern Italy.

Before any serious attempts were made to drain or fill up the marshes of the Maremma, various other sanitary experiments were tried. It was generally believed that the insalubrity of the province was the consequence, not the cause, of its depopulation, and that, if it were once densely inhabited, the ordinary operations of agriculture, and especially the maintenance of numerous domestic fires, would restore it to its ancient healthfulness. [Footnote: Macchiavelli advised the Government of Tuscany "to provide that men should restore the wholesomeness of the soil by cultivation, and purify the air by fires."--Salvagnoli, Memorie, p. 111.] In accordance with these views, settlers were invited from various parts of Italy, from Greece, and, after the accession of the Lorraine princes, from that country also, and colonized in the Maremma. To strangers coming from soils and skies so unlike those of the Tuscan marshes, the climate was more fatal than to the inhabitants of the neighboring districts, whose const.i.tutions had become in some degree inured to the local influences, or who at least knew better how to guard against them. The consequence very naturally was that the experiment totally failed to produce the desired effects, and was attended with a great sacrifice of life and a heavy loss to the treasury of the state.

The territory known as the Tuscan Maremma, ora maritime, or Maremme--for the plural form is most generally used--lies upon and near the western coast of Tuscany, and comprises about 1,900 square miles English, of which 500 square miles, or 320,000 acres, are plain and marsh including 45,500 acres of water surface, and about 290,000 acres are forest. One of the mountain peaks, that of Mount Amiata, rises to the height of 6,280 feet. The mountains of the Maremma are healthy, the lower hills much less so, as the malaria is felt at some points at the height of 1,000 feet, and the plains, with the exception of a few localities favorably situated on the seacoast, are in a high degree pestilential.

The fixed population is about 80,000, of whom one-sixth live on the plains in the winter and about one-tenth in the summer. Nine or ten thousand laborers come down from the mountains of the Maremma and the neighboring provinces into the plain, during the latter season, to cultivate and gather the crops.

Out of this small number of inhabitants and strangers, 35,619 were ill enough to require medical treatment between the 1st of June, 1840, and the 1st of June, 1841, and more than one-half the cases were of intermittent, malignant, gastric, or catarrhal fever. Very few agricultural laborers escaped fever, though the disease did not always manifest itself until they had returned to the mountains. In the province of Grosseto, which embraces nearly the whole of the Maremma, the annual mortality was 3.92 per cent., the average duration of life but 23.18 years, and 75 per cent. of the deaths were among persons engaged in agriculture.

The filling up of the low grounds and the partial separation of the waters of the sea and the land, which had been in progress since the year 1827, now began to show very decided effects upon the sanitary condition of the population. In the year ending June 1st, 1842, the number of the sick was reduced by more than 2,000, and the cases of fever by more than 4,000. The next year the cases of fever fell to 10,500, and in that ending June 1st, 1844, to 9,200. The political events of 1848, and the preceding and following years, occasioned the suspension of the works of improvement in the Maremma, but they were resumed after the revolution of 1859. I have spoken with some detail of the improvements in the Tuscan Maremma, because of their great relative importance, and because their history is well known; but like operations have been executed in the territory of Pisa and upon the coast of the duchy of Lucca. In the latter case they were confined princ.i.p.ally to prevention of the intermixing of fresh water with that of the sea. In 1741 sluices or lock-gates were constructed for this purpose, and the following year the fevers, which had been destructive to the coast population for a long time previous, disappeared altogether. In 1768 and 1769, the works having fallen to decay, the fevers returned in a very malignant form, but the rebuilding of the gates again restored the healthfulness of the sh.o.r.e. Similar facts recurred in 1784 and 1785, and again from 1804 to 1821. This long and repeated experience has at last impressed upon the people the necessity of vigilant attention to the sluices, which are now kept in constant repair. The health of the coast is uninterrupted, and Viareggio, the capital town of the district, is now much frequented for its sea-baths and its general salubrity, at a season when formerly it was justly shunned as the abode of disease and death. [Footnote: Giorgini, Sur les causes de l'Insalubrite de l'air dans le voisinage des marais, etc., lue a l'Academie des Sciences a Paris, le 12 Juillet, 1825. Reprinted in Salvagnoli, Rapporto, etc., appendice, p. 5, et seqq.]

Improvements in the Val di Chiana.

For twenty miles or more after the remotest headwaters of the Arno have united to form a considerable stream, this river flows south-eastwards to the vicinity of Arezzo. It here sweeps round to the north-west, and follows that course to near its junction with the Sieve, a few miles above Florence, from which point its general direction is westward to the sea. From the bend at Arezzo, a depression called the Val di Chiana runs south-eastwards until it strikes into the valley of the Paglia, a tributary of the Tiber, and thus connects the basin of the latter river with that of the Arno. In the Middle Ages, and down to the eighteenth century, the Val di Chiana was often overflowed and devastated by the torrents which poured down from the highlands, transporting great quant.i.ties of slime with their currents, stagnating upon its surface, and gradually converting it into a marshy and unhealthy district, which was at last very greatly reduced in population and productiveness. It had, in fact, become so desolate that even the swallow had deserted it. [Footnote: This curious fact is thus stated in the preface to Fos...o...b..oni (Memorie sopra la Val di Chiana, edition of 1835, p. xiii.), from which also I borrow most of the data hereafter given with respect to that valley: "It is perhaps not universally known, that the swallows, which come from the north [south] to spend the summer in our climate, do not frequent marshy districts with a malarious atmosphere. A proof of the restoration of salubrity in the Val di Chiana is furnished by these aerial visitors, which had never before been seen in those low grounds, but which have appeared within a few years at Forano and other points similarly situated."

Is the air of swamps destructive to the swallows, or is their absence in such localities merely due to the want of human habitations, near which this half-domestic bird loves to breed, perhaps because the house-fly and other insects which follow man are found only in the vicinity of his dwellings In almoust all European countries the swallow is protected, by popular opinion or superst.i.tion, from the persecution to which almost all other birds are subject. It is possible that this respect for the swallow is founded upon ancient observation of the fact just stated on the authority of Fos...o...b..oni. Ignorance mistakes the effect for the cause, and the absence of this bird may have been supposed to be the occasion, not the consequence, of the unhealthiness of particular localities. This opinion once adopted, the swallow would become a sacred bird, and in process of time fables and legends would be invented to give additional sanction to the prejudices which protected it. The Romans considered the swallow as consecrated to the Penates, or household G.o.ds, and according to Peretti (Le Serate del Villaggio, p.

168) the Lombard peasantry think it a sin to kill them, because they are le gallinelle del Signore, the chickens of the Lord.]

The bed of the Arno near Arezzo and that of the Paglia at the southern extremity of the Val di Chiana did not differ much in level. The general inclination of the valley was therefore small; it does not appear to have ever been divided into opposite slopes by a true watershed, and the position of the summit seems to have s.h.i.+fted according to the varying amount and place of deposit of the sediment brought down by the lateral streams which emptied into it. The length of its princ.i.p.al channel of drainage, and even the direction of its flow at any given point, were therefore fluctuating. Hence, much difference of opinion was entertained at different times with regard to the normal course of this stream, and, consequently, to the question whether it was to be regarded as properly an affluent of the Tiber or of the Arno.

The bed of the latter river at the bend has been eroded to the depth of thirty or forty feet, and that, apparently, at no very remote period. [Footnote: Able geologists infer from recent investigations, that, although the Arno flowed to the south within the pliocenic period, the direction of its course was changed at an earlier epoch than that supposed in the text.] If it were elevated to what was evidently original height, the current of the Arno would be so much above that of the Paglia as to allow of a regular flow from its channel to the latter stream, through the Val di Chiana, provided the bed of the valley had remained at the level which excavations prove it to have had a few centuries ago, before it was raised by the deposits I have mentioned.

These facts, together with the testimony of ancient geographers which scarcely admits of any other explanation, are thought to prove that all the waters of the Upper Arno were originally discharged through the Val di Chiana into the Tiber, and that a part of them still continued to flow, at least occasionally, in that direction down to the days of the Roman empire, and perhaps for some time later. The depression of the bed of the Arno, and the raising of that of the valley by the deposits of the lateral torrents, finally cut off the branch of the river which had flowed to the Tiber, and all its waters were turned into its present channel, though the drainage of the princ.i.p.al part of the Val di Chiana appears to have been in a south-eastwardly direction until within a comparatively recent period.