It is easy to see that the damage occasioned by such floods as I have

described must be almost incalculable, and it is by no means confined to

the effects produced by overflow and the mechanical force of the

superficial currents. In treating of the devastations of torrents in a

former chapter, I confined myself principally to the erosion of surface

and the transportation of mineral matter to lower grounds by them. The

general action of torrents, as there shown, tends to the ultimate

elevation of their beds by the deposit of the earth, gravel, and stone

conveyed by them; but until they have thus raised their outlets so as

sensibly to diminish the inclination of their channels--and sometimes

when extraordinary floods give the torrents momentum enough to sweep

away the accumulations which they have themselves heaped up--the swift

flow of their currents, aided by the abrasion of the rolling rocks and

gravel, scoops their beds constantly deeper, and they consequently not

only undermine their banks, but frequently sap the most solid

foundations which the art of man can build for the support of bridges

and hydraulic structures.[352]

In the inundation of 1857, the Ardeche destroyed a stone bridge near La

Beaume, which had been built about eighty years before. The resistance

of the piers, which were erected on piles, the channel at that point

being of gravel, produced an eddying current that washed away the bed of

the river above them, and the foundation, thus deprived of lateral

support, yielded to the weight of the bridge, and the piles and piers

fell up stream.

By a curious law of compensation, the stream which, at flood, scoops out

cavities in its bed, often fills them up again as soon as the diminished

velocity of the current allows it to let fall the sand and gravel with

which it is charged, so that when the waters return to their usual

channel, the bottom shows no sign of having been disturbed. In a flood

of the Escontay, a tributary of the Rhone, in 1846, piles driven sixteen

feet into its gravelly bed for the foundation of a pier were torn up and

carried off, and yet, when the river had fallen to low-water mark, the

bottom at that point appeared to have been raised higher than it was

before the flood, by new deposits of sand and gravel, while the cut

stones of the half-built pier were found buried to a great depth, in the

excavation which the water had first washed out. The gravel with which

rivers thus restore the level of their beds is principally derived from

the crushing of the rocks brought down by the mountain torrents, and the

destructive effects of inundations are immensely diminished by this

reduction of large stones to minute fragments. If the blocks hurled down

from the cliffs were transported unbroken to the channels of large

rivers, the mechanical force of their movement would be irresistible.

They would overthrow the strongest barriers, spread themselves over a

surface as wide as the flow of the waters, and convert the most smiling

valleys into scenes of the wildest desolation.

c. _Crushing Force of Torrents._

There are few operations of nature where the effect seems more

disproportioned to the cause than in the comminution of rock in the

channel of swift waters. Igneous rocks are generally so hard as to be

wrought with great difficulty, and they bear the weight of enormous

superstructures without yielding to the pressure; but to the torrent

they are as wheat to the millstone. The streams which pour down the

southern scarp of the Mediterranean Alps along the Riviera di Ponente,

near Genoa, have short courses, and a brisk walk of a couple of hours or

even less takes you from the sea beach to the headspring of many of

them. In their heaviest floods, they bring rounded masses of serpentine

quite down to the sea, but at ordinary high water their lower course is

charged only with finely divided particles of that rock. Hence, while,

near their sources, their channels are filled with pebbles and angular

fragments, intermixed with a little gravel, the proportions are reversed

near their mouths, and, just above the points where their outlets are

partially choked by the rolling shingle of the beach, their beds are

composed of sand and gravel to the almost total exclusion of pebbles.

The greatest depth of the basin of the Ardeche is seventy-five miles,

but most of its tributaries have a much shorter course. "These

affluents," says Mardigny, "hurl into the bed of the Ardeche enormous

blocks of rock, which this river, in its turn, bears onward, and grinds

down, at high water, so that its current rolls only gravel at its

confluence with the Rhone."[353]

Guglielmini argued that the gravel and sand of the beds of running

streams were derived from the trituration of rocks by the action of the

currents, and inferred that this action was generally sufficient to

reduce hard rock to sand in its passage from the source to the outlet of

rivers. Frisi controverted this opinion, and maintained that river sand

was of more ancient origin, and he inferred from experiments in

artificially grinding stones that the concussion, friction, and

attrition of rock in the channel of running waters were inadequate to

its comminution, though he admitted that these same causes might reduce

silicious sand to a fine powder capable of transportation to the sea by

the currents.[354] Frisi's experiments were tried upon rounded and

polished river pebbles, and prove nothing with regard to the action of

torrents upon the irregular, more or less weathered, and often cracked

and shattered rocks which lie loose in the ground at the head of

mountain valleys. The fury of the waters and of the wind which

accompanies them in the floods of the French Alpine torrents is such,

that large blocks of stone are hurled out of the bed of the stream to

the height of twelve or thirteen feet. The impulse of masses driven with

such force overthrows the most solid masonry, and their concussion

cannot fail to be attended with the crushing of the rocks

themselves.[355]

d. _Inundations of 1856 in France._

The month of May, 1856, was remarkable for violent and almost

uninterrupted rains, and most of the river basins of France were

inundated to an extraordinary height. In the valleys of the Loire and

its affluents, about a million of acres, including many towns and

villages, were laid under water, and the amount of pecuniary damage was

almost incalculable.[356] The flood was not less destructive in the

valley of the Rhone, and in fact an invasion by a hostile army could

hardly have been more disastrous to the inhabitants of the plains than

was this terrible deluge. There had been a flood of this latter river in

the year 1840, which, for height and quantity of water, was almost as

remarkable as that of 1856, but it took place in the month of November,

when the crops had all been harvested, and the injury inflicted by it

upon agriculturists was, therefore, of a character to be less severely

and less immediately felt than the consequences of the inundation of

1856.[357]

In the fifteen years between these two great floods, the population and

the rural improvements of the river valleys had much increased, common

roads, bridges, and railways had been multiplied and extended, telegraph

lines had been constructed, all of which shared in the general ruin, and

hence greater and more diversified interests were affected by the

catastrophe of 1856 than by any former like calamity. The great flood of

1840 had excited the attention and roused the sympathies of the French

people, and the subject was invested with new interest by the still more

formidable character of the inundations of 1856. It was felt that these

scourges had ceased to be a matter of merely local concern, for,

although they bore most heavily on those whose homes and fields were

situated within the immediate reach of the swelling waters, yet they

frequently destroyed harvests valuable enough to be a matter of national

interest, endangered the personal security of the population of

important political centres, interrupted communication for days and even

weeks together on great lines of traffic and travel--thus severing as

it were all Southwestern France from the rest of the empire--and finally

threatened to produce great and permanent geographical changes. The

well-being of the whole commonwealth was seen to be involved in

preventing the recurrence, and in limiting the range of such

devastations. The Government encouraged scientific investigation of the

phenomena and their laws. Their causes, their history, their immediate

and remote consequences, and the possible safeguards to be employed

against them, have been carefully studied by the most eminent

physicists, as well as by the ablest theoretical and practical engineers

of France. Many hitherto unobserved facts have been collected, many new

hypotheses suggested, and many plans, more or less original in

character, have been devised for combating the evil; but thus far, the

most competent judges are not well agreed as to the mode, or even the

possibility, of applying a remedy.

e. _Remedies against Inundations._

Perhaps no one point has been more prominent in the discussions than the

influence of the forest in equalizing and regulating the flow of the

water of precipitation. As we have already seen, opinion is still

somewhat divided on this subject, but the conservative action of the

woods in this respect has been generally recognized by the public of

France, and the Government of the empire has made this principle the

basis of important legislation for the protection of existing forests,

and for the formation of new. The clearing of woodland, and the

organization and functions of a police for its protection, are regulated

by a law bearing date June 18th, 1859, and provision was made for

promoting the restoration of private woods by a statute adopted on the

28th of July, 1860. The former of these laws passed the legislative body

by a vote of 246 against 4, the latter with but a single negative voice.

The influence of the government, in a country where the throne is so

potent as in France, would account for a large majority, but when it is

considered that both laws, the former especially, interfere very

materially with the rights of private domain, the almost entire

unanimity with which they were adopted is proof of a very general

popular conviction, that the protection and extension of the forests is

a measure more likely than any other to check the violence, if not to

prevent the recurrence, of destructive inundations. The law of July

28th, 1860, appropriated 10,000,000 francs, to be expended, at the rate

of 1,000,000 francs per year, in executing or aiding the replanting of

woods. It is computed that this appropriation will secure the creation

of new forest to the extent of about 250,000 acres, or one eleventh part

of the soil where the restoration of the forest is thought feasible and,

at the same time, specially important as a security against the evils

ascribed in a great measure to its destruction.

The provisions of the laws in question are preventive rather than

remedial; but some immediate effect may be expected to result from them,

particularly if they are accompanied with certain other measures, the

suggestion of which has been favorably received. The strong repugnance

of the mountaineers to the application of a system which deprives them

of a part of their pasturage--for the absolute exclusion of domestic

animals is indispensable to the maintenance of an existing forest and to

the formation of a new--is the most formidable obstacle to the execution

of the laws of 1859-'60. It is proposed to compensate this loss by a

cheap system of irrigation of lower pasture grounds, consisting in

little more than in running horizontal furrows along the hillsides, thus

converting the scarp of the hills into a succession of small terraces

which, when once turfed over, are very permanent. Experience is said to

have demonstrated that this simple process suffices to retain the water

of rains, of snows, and of small springs and rivulets, long enough for

the irrigation of the soil, thus increasing its product of herbage in a

fivefold proportion, and that it partially checks the too rapid flow of

surface water into the valleys, and, consequently, in some measure

obviates one of the most prominent causes of inundations.[358] It is

evident that, if such results are produced by this method, its

introduction upon an extensive scale must also have the same climatic

effects as other systems of irrigation.

Whatever may be the ultimate advantages of reclothing a large extent of

the territory of France with wood, or of so shaping its surface as to

prevent the too rapid flow of water over it, the results to be obtained

by such processes can be realized in an adequate measure only after a

long succession of years. Other steps must be taken, both for the

immediate security of the lives and property of the present generation,

and for the prevention of yet greater and remoter evils which are

inevitable unless means to obviate them are found before it is forever

too late. The frequent recurrence of inundations like those of 1856, for

a single score of years, in the basins of the Rhone and the Loire, with

only the present securities against them, would almost depopulate the

valleys of those rivers, and produce physical revolutions in them,

which, like revolutions in the political world, could never be made to

"go backward."

Destructive inundations are seldom, if ever, produced by precipitation

within the limits of the principal valley, but almost uniformly by

sudden thaws or excessive rains on the mountain ranges where the

tributaries take their rise. It is therefore plain that any measures

which shall check the flow of surface waters into the channels of the

affluents, or which shall retard the delivery of such waters into the

principal stream by its tributaries, will diminish in the same

proportion the dangers and the evils of inundation by great rivers. The

retention of the surface waters upon or in the soil can hardly be

accomplished except by the methods already mentioned, replanting of

forests, and furrowing or terracing. The current of mountain streams can

be checked by various methods, among which the most familiar and obvious

is the erection of barriers or dams across their channels, at points

convenient for forming reservoirs large enough to retain the superfluous

waters of great rains and thaws. Besides the utility of such basins in

preventing floods, the construction of them is recommended by very

strong considerations, such as the meteorological effects of increased

evaporable surface, the furnishing of a constant supply of water for

agricultural and mechanical purposes, and, finally, their value as ponds

for breeding and rearing fish, and, perhaps, for cultivating aquatic

vegetables.

The objections to the general adoption of the system of reservoirs are

these: the expense of their construction and maintenance; the reduction

of cultivable area by the amount of surface they must cover; the

interruption they would occasion to free communication; the probability

that they would soon be filled up with sediment, and the obvious fact

that when full of earth or even water, they would no longer serve their

principal purpose; the great danger to which they would expose the

country below them in case of the bursting of their barriers;[359] the

evil consequences they would occasion by prolonging the flow of

inundations in proportion as they diminished their height; the injurious

effects it is supposed they would produce upon the salubrity of the

neighboring districts; and, lastly, the alleged impossibility of

constructing artificial basins sufficient in capacity to prevent, or in

any considerable measure to mitigate, the evils they are intended to

guard against.

The last argument is more easily reduced to a numerical question than

the others. The mean and extreme annual precipitation of all the basins

where the construction of such works would be seriously proposed is

already approximately known by meteorological tables, and the quantity

of water, delivered by the greatest floods which have occurred within

the memory of man, may be roughly estimated from their visible traces.

From these elements, or from recorded observations, the capacity of the

necessary reservoirs can be calculated. Let us take the case of the

Ardeche. In the inundation of 1857, that river poured into the Rhone

1,305,000,000 cubic yards of water in three days. If we suppose that

half this quantity might have been suffered to flow down its channel

without inconvenience, we shall have about 650,000,000 cubic yards to

provide for by reservoirs. The Ardeche and its principal affluent, the

Chassezac, have, together, about twelve considerable tributaries rising

near the crest of the mountains which bound the basin. If reservoirs of

equal capacity were constructed upon all of them, each reservoir must be

able to contain 54,000,000 cubic yards, or, in other words, must be

equal to a lake 3,000 yards long, 1,000 yards wide, and 18 yards deep,

and besides, in order to render any effectual service, the reservoirs

must all have been empty at the commencement of the rains which produced

the inundation.

Thus far, I have supposed the swelling of the waters to be uniform

throughout the whole basin; but such was by no means the fact in the

inundation of 1857, for the rise of the Chassezac, which is as large as

the Ardeche proper, did not exceed the limits of ordinary floods, and

the dangerous excess came solely from the headwaters of the latter

stream. Hence reservoirs of double the capacity I have supposed would

have been necessary upon the tributaries of that river, to prevent the

injurious effects of the inundation. It is evident that the construction

of reservoirs of such magnitude for such a purpose is financially, if

not physically, impracticable, and when we take into account a point I

have just suggested, namely, that the reservoirs must be empty at all

times of apprehended flood, and, of course, their utility limited almost

solely to the single object of preventing inundations, the total

inapplicability of such a measure in this particular case becomes still

more glaringly manifest.

Another not less conclusive fact is that the valleys of all the upland

tributaries of the Ardeche descend so rapidly, and have so little

lateral expansion, as to render the construction of capacious reservoirs

in them quite impracticable. Indeed, engineers have found but two points

in the whole basin suitable for that purpose, and the reservoirs

admissible at these would have only a joint capacity of about 70,000,000

cubic yards, or less than one ninth part of what I suppose to be

required. The case of the Ardeche is no doubt an extreme one, both in

the topographical character of its basin and in its exposure to

excessive rains; but all destructive inundations are, in a certain

sense, extreme cases also, and this of the Ardeche serves to show that

the construction of reservoirs is not by any means to be regarded as a

universal panacea against floods.

Nor, on the other hand, is this measure to be summarily rejected. Nature

has adopted it on a great scale, on both flanks of the Alps, and on a

smaller, on those of the Adirondacks and lower chains, and in this as in

many other instances, her processes may often be imitated with

advantage. The validity of the remaining objections to the system under

discussion depends on the topography, geology, and special climate of

the regions where it is proposed to establish such reservoirs. Many

upland streams present numerous points where none of these objections,

except those of expense and of danger from the breaking of dams, could

have any application. Reservoirs may be so constructed as to retain the

entire precipitation of the heaviest thaws and rains, leaving only the

ordinary quantity to flow along the channel; they may be raised to such

a height as only partially to obstruct the surface drainage; or they may

be provided with sluices by means of which their whole contents can be

discharged in the dry season and a summer crop be grown upon the ground

they cover at high water. The expediency of employing them and the mode

of construction depend on local conditions, and no rules of universal

applicability can be laid down on the subject.

It is remarkable that nations which we, in the false pride of our modern

civilization, so generally regard as little less than barbarian, should

have long preceded Christian Europe in the systematic employment of

great artificial basins for the various purposes they are calculated to

subserve. The ancient Peruvians built strong walls, of excellent

workmanship, across the channels of the mountain sources of important

streams, and the Arabs executed immense works of similar description,

both in the great Arabian peninsula and in all the provinces of Spain

which had the good fortune to fall under their sway. The Spaniards of

the fifteenth and sixteenth centuries, who, in many points of true

civilization and culture, were far inferior to the races they subdued,

wantonly destroyed these noble monuments of social and political wisdom,

or suffered them to perish, because they were too ignorant to appreciate

their value, or too unskilful as practical engineers to be able to

maintain them, and some of their most important territories were soon

reduced to sterility and poverty in consequence.

Another method of preventing or diminishing the evils of inundation by

torrents and mountain rivers, analogous to that employed for the

drainage of lakes, consists in the permanent or occasional diversion of

their surplus waters, or of their entire currents, from their natural

courses, by tunnels or open channels cut through their banks. Nature, in

many cases, resorts to a similar process. Most great rivers divide

themselves into several arms in their lower course, and enter the sea by

different mouths. There are also cases where rivers send off lateral

branches to convey a part of their waters into the channel of other

streams.[360] The most remarkable of these is the junction between the

Amazon and the Orinoco by the natural canal of the Cassiquiare and the

Rio Negro. In India, the Cambodja and the Menam are connected by the

Anam; the Saluen and the Irawaddi by the Panlaun. There are similar

examples, though on a much smaller scale, in Europe. The Tornea and the

Calix rivers in Lapland communicate by the Tarando, and in Westphalia,

the Else, an arm of the Haase, falls into the Weser.

The change of bed in rivers by gradual erosion of their banks is

familiar to all, but instances of the sudden abandonment of a primitive

channel are by no means wanting. At a period of unknown antiquity, the

Ardeche pierced a tunnel 200 feet wide and 100 high, through a rock, and

sent its whole current through it, deserting its former bed, which

gradually filled up, though its course remained traceable. In the great

inundation of 1827, the tunnel proved insufficient for the discharge of

the water, and the river burst through the obstructions which had now

choked up its ancient channel, and resumed its original course.[361]

It was probably such facts as these that suggested to ancient engineers

the possibility of like artificial operations, and there are numerous

instances of the execution of works for this purpose in very remote

ages. The Bahr Jusef, the great stream which supplies the Fayoum with

water from the Nile, has been supposed, by some writers, to be a natural

channel; but both it and the Bahr el Wady are almost certainly

artificial canals constructed to water that basin, to regulate the level

of Lake Moeris, and possibly, also, to diminish the dangers resulting

from excessive inundations of the Nile, by serving as waste-weirs to

discharge a part of its surplus waters. Several of the seven ancient

mouths of the Nile are believed to be artificial channels, and Herodotus

even asserts that King Menes diverted the entire course of that river

from the Libyan to the Arabian side of the valley. There are traces of

an ancient river bed along the western mountains, which give some

countenance to this statement. But it is much more probable that the

works of Menes were designed rather to prevent a natural, than to

produce an artificial, change in the channel of the river.

Two of the most celebrated cascades in Europe, those of the Teverone at

Tivoli and of the Velino at Terni, owe, if not their existence, at least

their position and character, to the diversion of their waters from

their natural beds into new channels, in order to obviate the evils

produced by their frequent floods. Remarkable works of the same sort

have been executed in Switzerland, in very recent times. Until the year

1714, the Kander, which drains several large Alpine valleys, ran, for a

considerable distance, parallel with the Lake of Thun, and a few miles

below the city of that name emptied into the river Aar. It frequently

flooded the flats along the lower part of its course, and it was

determined to divert it into the Lake of Thun. For this purpose, two

parallel tunnels were cut through the intervening rock, and the river

turned into them. The violence of the current burst up the roof of the

tunnels, and, in a very short time, wore the new channel down not less

than one hundred feet, and even deepened the former bed at least fifty

feet, for a distance of two or three miles above the tunnel. The lake

was two hundred feet deep at the point where the river was conducted

into it, but the gravel and sand carried down by the Kander has formed

at its mouth a delta containing more than a hundred acres, which is

still advancing at the rate of several yards a year. The Linth, which

formerly sent its waters directly to the Lake of Zurich, and often

produced very destructive inundations, was turned into the Wallensee

about forty years ago, and in both these cases a great quantity of

valuable land was rescued both from flood and from insalubrity.

In Switzerland, the most terrible inundations often result from the

damming up of deep valleys by ice slips or by the gradual advance of

glaciers, and the accumulation of great masses of water above the

obstructions. The ice is finally dissolved by the heat of summer or the

flow of warm waters, and when it bursts, the lake formed above is

discharged almost in an instant, and all below is swept down to certain

destruction. In 1595, about a hundred and fifty lives and a great amount

of property were lost by the eruption of a lake formed by the descent of

a glacier into the valley of the Drance, and a similar calamity laid

waste a considerable extent of soil in the year 1818. On this latter

occasion, the barrier of ice and snow was 3,000 feet long, 600 thick,

and 400 high, and the lake which had formed above it contained not less

than 800,000,000 cubic feet. A tunnel was driven through the ice, and

about 300,000,000 cubic feet of water safely drawn off by it, but the

thawing of the walls of the tunnel rapidly enlarged it, and before the

lake was half drained, the barrier gave way and the remaining

500,000,000 cubic feet of water were discharged in half an hour. The

recurrence of these floods has since been prevented by directing streams

of water, warmed by the sun, upon the ice in the bed of the valley, and

thus thawing it before it accumulates in sufficient mass to threaten

serious danger.

In the cases of diversion of streams above mentioned, important

geographical changes have been directly produced by those operations. By

the rarer process of draining glacier lakes, natural eruptions of water,

which would have occasioned not less important changes in the face of

the earth, have been prevented by human agency.

The principal means hitherto relied upon for defence against river

inundations has been the construction of dikes along the banks of the

streams, parallel to the channel and generally separated from each other

by a distance not much greater than the natural width of the bed.[362]

If such walls are high enough to confine the water and strong enough to

resist its pressure, they secure the lands behind them from all the

evils of inundation except those resulting from infiltration; but such

ramparts are enormously costly in original construction and maintenance,

and, as we have already seen, the filling up of the bed of the river in

its lower course, by sand and gravel, involves the necessity of

occasionally incurring new expenditures in increasing the height of the

banks.[363] They are attended, too, with some collateral disadvantages.

They deprive the earth of the fertilizing deposits of the waters, which

are powerful natural restoratives of soils exhausted by cultivation;

they accelerate the rapidity and transporting power of the current at

high water by confining it to a narrower channel, and it consequently

conveys to the sea the earthy matter it holds in suspension, and chokes

up harbors with a deposit which it would otherwise have spread over a

wide surface; they interfere with roads and the convenience of river

navigation, and no amount of cost or care can secure them from

occasional rupture, in case of which the rush of the waters through the

breach is more destructive than the natural flow of the highest

inundation.[364]

For these reasons, many experienced engineers are of opinion that the

system of longitudinal dikes ought to be abandoned, or, where that

cannot be done without involving too great a sacrifice of existing

constructions, their elevation should be much reduced, so as to present

no obstruction to the lateral spread of extraordinary floods, and they

should be provided with sluices to admit the water without violence

whenever they are likely to be overflowed. Where dikes have not been

erected, and where they have been reduced in height, it is proposed to

construct, at convenient intervals, transverse embankments of moderate

height running from the banks of the river across the plains to the

hills which bound them. These measures, it is argued, will diminish the

violence of inundations by permitting the waters to extend themselves

over a greater surface and thus retarding the flow of the river

currents, and will, at the same time, secure the deposit of fertilizing

slime upon all the soil covered by the flood.

Rozet, an eminent French engineer, has proposed a method of diminishing

the ravages of inundations, which aims to combine the advantages of all

other systems, and at the same time to obviate the objections to which

they are all more or less liable.[365] The plan of Rozet is recommended

by its simplicity and cheapness as well as its facility and rapidity of

execution, and is looked upon with favor by many persons very competent

to judge in such matters. He proposes to commence with the amphitheatres

in which mountain torrents so often rise, by covering their slopes and

filling their beds with loose blocks of rock, and by constructing at

their outlets, and at other narrow points in the channels of the

torrents, permeable barriers of the same material promiscuously heaped

up, much according to the method employed by the ancient Romans in their

northern provinces for a similar purpose. By this means, he supposes,

the rapidity of the current would be checked, and the quantity of

transported pebbles and gravel much diminished.

When the stream has reached that part of its course where it is bordered

by soil capable of cultivation, and worth the expense of protection, he

proposes to place along one or both sides of the stream, according to

circumstances, a line of cubical blocks of stone or pillars of masonry

three or four feet high and wide, and at the distance of about eleven

yards from each other. The space between the two lines, or between a

line and the opposite high bank, would, of course, be determined by

observation of the width of the swift-water current at high floods. As

an auxiliary measure, small ditches and banks, or low walls of pebbles,

should be constructed from the line of blocks across the grounds to be

protected, nearly at right angles to the current, but slightly inclining

downward, and at convenient distances from each other. Rozet thinks the

proper interval would be 300 yards, and it is evident that, if he is

right in his main principle, hedges, rows of trees, or even common

fences, would in many cases answer as good a purpose as banks and

trenches or low walls. The blocks or pillars of stone would, he

contends, check the lateral currents so as to compel them to let fall

all their pebbles and gravel in the main channel--where they would be

rolled along until ground down to sand or silt--and the transverse

obstructions would detain the water upon the soil long enough to secure

the deposit of its fertilizing slime. Numerous facts are cited in

support of the author's views, and I imagine there are few residents of

rural districts whose own observation will not furnish testimony

confirmatory of their soundness.[366]

The deposit of slime by rivers upon the flats along their banks not only

contributes greatly to the fertility of the soil thus flowed, but it

subserves a still more important purpose in the general economy of

nature. All running streams begin with excavating channels for

themselves, or deepening the natural depressions in which they

flow;[367] but in proportion as their outlets are raised by the solid

material transported by their currents, their velocity is diminished,

they deposit gravel and sand at constantly higher and higher points, and

so at last elevate, in the middle and lower part of their course, the

beds they had previously scooped out.[368] The raising of the channels

is compensated in part by the simultaneous elevation of their banks and

the flats adjoining them, from the deposit of the finer particles of

earth and vegetable mould brought down from the mountains, without which

elevation the low grounds bordering all rivers would be, as in many

cases they in fact are, mere morasses.

All arrangements which tend to obstruct this process of raising the

flats adjacent to the channel, whether consisting in dikes which confine

the waters, and, at the same time, augment the velocity of the current,

or in other means of producing the last-mentioned effect, interfere with

the restorative economy of nature, and at last occasion the formation of

marshes where, if left to herself, she would have accumulated

inexhaustible stores of the richest soil, and spread them out in plains

above the reach of ordinary floods.[369]

_Consequences if the Nile had been Diked._

If a system of continuous lateral dikes, like those of the Po, had been

adopted in Egypt in the early dynasties, when the power and the will to

undertake the most stupendous material enterprises were so eminently

characteristic of the government of that country, and the waters of the

annual inundation consequently prevented from flooding the land, it is

conceivable that the productiveness of the small area of cultivable soil

in the Nile valley might have been long kept up by artificial irrigation

and the application of manures. But nature would have rebelled at last,

and centuries before our time the mighty river would have burst the

fetters by which impotent man had vainly striven to bind his swelling

floods, the fertile fields of Egypt would have been converted into dank

morasses, and then, perhaps, in some distant future, when the expulsion

of man should have allowed the gradual restoration of the primitive

equilibrium, would be again transformed into luxuriant garden and plough

land. Fortunately, the "wisdom of Egypt" taught her children better

things. They invited and welcomed, not repulsed, the slimy embraces of

Nilus, and his favors have been, from the hoariest antiquity, the

greatest material blessing ever bestowed upon a people.[370]

The valley of the Po has probably not been cultivated or inhabited so

long as that of the Nile, but embankments have been employed on its

lower course for at least two thousand years, and for many centuries

they have been connected in a continuous chain. I have pointed out in a

former chapter the effects produced on the geography of the Adriatic by

the deposit of river sediment in the sea at the mouths of the Po, the

Adige, and the Brenta. If these rivers had been left unconfined, like

the Nile, and allowed to spread their muddy waters at will, according to

the laws of nature, the slime they have carried to the coast would have

been chiefly distributed over the plains of Lombardy. Their banks would

have risen as fast as their beds, the coast line would not have been

extended so far into the Adriatic, and, the current of the streams being

consequently shorter, the inclination of their channel and the rapidity

of their flow would not have been so greatly diminished. Had man spared

a reasonable proportion of the forests of the Alps, and not attempted to

control the natural drainage of the surface, the Po would resemble the

Nile in all its essential characteristics, and, in spite of the

difference of climate, perhaps be regarded as the friend and ally, not

the enemy and the invader, of the population which dwells upon its

banks.[371]

The Nile is larger than all the rivers of Lombardy together,[372] it

drains a basin twenty 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, not exceed the truth if we suppose the annually

inundated surface of Egypt to have been elevated, upon an average, ten

feet, within the last 5,000 years, or twice and a half the period during

which the history of the Po is known to us.[373]

We may estimate the present actually cultivated area of Egypt at about

5,500 square statute miles. As I have computed in a note on page 372,

that area is not more than half as extensive as 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 extent of soil it does not now reach. We may, then, adopt a

mean between the two quantities, and we shall probably come near the

truth if we assume the convenient number of 7,920 square statute miles

as the average measure of the inundated land during the historical

period. Taking the deposit on this surface at ten feet, the river

sediment let fall on the soil of Egypt within the last fifty centuries

would amount to fifteen cubic miles.

Had the Nile been banked in, like the Po, all this deposit, except that

contained in the water diverted by canals or otherwise drawn from the

river for irrigation and other purposes, would have been carried out to

sea.[374] This would have been a considerable quantity; for the Nile

holds earth in suspension even at low water, a much larger proportion

during the flood, and irrigation must have been carried on during the

whole year. The precise amount which would have been thus distributed

over the soil is matter of conjecture, but three cubic miles is

certainly a liberal estimate. This would leave twelve cubic miles as the

quantity which embankments would have compelled the Nile to transport to

the Mediterranean 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.[375]

_Deposits of the Tuscan Rivers._

The Arno, and all the rivers rising on the western slopes and spurs of

the Apennines, carry down immense quantities 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 quantity 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 seaward 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 mart of

Populonia has not 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.[376]

It is, in general, true, that the intervention of man has hitherto

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 grass

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 class of cases.

The valleys and shores of Tuscany form, however, a striking exception to

this remark. The success 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 noblest, 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 a very active cause of the development of malarious

influences.[377]

_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 southeastward to

the vicinity of Arezzo. It here sweeps round to the northwest, 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 southeastward 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

quantities 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.[378]

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 shifted

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 principal 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. If

it were elevated to what was evidently its 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 and of the

Arno itself, 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 principal drainage of the Val di Chiana appears to have been

in a southeastwardly direction until within a comparatively recent

period.

In the sixteenth century, the elevation of the bed of the valley had

become so considerable, that in 1551, at a point about ten miles south

of the Arno, it was found to be not less than one hundred and thirty

feet above that river; then followed a level of ten miles, and then a

continuous descent to the Paglia. Along the level portion of the valley

was a boatable channel, and lakes, sometimes a mile or even two miles in

breadth, had formed at various points farther south. At this period, the

drainage of the summit level might easily have been determined in either

direction, and the opposite descents of the valley made to culminate at

the north or at the south end of the level. In the former case, the

watershed would have been ten miles south of the Arno; in the latter,

twenty miles, and the division would have been not very unequal.

Various schemes were suggested at this time for drawing off the stagnant

waters, as well as for the future regular drainage of the valley, and

small operations for those purposes were undertaken with partial

success; but it was feared that the discharge of the accumulated waters

into the Tiber would produce a dangerous inundation, while the diversion

of the drainage into the Arno would increase the violence of the floods

to which that river was very subject, and no decisive steps were taken.

In 1606, an engineer whose name has not been preserved proposed, as the

only possible method of improvement, the piercing of a tunnel through

the hills bounding the valley on the west to convey its waters to the

Ombrone, but the expense and other objections prevented the adoption of

this project.[379] The fears of the Roman Government for the security of

the valley of the Tiber had induced it to construct barriers across that

part of the channel which lay within its territory, and these

obstructions, though not specifically intended for that purpose,

naturally promoted the deposit of sediment and the elevation of the bed

of the valley in their neighborhood. The effect of this measure and of

the continued spontaneous action of the torrents was, that the northern

slope, which in 1551 had commenced at the distance of ten miles from the

Arno, was found in 1605 to begin, nearly thirty miles south of that

river, and in 1645 it had been removed about six miles farther in the

same direction.[380]

In the seventeenth century, the Tuscan and Papal Governments consulted

Galileo, Torricelli, Castelli, Cassini, Viviani, and other distinguished

philosophers and engineers, on the possibility of reclaiming the valley

by a regular artificial drainage. Most of these eminent physicists were

of opinion that the measure was impracticable, though not altogether for

the same reasons; but they seem to have agreed in thinking that the

opening of such channels, in either direction, as would give the current

a flow sufficiently rapid to drain the lands properly, would dangerously

augment the inundations of the river--whether the Tiber or the

Arno--into which the waters should be turned. The general improvement of

the valley was now for a long time abandoned, and the waters were

allowed to spread and stagnate until carried off by partial drainage,

infiltration, and evaporation. Torricelli had contended that the slope

of a large part of the valley was too small to allow it to be drained by

ordinary methods, and that no practicable depth and width of canal would

suffice for that purpose. It could be laid dry, he thought, only by

converting its surface into an inclined plane, and he suggested that

this might be accomplished by controlling the flow of the numerous

torrents which pour into it, so as to force them to deposit their

sediment at the pleasure of the engineer, and, consequently, to elevate

the level of the area over which it should be spread.[381] This plan

did not meet with immediate general acceptance, but it was soon adopted

for local purposes at some points in the southern part of the valley,

and it gradually grew in public favor and was extended in application

until its final triumph a hundred years later.

In spite of these encouraging successes, however, the fear of danger to

the valley of the Arno and the Tiber, and the difficulty of an agreement

between Tuscany and Rome--the boundary between which states crossed the

Val di Chiana not far from the halfway point between the two rivers--and

of reconciling other conflicting interests, prevented the resumption of

the projects for the general drainage of the valley until after the

middle of the eighteenth century. In the mean time the science of

hydraulics had become better understood, and the establishment of the

natural law according to which the velocity of a current of water, and

of course the proportional quantity discharged by it in a given time,

are increased by increasing its mass, had diminished if not dissipated

the fear of exposing the banks of the Arno to greater danger from

inundations by draining the Val di Chiana into it.

The suggestion of Torricelli was finally adopted as the basis of a

comprehensive system of improvement, and it was decided to continue and

extend the inversion of the original flow of the waters, and to turn

them into the Arno from a point as far to the south as should be found

practicable. The conduct of the works was committed to a succession of

able engineers who, for a long series of years, were under the general

direction of the celebrated philosopher and statesman Fossombroni, and

the success has fully justified the expectations of the most sanguine

advocates of the scheme. The plan of improvement embraced two branches:

the one, the removal of certain obstructions in the bed of the Arno,

and, consequently, the further depression of the channel of that river,

in certain places, with the view of increasing the rapidity of its

current; the other, the gradual filling up of the ponds and swamps, and

raising of the lower grounds of the Val di Chiana, by directing to

convenient points the flow of the streams which pour down into it, and

there confining their waters by temporary dams until the sediment was

deposited where it was needed. The economical result of these operations

has been, that in 1835 an area of more than four hundred and fifty

square miles of pond, marsh, and damp, sickly low grounds had been

converted into fertile, healthy and well-drained soil, and,

consequently, that so much territory has been added to the agricultural

domain of Tuscany.

But in our present view of the subject, the geographical revolution

which has been accomplished is still more interesting. The climatic

influence of the elevation and draining of the soil must have been

considerable, though I do not know that an increase or a diminution of

the mean temperature or precipitation in the valley has been established

by meteorological observation. There is, however, in the improvement of

the sanitary condition of the Val di Chiana, which was formerly

extremely unhealthy, satisfactory proof of a beneficial climatic change.

The fevers, which not only decimated the population of the low grounds

but infested the adjacent hills, have ceased their ravages, and are now

not more frequent than in other parts of Tuscany. The strictly

topographical effect of the operations in question, besides the

conversion of marsh into dry surface, has been the inversion of the

inclination of the valley for a distance of thirty-five miles, so that

this great plain which, within a comparatively short period, sloped and

drained its waters to the south, now inclines and sends its drainage to

the north. The reversal of the currents of the valley has added to the

Arno a new tributary equal to the largest of its former affluents, and a

most important circumstance connected with this latter fact is, that the

increase of the volume of its waters has accelerated their velocity in a

still greater proportion, and, instead of augmenting the danger from its

inundations, has almost wholly obviated that source of apprehension.

Between the beginning of the fifteenth century and the year 1761,

thirty-one destructive floods of the Arno are recorded; between 1761,

when the principal streams of the Val di Chiana were diverted into that

river, and 1835, not one.[382]

_Improvements in the Tuscan Maremme._

In the improvements of the Tuscan Maremma, more formidable difficulties

have been encountered. The territory to be reclaimed was more extensive;

the salubrious places of retreat for laborers and inspectors were more

remote; the courses of the rivers to be controlled were longer and their

natural inclination less rapid; some of them, rising in wooded regions,

transported comparatively little earthy matter,[383] and above all,

A like example is observed in the Anapus near Syracuse, which, below the

junction of its two branches, is narrower, though swifter than either of

them, and such cases are by no means unfrequent. The immediate effect of

the confluence of two rivers upon the current below depends upon local

circumstances, and especially upon the angle of incidence. If the two

nearly coincide in direction, so as to include a small angle, the joint

current will have a greater velocity than the slower confluent, perhaps

even than either of them. If the two rivers run in transverse, still

more if they flow in more or less opposite directions, the velocity of

the principal branch will be retarded both above and below the junction,

and at high water it may even set back the current of the affluent.

On the other hand, the diversion of a considerable branch from a river

retards its velocity below the point of separation, and here a deposit

of earth in its channel immediately begins, which has a tendency to turn

the whole stream into the new bed. "Theory and the authority of all

hydrographical writers combine to show that the channels of rivers

undergo an elevation of bed below a canal of diversion."--Letter of

FOSSOMBRONI, in SALVAGNOLI, _Raccolta di Documenti_, p. 32. See the

early authorities and discussions on the principle stated in the text,

in FRISI, _Del modo di regolare i Fiumi e i Torrenti_, libro iii, capit.

i. 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.[384]

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 passages, 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,[385] 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 Maremme, 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.[386] 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 Maremme. 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 constitutions 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 maritima_, 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, and are now in successful

progress.

I have spoken, with some detail, of the improvements in the Val di

Chiana and 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 principally 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 shore. 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.[387]

It is now a hundred years since the commencement of the improvements in

the Val di Chiana, and those of the Maremma have been in more or less

continued operation for above a generation. They have, as we have seen,

produced important geographical changes in the surface of the earth and

in the flow of considerable rivers, and their effects have been not less

conspicuous in preventing other changes, of a deleterious character,

which would infallibly have taken place if they had not been arrested by

the improvements in question. It has been already stated that, in order

to prevent the overflow of the valley of the Tiber by freely draining

the Val di Chiana into it, the Papal authorities, long before the

commencement of the Tuscan works, constructed strong barriers near the

southern end of the valley, which detained the waters of the wet season

until they could be gradually drawn off into the Paglia. They

consequently deposited most of their sediment in the Val di Chiana and

carried down comparatively little earth to the Tiber. The lateral

streams contributing the largest quantities of sedimentary matter to the

Val di Chiana originally flowed into that valley near its northern end;

and the change of their channels and outlets in a southern direction, so

as to raise that part of the valley by their deposits and thereby

reverse its drainage, was one of the principal steps in the process of

improvement.

We have seen that the north end of the Val di Chiana near the Arno had

been raised by spontaneous deposit of sediment to such a height as to

interpose a sufficient obstacle to all flow in that direction. If, then,

the Roman dam had not been erected, or the works of the Tuscan

Government undertaken, the whole of the earth, which has been arrested

by those works and employed to raise the bed and reverse the declivity

of the valley, would have been carried down to the Tiber and thence into

the sea. The deposit thus created, would, of course, have contributed to

increase the advance of the shore at the mouth of that river, which has

long been going on at the rate of three metres and nine tenths (twelve

feet and nine inches) per annum.[388] It is evident that a quantity of

earth, sufficient to effect the immense changes I have described in a

wide valley more than thirty miles long, if deposited at the outlet of

the Tiber, would have very considerably modified the outline of the

coast, and have exerted no unimportant influence on the flow of that

river, by raising its point of discharge and lengthening its channel.

The sediment washed into the marshes of the Maremme is not less than

12,000,000 cubic yards per annum. The escape of this quantity into the

sea, which is now almost wholly prevented, would be sufficient to

advance the coast line fourteen yards per year, for a distance of forty

miles, computing the mean depth of the sea near the shore at twelve

yards. It is true that in this case, as well as in that of other rivers,

the sedimentary matter would not be distributed equally along the shore,

and much of it would be carried out into deep water, or perhaps

transported by the currents to distant coasts. The immediate effects of

the deposit, therefore, would not be so palpable as they appear in this

numerical form, but they would be equally certain, and would infallibly

manifest themselves, first, perhaps, at some remote point, and afterward

at or near the outlets of the rivers which produced them.

_Obstruction of River Mouths._

The mouths of a large proportion of the streams known to ancient

internal navigation are already blocked up by sandbars or fluviatile

deposits, and the maritime approaches to river harbors frequented by the

ships of Phenicia and Carthage and Greece and Rome are shoaled to a

considerable distance out to sea. The inclination 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 less 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 these 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.[389]

In their upper course, where the current is swiftest, they are most

heavily charged with coarse rolled or suspended matter, and this, in

floods, they deposit on their shores 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 increases 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 quantity, 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 quantity 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

shore, 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, until it finds a resting place in the

northeastern angle of the Mediterranean.[390] 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,

and to fill up the harbors made famous by Phenician commerce.

_Subterranean Waters._

I have frequently alluded to a branch of geography, the importance of

which is but recently adequately recognized--the subterranean waters of

the earth considered as stationary reservoirs, as flowing currents, and

as filtrating fluids. The earth drinks in moisture by direct absorption

from the atmosphere, by the deposition of dew, by rain and snow, by

percolation from rivers and other superficial bodies of water, and

sometimes by currents flowing into caves or smaller visible

apertures.[391] Some of this humidity is exhaled again by the soil,

some is taken up by organic growths and by inorganic compounds, some

poured out upon the surface by springs and either immediately evaporated

or carried down to larger streams and to the sea, some flows by

subterranean courses into the bed of fresh-water rivers[392] or of the

ocean, and some remains, though even here not in forever motionless

repose, to fill deep cavities and underground channels.[393] In every

case the aqueous vapors of the air are the ultimate source of supply,

and all these hidden stores are again returned to the atmosphere by

evaporation.

The proportion of the water of precipitation taken up by direct

evaporation from the surface of the ground seems to have been generally

exaggerated, sufficient allowance not being made for moisture carried

downward, or in a lateral direction, by infiltration or by crevices in

the superior rocky or earthy strata. According to Wittwer, Mariotte

found that but one sixth of the precipitation in the basin of the Seine

was delivered into the sea by that river, "so that five sixths remained

for evaporation and consumption by the organic world."[394]

Lieutenant Maury--whose scientific reputation, though fallen, has not

quite sunk to the level of his patriotism--estimates the annual amount

of precipitation in the valley of the Mississippi at 620 cubic miles,

the discharge of that river into the sea at 107 cubic miles, and

concludes that "this would leave 513 cubic miles of water to be

evaporated from this river basin annually."[395] In these and other like

computations, the water carried down into the earth by capillary and

larger conduits is wholly lost sight of, and no thought is bestowed upon

the supply for springs, for common and artesian wells, and for

underground rivers, like those in the great caves of Kentucky, which may

gush up in fresh-water currents at the bottom of the Caribbean Sea, or

rise to the light of day in the far-off peninsula of Florida.

The progress of the emphatically modern science of geology has corrected

these erroneous views, because the observations on which it depends have

demonstrated not only the existence, but the movement, of water in

nearly all geological formations, have collected evidence of the

presence of large reservoirs at greater or less depths beneath surfaces

of almost every character, and have investigated the rationale of the

attendant phenomena. The distribution of these waters has been minutely

studied with reference to a great number of localities, and though the

actual mode of their vertical and horizontal transmission is still

involved in much doubt, the laws which determine their aggregation are

so well understood, that, when the geology of a given district is known,

it is not difficult to determine at what depth water will be reached by

the borer, and to what height it will rise.

The same principles have been successfully applied to the discovery of

small subterranean collections or currents of water, and some persons

have acquired, by a moderate knowledge of the superficial structure of

the earth combined with long practice, a skill in the selection of

favorable places for digging wells which seems to common observers

little less than miraculous. The Abbe Paramelle--a French ecclesiastic

who devoted himself for some years to this subject and was extensively

employed as a well-finder--states, in his work on Fountains, that in the

course of thirty-four years he had pointed out more than ten thousand

subterranean springs, and though his geological speculations were often

erroneous, the highest scientific authorities in Europe have testified

to the great practical value of his methods, and the almost infallible

certainty of his predictions.[396]

Babinet quotes a French proverb, "Summer rain wets nothing," and

explains it as meaning that the water of such rains is "almost totally

taken up by evaporation." "The rains of summer," he adds, "however

abundant they may be, do not penetrate the soil to a greater depth than

15 or 20 centimetres. In summer the evaporating power of the heat is

five or six times as great as in winter, and this power is exerted by an

atmosphere capable of containing five times as much vapor as in winter."

"A stratum of snow which prevents evaporation [from the soil] causes

almost all the water that composes it to filter down into the earth, and

form a reserve for springs, wells, and rivers which could not be

supplied by any amount of summer rain." "This latter--useful, indeed

like dew, to vegetation--does not penetrate the soil and accumulate a

store to feed springs and to be brought up by them to the open

air."[397] This conclusion, however applicable it may be to the climate

and soil of France, is too broadly stated to be accepted as a general

truth, and in countries where the precipitation is small in the winter

months, familiar observation shows that the quantity of water yielded by

deep wells and natural springs depends not less on the rains of summer

than on those of the rest of the year, and, consequently, that much of

the precipitation of that season must find its way to strata too deep to

lose water by evaporation.

The supply of subterranean reservoirs and currents, as well as of

springs, is undoubtedly derived chiefly from infiltration, and hence it

must be affected by all changes of the natural surface that accelerate

or retard the drainage of the soil, or that either promote or obstruct

evaporation from it. It has sufficiently appeared from what has gone

before, that the spontaneous drainage of cleared ground is more rapid

than that of the forest, and consequently, that the felling of the

woods, as well as the draining of swamps, deprives the subterranean

waters of accessions which would otherwise be conveyed to them by

infiltration. The same effect is produced by artificial contrivances for

drying the soil either by open ditches or by underground pipes or

channels, and in proportion as the sphere of these operations is

extended, the effect of them cannot fail to make itself more and more

sensibly felt in the diminished supply of water furnished by wells and

running springs.[398]

It is undoubtedly true that loose soils, stripped of vegetation and

broken up by the plough or other processes of cultivation, may, until

again carpeted by grasses or other plants, absorb more rain and snow

water than when they were covered by a natural growth; but it is also

true that the evaporation from such soils is augmented in a still

greater proportion. Rain scarcely penetrates beneath the sod of grass

ground, but runs off over the surface; and after the heaviest showers a

ploughed field will often be dried by evaporation before the water can

be carried off by infiltration, while the soil of a neighboring grove

will remain half saturated for weeks together. Sandy soils frequently

rest on a tenacious subsoil, at a moderate depth, as is usually seen in

the pine plains of the United States, where pools of rain water collect

in slight depressions on the surface of earth, the upper stratum of

which is as porous as a sponge. In the open grounds such pools are very

soon dried up by the sun and wind; in the woods they remain unevaporated

long enough for the water to diffuse itself laterally until it finds, in

the subsoil, crevices through which it may escape, or slopes which it

may follow to their outcrop or descend along them to lower strata.

The readiness with which water not obstructed by impermeable strata

diffuses itself through the earth in all directions--and, consequently,

the importance of keeping up the supply of subterranean reservoirs--find

a familiar illustration in the effect of paving the ground about the

stems of vines and trees. The surface earth around the trunk of a tree

may be made perfectly impervious to water, by flag stones and cement,

for a distance greater than the spread of the roots; and yet the tree

will not suffer for want of moisture, except in droughts severe enough

sensibly to affect the supply in deep wells and springs. Both forest and

fruit trees grow well in cities where the streets and courts are closely

paved, and where even the lateral access of water to the roots is more

or less obstructed by deep cellars and foundation walls. The deep-lying

veins and sheets of water, supplied by infiltration from above, send up

moisture by capillary attraction, and the pavement prevents the soil

beneath it from losing its humidity by evaporation. Hence, city-grown

trees find moisture enough for their roots, and though plagued with

smoke and dust, often retain their freshness while those planted in the

open fields, where sun and wind dry up the soil faster than the

subterranean fountains can water it, are withering from drought. Without

the help of artificial conduit or of water carrier, the Thames and the

Seine refresh the ornamental trees that shade the thoroughfares of

London and of Paris, and beneath the hot and reeking mould of Egypt, the

Nile sends currents to the extremest border of its valley.[399]

_Artesian Wells._

The existence of artesian wells depends upon that of subterranean

reservoirs and rivers, and the supply yielded by borings is regulated by

the abundance of such sources. The waters of the earth are, in many

cases, derived from superficial currents which are seen to pour into

chasms opened, as it were, expressly for their reception; and in others

where no apertures in the crust of the earth have been detected, their

existence is proved by the fact that artesian wells sometimes bring up

from great depths seeds, leaves, and even living fish, which must have

been carried down through channels large enough to admit a considerable

stream. But in general, the sheets and currents of water reached by deep

boring appear to be primarily due to infiltration from highlands where

the water is first collected in superficial or subterranean reservoirs.

By means of channels conforming to the dip of the strata, these

reservoirs communicate with the lower basins, and exert upon them a

fluid pressure sufficient to raise a column to the surface, whenever an

orifice is opened.[400] The water delivered by an artesian well is,

therefore, often derived from distant sources, and may be wholly

unaffected by geographical or meteorological changes in its immediate

neighborhood, while the same changes may quite dry up common wells and

springs which are fed only by the local infiltration of their own narrow

basins.

In most cases, artesian wells have been bored for purely economical or

industrial purposes, such as to obtain good water for domestic use or

for driving light machinery, to reach saline or other mineral springs,

and recently, in America, to open fountains of petroleum or rock oil.

The geographical and geological effects of such abstraction of fluids

from the bowels of the earth are too remote and uncertain to be here

noticed;[401] but artesian wells have lately been employed in Algeria

for a purpose which has even now a substantial, and may hereafter

acquire a very great geographical importance. It was observed by many

earlier as well as recent travellers in the East, among whom Shaw

deserves special mention, that the Libyan desert, bordering upon the

cultivated shores of the Mediterranean, appeared in many places to rest

upon a subterranean lake at an accessible distance below the surface.

The Moors are vaguely said to have _bored_ artesian wells down to this

reservoir, to obtain water for domestic use and irrigation, but I do

not find such wells described by any trustworthy traveller, and the

universal astonishment and incredulity with which the native tribes

viewed the operations of the French engineers sent into the desert for

that purpose, are a sufficient proof that this mode of reaching the

subterranean waters was new to them. They were, however, aware of the

existence of water below the sands, and were dexterous in digging

wells--square shafts lined with a framework of palm-tree stems--to the

level of the sheet. The wells so constructed, though not technically

artesian wells, answer the same purpose; for the water rises to the

surface and flows over it as from a spring.[402]

These wells, however, are too few and too scanty in supply to serve any

other purposes than the domestic wells of other countries, and it is but

recently that the transformation of desert into cultivable land by this

means has been seriously attempted. The French Government has bored a

large number of artesian wells in the Algerian desert within a few

years, and the native sheikhs are beginning to avail themselves of the

process. Every well becomes the nucleus of a settlement proportioned to

the supply of water, and before the end of the year 1860, several nomade

tribes had abandoned their wandering life, established themselves around

the wells, and planted more than 30,000 palm trees, besides other

perennial vegetables.[403] The water is found at a small depth,

generally from 100 to 200 feet, and though containing too large a

proportion of mineral matter to be acceptable to a European palate, it

answers well for irrigation, and does not prove unwholesome to the

natives.

The most obvious use of artesian wells in the desert at present is that

of creating stations for the establishment of military posts and halting

places for the desert traveller; but if the supply of water shall prove

adequate for the indefinite extension of the system, it is probably

destined to produce a greater geographical transformation than has ever

been effected by any scheme of human improvement. The most striking

contrast of landscape scenery that nature brings near together in time

or place, is that between the greenery of the tropics, or of a northern

summer, and the snowy pall of leafless winter. Next to this in startling

novelty of effect, we must rank the sudden transition from the shady and

verdant oasis of the desert to the bare and burning party-colored ocean

of sand and rock which surrounds it.[404] The most sanguine believer in

indefinite human progress hardly expects that man's cunning will

accomplish the universal fufilment of the prophecy, "the desert shall

blossom as the rose," in its literal sense; but sober geographers have

thought the future conversion of the sand plains of Northern Africa into

fruitful gardens, by means of artesian wells, not an improbable

expectation. They have gone farther, and argued that, if the soil were

covered with fields and forests, vegetation would call down moisture

from the Libyan sky, and that the showers which are now wasted on the

sea, or so often deluge Southern Europe with destructive inundation,

would in part be condensed over the arid wastes of Africa, and thus,

without further aid from man, bestow abundance on regions which nature

seems to have condemned to perpetual desolation.

An equally bold speculation, founded on the well-known fact, that the

temperature of the earth and of its internal waters increases as we

descend beneath the surface, has suggested that artesian wells might

supply heat for industrial and domestic purposes, for hot-house

cultivation, and even for the local amelioration of climate. The success

with which Count Lardarello has employed natural hot springs for the

evaporation of water charged with boracic acid, and other fortunate

applications of the heat of thermal sources, lend some countenance to

the latter project; but both must, for the present, be ranked among the

vague possibilities of science, not regarded as probable future triumphs

of man over nature.

_Artificial Springs._

A more plausible and inviting scheme is that of the creation of

perennial springs by husbanding rain and snow water, storing it up in

artificial reservoirs of earth, and filtering it through purifying

strata, in analogy with the operations of nature. The sagacious

Palissy--starting from the theory that all springs are primarily derived

from precipitation, and reasoning justly on the accumulation and

movement of water in the earth--proposed to reduce theory to practice,

and to imitate the natural processes by which rain is absorbed by the

earth and given out again in running fountains. "When I had long and

diligently considered the cause of the springing of natural fountains

and the places where they be wont to issue," says he, "I did plainly

perceive, at last, that they do proceed and are engendered of nought but

the rains. And it is this, look you, which hath moved me to enterprise

the gathering together of rain water after the manner of nature, and the

most closely according to her fashion that I am able; and I am well

assured that by following the formulary of the Supreme Contriver of

fountains, I can make springs, the water whereof shall be as good and

pure and clear as of such which be natural."[405] Palissy discusses the

subject of the origin of springs at length and with much ability,

dwelling specially on infiltration, and, among other things, thus

explains the frequency of springs in mountainous regions: "Having well

considered the which, thou mayest plainly see the reason why there be

more springs and rivulets proceeding from the mountains than from the

rest of the earth; which is for no other cause but that the rocks and

mountains do retain the water of the rains like vessels of brass. And

the said waters falling upon the said mountains descend continually

through the earth, and through crevices, and stop not till they find

some place that is bottomed with stone or close and thick rocks; and

they rest upon such bottom until they find some channel or other manner

of issue, and then they flow out in springs or brooks or rivers,

according to the greatness of the reservoirs and of the outlets

thereof."[406]

After a full exposition of his theory, Palissy proceeds to describe his

method of creating springs, which is substantially the same as that

lately proposed by Babinet in the following terms: "Choose a piece of

ground containing four or five acres, with a sandy soil, and with a

gentle slope to determine the flow of the water. Along its upper line,

dig a trench five or six feet deep and six feet wide. Level the bottom

of the trench, and make it impermeable by paving, by macadamizing, by

bitumen, or, more simply and cheaply, by a layer of clay. By the side of

this trench dig another, and throw the earth from it into the first, and

so on until you have rendered the subsoil of the whole parcel

impermeable to rain water. Build a wall along the lower line with an

aperture in the middle for the water, and plant fruit or other low trees

upon the whole, to shade the ground and check the currents of air which

promote evaporation. This will infallibly give you a good spring which

will flow without intermission and supply the wants of a whole hamlet or

a large chateau."[407] Babinet states that the whole amount of

precipitation on a reservoir of the proposed area, in the climate of

Paris, would be about 13,000 cubic yards, not above one half of which,

he thinks, would be lost, and, of course, the other half would remain

available to supply the spring. I much doubt whether this expectation

would be realized in practice, in its whole extent; for if Babinet is

right in supposing that the summer rain is wholly evaporated, the winter

rains, being much less in quantity, would hardly suffice to keep the

earth saturated and give off so large a surplus.

The method of Palissy, though, as I have said, similar in principle to

that of Babinet, would be cheaper of execution, and, at the same time,

more efficient. He proposes the construction of relatively small

filtering receptacles, into which he would conduct the rain falling upon

a large area of rocky hillside, or other sloping ground not readily

absorbing water. This process would, in all probability, be a very

successful, as well as an inexpensive, mode of economizing atmospheric

precipitation, and compelling the rain and snow to form perennial

fountains at will.

_Economizing Precipitation._

The methods suggested by Palissy and by Babinet are of limited

application, and designed only to supply a sufficient quantity of water

for the domestic use of small villages or large private establishments.

Dumas has proposed a much more extensive system for collecting and

retaining the whole precipitation in considerable valleys, and storing

it in reservoirs, whence it is to be drawn for household and mechanical

purposes, for irrigation, and, in short, for all the uses to which the

water of natural springs and brooks is applicable. His plan consists in

draining both surface and subsoil, by means of conduits differing in

construction according to local circumstances, but in the main not

unlike those employed in improved agriculture, collecting the water in a

central channel, securing its proper filterage, checking its too rapid

flow by barriers at convenient points, and finally receiving the whole

in spacious covered reservoirs, from which it may be discharged in a

constant flow or at intervals as convenience may dictate.[408]

There is no reasonable doubt that a very wide employment of these

various contrivances for economizing and supplying water is practicable,

and the expediency of resorting to them is almost purely an economical

question. There appears to be no serious reason to apprehend collateral

evils from them, and in fact all of them, except artesian wells, are

simply indirect methods of returning to the original arrangements of

nature, or, in other words, of restoring the fluid circulation of the

globe; for when the earth was covered with the forest, perennial springs

gushed from the foot of every hill, brooks flowed down the bed of every

valley. The partial recovery of the fountains and rivulets which once

abundantly watered the face of the agricultural world seems practicable

by such means, even without any general replanting of the forests; and

the cost of one year's warfare, if judiciously expended in a combination

of both methods of improvement, would secure, to almost every country

that man has exhausted, an amelioration of climate, a renovated

fertility of soil, and a general physical improvement, which might

almost be characterized as a new creation.