consequently, in many cases, the evaporation from it, as well as the
refrigeration which attends all evaporation. [Footnote: The relative
evaporating action of earth and water is a very complicated problem, and
the results of observation on the subject are conflicting. Schubler
found that at Geneva the evaporation from bare loose earth, in the
months of December, January, and February, was from two and a half to
nearly six times as great as from a like surface of water in the other
months. The evaporation from water was from about once and a half to six
times as great as from earth. Taking the whole year together, the
evaporation from the two surfaces was 199 lines from earth and 536 lines
from water. Experiments by Van der Steer, at the Helder, in the years
1861 and 1862, showed, for the former year, an evaporation of 602.9
millimetres from water, 1399.6 millimetres from ground covered with
clover and other grasses; in 1862, the evaporation from water was 584.5
millimetres, from grassground, 875.5. --Wilhelm, Der Boden und das
Wasser, p. 57; Krecke, Het Klimaat van Nederland, ii., p. 111.
On the other hand, the evaporation from the Nile in Egypt and Nubia is
stated to be three times as great as that from an equal surface of the
soil which borders it.--Lombardini, Saggio Idrologico sul Nilo, Milano,
1864, and Appendix. The relative thermometrical conditions of land and
water in the same vicinity are constantly varying, and the hygrometrical
state of both is equally unstable. Consequently there is no general
formula to express the proportionate evaporation from fluid and solid
geographical surfaces.] On the other hand, if the volume of water
abstracted is great, its removal deprives its basin of an equalizing and
moderating influence; for large bodies of water take very slowly the
temperature of the air in contact with their surface, and are almost
constantly either sending off heat into the atmosphere or absorbing heat
from it. Besides, as we have seen, lakes in elevated positions discharge
more or less water by infiltration, and contribute it by the same
process to other lakes, to springs, and to rivulets, at lower levels.
Hence the draining of lakes, on a considerable scale, must modify both
the humidity and the temperature of the atmosphere of the neighboring
regions, and the permanent supply of ground-water for the lands lying
below them.
Meteorological Action of Marshes.
The shallow water of marshes, indeed, performs this latter function,
but, under ordinary circumstances, marshes exercise in but a very small
degree the compensating meteorological action which I have ascribed to
large expansions of deeper water. The direct rays of the sun and the
warmth of the atmosphere penetrate to the soil beneath, and raise the
temperature of the water which covers it; and there is usually a much
greater evaporation from marshes than from lakes in the same region
during the warmer half of the year. This evaporation implies
refrigeration, and consequently the diminution of evaporation by the
drainage of swamps tends to prevent the lowering of the atmospheric
temperature, and to lessen the frequency and severity of frosts.
Accordingly it is a fact of experience that, other things being equal,
dry soils, and the air in contact with them, are perceptibly warmer
during the season of vegetation, when evaporation is most rapid, than
moist lands and the atmospheric stratum resting upon them. Instrumental
observation on this special point has not yet been undertaken on a large
scale, but still we have thermometric data sufficient to warrant the
general conclusion, and the influence of drainage in diminishing the
frequency of frost appears to be even better established than a direct
increase of atmospheric temperature. The steep and dry uplands of the
Green Mountain range in New England often escape frosts when the
Indian-corn harvest on moister grounds, five hundred or even a thousand
feet lower, is destroyed or greatly injured by them. The neighborhood of
a marsh is sure to be exposed to late spring and early autumnal frosts,
but they cease to be feared after it is drained, and this is
particularly observable in very cold climates, as, for example, in
Lapland. [Footnote: "The simplest backwoodsman knows by experiences that
all cultivation is impossible in the neighborhood of bogs and marshes.
Why is a crop near the borders of a marsh out off by frost, while a
field upon a hillock, a few stone's throws from it, is spared "--Lars
Levi Laestadius, Om Uppoldingar Lappmarrken, pp. 69, 74.]
In England, under-drains are not generally laid below the reach of daily
variations of temperature, or below a point from which moisture, if not
carried off by the drains, might be brought to the surface by capillary
attraction, and evaporated by the heat of the sun. They, therefore, like
surface-drains, withdraw from local solar action much moisture which
would otherwise be vaporized by it, and, at the same time, by drying the
soil above them, they increase its effective hygroscopicity, and it
consequently absorbs from the atmosphere a greater quantity of water
than it did when, for want of under-drainage, the subsoil was always
humid, if not saturated. [Footnote: Mangon thinks that the diminution of
evaporation by agricultural drainage corresponds, in certain
circumstances, to five per cent. of the heat received from the sun by
the same surface in a year. He cites observations by Parkes, showing a
difference in temperature of 5.5 degrees (centigrade ) in favor of
drained, as compared with undrained, ground in the same
vicinity.--Instructions pratiques sur le Drainage, pp. 227, 228.
The diminution of evaporation is not the only mode in which
under-draining affects the temperature. The increased effective
hygroscopicity of the soil increases its absorbent action, and the
condensation of atmospheric vapor thus produced is attended with the
manifestation of heat.] Under-drains, then, contribute to the dryness as
well as to the warmth of the atmosphere, and, as dry ground is more
readily heated by the rays of the sun than wet, they tend also to raise
the mean, and especially the summer, temperature of the soil.
Effects of Draining Lake of Haarlem.
The meteorological influence of the draining of lakes and of humid soils
has not, so far as I know, received much attention from experimental
physicists; but we are not altogether without direct proof in support of
theoretical and a priori conclusions. Thermometrical observations have
been regularly made at Zwanenburg, near the northern extremity of the
Lake of Haarlem, for more than a century; and since 1845 a similiar
registry has been kept at the Helder, forty or fifty miles more to the
north. In comparing these two series of observations, it is found that
towards the end of 1852, when the draining of the lake was finished, and
the following summer had completely dried the newly exposed soil--and,
of course, greatly diminished the water-surface--a change took place in
the relative temperature of those two stations. Taking the mean of each
successive period of five days, from 1845 to 1852, both inclusive, the
temperature of Zwanenburg was thirty-three hundredths of a degree
centigrade LOWER than at the Helder. From the end of 1852 the
thermometer at Zwanenburg has stood, from the 11th of April to the 20th
of September, twenty-two hundredths of a degree HIGHER than that at
Helder; but from the 14th of October to the 17th of March, it has marked
one-tenth of a degree LOWER than its mean between the same dates before
1853. [Footnote: Krecke, Het Klimaat van Nederland, ii., p. 64.]
There is no reason to doubt that these differences are due to the
draining of the lake. In summer, solar irradiation has acted more
powerfully on the now exposed earth and of course on the air in contact
with it; and there is no longer a large expanse of water still retaining
and of course imparting something of the winter temperature; in winter,
the earth has lost more heat by radiation than when covered by water and
the influence of the lake, as a reservoir of warmth accumulated in
summer and gradually given out in winter, was of course lost by its
drainage. Doubtless the quantity of moisture contained in the atmosphere
has been modified by the same cause, but it does not appear that
observations have been made upon this point. Facts lately observed by
Glaisher tend to prove an elevation of not far from two degrees in the
mean temperature of England during the course of the last hundred years.
For reasons which I have explained elsewhere, the early observations
upon which these conclusions are founded do not deserve entire
confidence; but admitting the fact of the alleged elevation, its most
probable explanation would be found in the more thorough draining of the
soil by superficial and by subterranean conduits.
So far as respects the immediate improvement of soil and climate, and
the increased abundance of the harvests, the English system of surface
and subsoil drainage has fully justified the eulogiums of its advocates;
but its extensive adoption appears to have been attended with some
altogether unforeseen and undesirable consequences, very analogous to
those which I have described as resulting from the clearing of the
forests. The under-drains carry off very rapidly the water imbibed by
the soil from precipitation, and through infiltration from neighboring
springs or other sources of supply. Consequently, in wet seasons, or
after heavy rains, a river bordered by artificially drained lands
receives in a few hours, from superficial and from subterranean
conduits, an accession of water which, in the natural state of the
earth, would have reached it only by small instalments after percolating
through hidden paths for weeks or even months, and would have furnished
perennial and comparatively regular contributions, instead of swelling
deluges, to its channel. Thus, when human impatience rashly substitutes
swiftly acting artificial contrivances for the slow methods by which
nature drains the surface and superficial strata of a river-basin, the
original equilibrium is disturbed, the waters of the heavens are no
longer stored up in the earth to be gradually given out again, but are
hurried out of man's domain with wasteful haste; and while the
inundations of the river are sudden and disastrous, its current, when
the drains have run dry, is reduced to a rivulet, it ceases to supply
the power to drive the machinery for which it was once amply sufficient,
and scarcely even waters the herds that pasture upon its margin.
The water of subterranean currents and reservoirs, as well as that of
springs and common wells, is doubtless principally furnished by
infiltration, and hence its quantity must vary with every change of
natural surface which tends to accelerate or to retard the drainage of
the surface-soil. The drainage of marshes, therefore, and all other
methods of drying the superficial strata, whether by open ditches or by
underground tubes or drains, has the same effect as clearing off the
forest in depriving the subterranean waters of accessions which they
would otherwise receive by infiltration, and in proportion as the sphere
of such operation is extended, their influence will make itself felt in
the diminished supply of water in springs and wells. [Footnote: Babinet
condemns the general draining of marshes. "Draining," says he, "has been
much in vogue for some years, and it has been a special object to dry
and fertilize marshy grounds. I believe that excessive dryness is thus
produced, and that other soils in the neighborhood are sterilized in
proportion."--Etudes et Lectures, iv., p. 118.
"The extent of soil artificially dried by drainage is constantly
increasing, and the water received by the surface from precipitation
flows off by new channels, and is in general carried off more rapidly
than before. Must not this fact exercise an influence on the regime of
springs whose basin of supply thus undergoes a more or less complete
transformation "--Bernhard Cotta, Preface to Paramelle, Quellenkunde, p.
vii., viii.
The effects of agricultural drainage are perceptible at great depths. It
has been observed in Cornwall that deep mines are more free from water
in well-drained districts than in those where drainage is not generally
practised.--Esquiros, Revue des Deux Mondes, 15 Nov., 1863, p. 430.
See also Asbjornsen, Torv og Torvdrift, p. 31.]
Geographical and Meteorological Effects of Aqueducts, Reservoirs, and
Canals.
Many of the great processes of internal improvement, such as aqueducts
for the supply of great cities, railroad cuts and embankments, and the
like, divert water from its natural channels and affect its distribution
and ultimate discharge. The collecting of the waters of a considerable
district into reservoirs, to be thence carried off by means of
aqueducts, as, for example, in the forest of Belgrade, near
Constantinople, deprives the grounds originally watered by the springs
and rivulets of the necessary moisture, and reduces them to barrenness.
[Footnote: See a very interesting paper on the Water-Supply of
Constantinople, by Mr. Homes, of the New York State Library, in the
Albany Argus of June 6, 1872. The system of aqueducts for the supply of
water to that city was commenced by Constantine, and the great aqueduct,
frequently ascribed to Justinian, which is 840 feet long and 112 feet
high, is believed to have been constructed during the reign of the
former emperor.] Similar effects must have followed from the
construction of the numerous aqueducts which supplied ancient Rome with
such a profuse abundance of water. [Footnote: The unhealthiness of the
Roman Campagna is ascribed by many mediaeval as well as later writers to
the escape of water from the ancient aqueducts, which had fallen out of
repair from neglect, or been broken down by enemies in the sieges of
Rome.] On the other hand, the filtration of water through the banks or
walls of an aqueduct carried upon a high level across low ground, often
injures the adjacent soil, and is prejudicial to the health of the
neighboring population; and it has been observed in Switzerland and
elsewhere, that fevers have been produced by the stagnation of the water
in excavations from which earth had been taken to form embankments for
railways.
If we consider only the influence of physical improvements on civilized
life, we shall perhaps ascribe to navigable canals a higher importance,
or at least a more diversified influence, than to aqueducts or to any
other works of man designed to control the waters of the earth, and to
affect their distribution. They bind distant regions together by social
ties, through the agency of the commerce they promote; they facilitate
the transportation of military stores and engines, and of other heavy
material connected with the discharge of the functions of government;
they encourage industry by giving marketable value to raw material and
to objects of artificial elaboration which would otherwise be worthless
on account of the cost of conveyance; they supply from their surplus
waters means of irrigation and of mechanical power; and, in many other
ways, they contribute much to advance the prosperity and civilization of
nations. Nor are they wholly without geographical importance. They
sometimes drain lands by conveying off water which would otherwise
stagnate on the surface, and, on the other hand, like aqueducts, they
render the neighboring soil cold and moist by the percolation of water
through their embankments; [Footnote: Sismondi, speaking of the Tuscan
canals, observes: "But inundations are not the only damage caused by the
waters to the plains of Tuscany. Raised, as the canals are, above the
soil, the water percolates through their banks, penetrates every
obstruction, and, in spite of all the efforts of industry, sterilizes
and turns to morasses fields which nature and the richness of the soil
seemed to have designed for the most abundant harvests. In ground thus
pervaded with moisture, or rendered COLD, as the Tuscans express it, by
the filtration of the canal-water, the vines and the mulberries, after
having for a few years yielded fruit of a saltish taste, rot and perish.
The wheat decays in the ground, or dies as soon as it sprouts. Winter
crops are given up, and summer cultivation tried for a time; but the
increasing humidity, and the saline matter communicated to the
earth--which affects the taste of all its products, even to the grasses,
which the cattle refuse to touch--at last compel the husbandman to
abandon his fields and leave uncultivated a soil that no longer repays
his labor."--Tableau de l'Agriculture Toscane, pp. 11, 12.] they dam up,
check, and divert the course of natural currents, and deliver them at
points opposite to, or distant from, their original outlets; they often
require extensive reservoirs to feed them thus retaining through the
year accumulations of water--which would otherwise run off, or evaporate
in the dry season--and thereby enlarging the evaporable surface of the
country; and we have already seen that they interchange the flora and
the fauna of provinces widely separated by nature. All these modes of
action certainly influence climate and the character of terrestrial
surface, though our means of observation are not yet perfected enough to
enable us to appreciate and measure their effects.
Antiquity of Irrigation.
We know little of the history of the extinct civilizations which
preceded the culture of the classic ages, and no nation has, in modern
times, spontaneously emerged from barbarian and created for itself the
arts of social life. [Footnote: I ought perhaps to except the Mexicans
and the Peruvians, whose arts and institutions are not yet shown to be
historically connected with those of any more ancient people. The
lamentable destruction of so many memorials of these tribes, by the
ignorance and bigotry of the so-called Christian barbarians who
conquered them, has left us much in the dark as to many points of their
civilization; but they seem to have reached that stage where continued
progress in knowledge and in power over nature is secure, and a few more
centuries of independence might have brought them to originate for
themselves most of the great inventions which the last four centuries
have bestowed upon man.] The improvements of the savage races whose
history we can distinctly trace are borrowed and imitative, and our
theories as to the origin and natural development of industrial art are
conjectural. Of course, the relative antiquity of particular branches of
human industry depends much upon the natural character of soil, climate,
and spontaneous vegetable and animal life in different countries; and
while the geographical influence of man would, under given
circumstances, be exerted in one direction, it would, under different
conditions, act in an opposite or a diverging line. I have given some
reasons for thinking that in the climates to which our attention has
been chiefly directed, man's first interference with the natural
arrangement and disposal of the waters was in the way of drainage of
surface. But if we are to judge from existing remains alone, we should
probably conclude that irrigation is older than drainage; for, in the
regions regarded by general tradition as the cradle of the human race,
we find traces of canals evidently constructed for the former purpose at
a period long preceding the ages of which we have any written memorials.
There are, in ancient Armenia, extensive districts which were already
abandoned to desolation at the earliest historical epoch, but which, in
a yet remoter antiquity, had been irrigated by a complicated and highly
artificial system of canals, the lines of which can still be followed;
and there are, in all the highlands where the sources of the Euphrates
rise, in Persia, in Egypt, in India, and in China, works of this sort
which must have been in existence before man had begun to record his own
annals.
In warm countries, such as most of those just mentioned, the effects I
have described as usually resulting from the clearing of the forests
would very soon follow. In such climates, the rains are inclined to be
periodical; they are also violent, and for these reasons the soil would
be parched in summer and liable to wash in winter. In these countries,
therefore, the necessity for irrigation must soon have been felt, and
its introduction into mountainous regions like Armenia must have been
immediately followed by a system of terracing, or at least scarping the
hillsides. Pasture and meadow, indeed, may be irrigated even when the
surface is both steep and irregular, as may be observed abundantly on
the Swiss as well as on the Piedmontese slope of the Alps; but in dry
climates, ploughland and gardens on hilly grounds require terracing,
both for supporting the soil and for administering water by irrigation,
and it should be remembered that terracing, of itself, even without
special arrangements for controlling the distribution of water, prevents
or at least checks the flow of rain-water, and gives it time to sink
into the ground instead of running off over the surface.
The summers in Egypt, in Syria, and in Asia Minor and even Rumelia, are
almost rainless. In such climates, the neccssity of irrigation is
obvious, and the loss of the ancient means of furnishing it helps to
explain the diminished fertility of most of the countries in question.
[Footnote: In Egypt, evaporation and absorption by the earth are so
rapid, that all annual crops require irrigation during the whole period
of their growth. As fast as the water retires by the subsidence of the
annual inundation, the seed is sown upon the still moist, uncovered
soil, and irrigation begins at once. Upon the Nile, you hear the
creaking of the water-wheels, and sometimes the movement of steam-pumps,
through the whole night, while the poorer cultivators unceasingly ply
the simple shadoof, or bucket-and-sweep, laboriously raising the water
from trough to trough by as many as six or seven stages when the river
is low. The bucket is of flexible leather, with a stiff rim, and is
emptied into the trough, not by inverting it like a wooden bucket, but
by putting the hand beneath and pushing the bottom up till the water all
runs out over the brim, or, in other words, by turning the vessel inside
out.
The quantity of water thus withdrawn from the Nile is enormous. Most of
this is evaporated directly from the surface or the superficial strata,
but some moisture percolates down and oozes through the banks into the
river again, while a larger quantity sinks till it joins the slow
current of infiltration by which the Nile water pervades the earth of
the valley to the distance, at some points, of not less than fifty
miles.] The surface of Palestine, for example, is composed, in a great
measure, of rounded limestone hills, once, no doubt, covered with
forests. These were partially removed before the Jewish conquest.
[Footnote: "Forests," "woods," and "groves," are frequently mentioned in
the Old Testament as existing at particular places, and they are often
referred to by way of illustration, as familiar objects. "Wood" is twice
spoken of as a material in the New Testament, but otherwise--at least
according to Cruden--not one of the above words occurs in that volume.
In like manner, while the box, the cedar, the fir, the oak, the pine,
"beams," and "timber," are very frequently mentioned in the Old
Testament, not one of these words is found in the New, EXCEPT the case
of the "beam in the eye," in the parable in Matthew and Luke.
This interesting fact, were other evidence wanting, would go far to
prove that a great change had taken place in this respect between the
periods when the Old Testament and the New were respectively composed;
for the scriptural writers, and the speakers introduced into their
narratives, are remarkable for their frequent allusions to the natural
objects and the social and industrial habits which characterized their
ages and their country.] When the soil began to suffer from drought,
reservoirs to retain the waters of winter were hewn in the rock near the
tops of the hills, and the declivities were terraced. So long as the
cisterns were in good order, and the terraces kept up, the fertility of
Palestine was unsurpassed, but when misgovernment and foreign and
intestine war occasioned the neglect or destruction of these
works--traces of which still meet the traveller's eye at every
step,--when the reservoirs were broken and the terrace walls had fallen
down, there was no longer water for irrigation in summer, the rains of
winter soon washed away most of the thin layer of earth upon the rocks,
and Palestine was reduced almost to the condition of a desert.
The course of events has been the same in Idumaea. The observing
traveller discovers everywhere about Petra, particularly if he enters
the city by the route of Wadi Ksheibeh, very extensive traces of ancient
cultivation, and upon the neighboring ridges are the ruins of numerous
cisterns evidently constructed to furnish a supply of water for
irrigation. [Footnote: One of these on Mount Hor, two stories deep, is
in such good preservation, although probably not repaired for many
centuries, that I found ten feet of water in it in June, 1851.] In
primitive ages, the precipitation of winter in these hilly countries
was, in great part, retained for a time in the superficial soil, first
by the vegetable mould of the forests, and then by the artificial
arrangements I have described. The water imbibed by the earth was partly
taken up by direct evaporation, partly absorbed by vegetation, and
partly carried down by infiltration to subjacent strata which gave it
out in springs at lower levels, and thus a fertility of soil and a
condition of the atmosphere were maintained sufficient to admit of the
dense population that once inhabited those now arid wastes. At present,
the rain-water runs immediately off from the surface and is carried down
to the sea, or is drunk up by the sands of the wadis, and the hillsides
which once teemed with plenty are bare of vegetation, and seared by the
scorching winds of the desert.
In fact, climatic conditions render irrigation a necessity in all the
oriental countries which have any importance in ancient or in modern
history, and there can be no doubt that this diffusion of water over
large surfaces has a certain reaction on climate. Some idea of the
extent of artificially watered soil in India may be formed from the fact
that in fourteen districts of the Presidency of Madras, not less than
43,000 reservoirs, constructed by the ancient native rulers for the
purpose of irrigation, are now in use, and that there are in those
districts at least 10,000 more which are in ruins and useless. These
reservoirs are generally formed by damming the outlets of natural
valleys; and the dams average half a mile in length, though some of them
are thirty miles long and form ponds covering from 37,000 to 50,000
acres. The areas of these reservoirs alone considerably increase the
water-surface, and each one of them irrigates an extent of cultivated
ground much larger than itself. Hence there is a great augmentation of
humid surface from those constructions. [Footnote: The present
government of India obtains the same result more economically and
advantageously by constructing in many provinces of that vast empire
canals of great length and capacity, which not only furnish a greater
supply of water than the old reservoirs, but so distribute it as to
irrigate a larger area than could be watered by any system of artificial
basins. The excavacations for the Ganges Canal were nearly equal to
those for the Suez Canal, falling little short of 100,000 cubic yards,
without counting feeders and accessory lines amounting to a length of
3,000 miles. This canal, according to a recent article in the London
Times, waters a tract of land 320 miles long by 50 broad. The Jumna
Canal, 130 miles long, with 608 miles of distributing branches, waters a
territory 120 miles long with a breadth of 15 miles.
Other statements estimate the amount of land actually under irrigation
in British India at 6,000,000 acres, and add that canals now in
construction will water as much more. The Indian irrigation canals are
generally navigable, some of them by boats of large tonnage, and the
canals return a net revenue of from five to twenty per cent. on their
cost.]
The cultivable area of Egypt, or the space between desert and desert
where cultivation would be possible, is now estimated at ten thousand
square statute miles. [Footnote: The area which the waters of the Nile,
left to themselves, would now cover is greater than it would have been
in ancient times, because the bed of the river has been elevated, and
consequently the lateral spread of the inundation increased. See Smith's
Dictionary of Geography, article "Aegyptus". But the industry of the
Egyptians in the days of the Pharaohs and the Ptolemies carried the
Nile-water to large provinces, which have now been long abandoned and
have relapsed into the condition of desert. "Anciently," observes the
writer of the article "Egypt" in Smith's Dictionary of the Bible, "2,735
square miles more [about 3,700 square statute miles] may have been
cultivated. In the best days of Egypt, probably all the land was
cultivated that could be made available for agricultural purposes, and
hence we may estimate the ancient arable area of that country at not
less than 11,000 square statute miles, or fully double its present
extent."
According to an article in the Bollettino della Societa Geografica
Italiana, vol. v., pt. iii., p. 210, the cultivated soil of Egypt in
1869 amounted to 4,500,000 acres, and the remaining soil capable of
cultivation was estimated at 2,000,000 acres.] Much of the surface,
though not out of the reach of irrigation, lies too high to be
economically watered, and irrigation and cultivation are therefore at
present confined to an area of seven thousand square miles, nearly the
whole of which is regularly and constantly watered when not covered by
the inundation, except in the short interval between the harvest and the
rise of the waters. For nearly half of the year, then, irrigation adds
seven thousand square miles to the humid surface of the Nile valley, or,
in other words, more than decuples the area from which an appreciable
quantity of moisture would otherwise be evaporated; for after the Nile
has retired within its banks, its waters by no means cover one-tenth of
the space just mentioned.
The Nile receives not a single tributary in its course below Khartoum;
there is not so much as one living spring in the whole land, [Footnote:
The so-called spring at Heliopolis is only a thread of water infiltrated
from the Nile or the canals.] and, with the exception of a narrow strip
of coast, where the annual precipitation is said to amount to six
inches, the fall of rain in the territory of the Pharaohs is not two
inches in the year. The subsoil of the whole valley is pervaded with
moisture by infiltration from the Nile, and water can everywhere be
found at the depth of a few feet. Were irrigation suspended, and Egypt
abandoned, as in that case it must be, to the operations of nature,
there is no doubt that trees, the roots of which penetrate deeply, would
in time establish themselves on the deserted soil, fill the valley with
verdure, and perhaps at last temper the climate, and even call down
abundant rain from the heavens. [Footnote: The date and the doum palm,
the sont and many other acacias, the caroub, the sycamore and other
trees grow in Egypt without irrigation, and would doubtless spread
through the entire valley in a few years.] But the immediate effect of
discontinuing irrigation would be, first, an immense reduction of the
evaporation from the valley in the dry season, and then a greatly
augmented dryness and heat of the atmosphere. Even the almost constant
north wind--the strength of which would be increased in consequence of
these changes--would little reduce the temperature of the narrow cleft
between the burning mountains which hem in the channel of the Nile, so
that a single year would transform the most fertile of soils to the most
barren of deserts, and render uninhabitable a territory that irrigation
makes capable of sustaining as dense a population as has ever existed in
any part of the world. [Footnote: Wilkinson states that the total
population, which, two hundred years ago, was estimated at 4,000,000,
amounted till lately to only about 1,800,000 souls, having been reduced
since the year 1800 from 2,500,000 to less than 2,000,000.--Handbook for
Travellers in Egypt. p. 10. The population at the end of the year 1869
is computed at 5,215,000.--Bollettino della Soc. Geog. Ital., vol. v.,
pt. iii., p. 215. This estimate doubtless includes countries bordering
on the upper Nile not embraced in Wilkinson's statistics.] Whether man
found the valley of the Nile a forest, or such a waste as I have just
described, we do not historically know. In either case, he has not
simply converted a wilderness into a garden, but has unquestionably
produced extensive climatic change. [Footnote: Ritter supposes Egypt to
have been a sandy desert when it was first occupied by man. "The first
inhabitant of the sandy valley of the Nile was a desert-dweller, as his
neighbors right and left, the Libyan, the nomade Arab, still are. But
the civilized people of Egypt transformed, by canals, the waste into the
richest granary of the world; they liberated themselves from the
shackles of the rock and sand desert, in the midst of which, by a wise
distribution of the fluid through the solid geographical form, by
irrigation in short, they created a region of culture most rich in
historical monuments."--Einleitung zur allgemeinen vergleichenden
Geographie, pp. 165, 166.
This view seems to me highly improbable; for great rivers, in warm
climates, are never bordered by sandy plains. A small stream may be
swallowed up by sands, but if the volume of water is too large to be
carried off by evaporation or drank up by absorption, it saturates its
banks with moisture, and unless resisted by art, converts them into
marshes covered with aquatio vegetation. By canals and embankments, man
has done much to modify the natural distribution of the waters of the
Nile; yet the annual inundation is not his work, and the river must have
overflowed its banks and carried spontaneous vegetation with its waters,
as well before as since Egypt was first occupied by the human family.
There is, indeed, some reason to suppose that man lived upon the banks
of the Nile when its channel was much lower, and the spread of its
inundations much narrower, than at present; but wherever its flood
reached, there the forest would propagate itself, and its shores would
certainly have been morasses rather than sands.
The opinions of Ritter on this subject are not only improbable, but they
are contradictory to the little historical testimony we possess.
Herodotus informs us in Euterpe that except the province of Thebes, all
Egypt, that is to say, the whole of the Delta and of middle Egypt
extending to Hemopolis Magna in N. L. 27 degrees 45 minutes, was
originally a morass. This morass was doubtless in great part covered
with trees, and hence, in the most ancient hieroglyphical records, a
tree is the sign for the cultivated land between the desert and the
channel of the Nile. In all probability, the real change effected by
human art in the superficial geography of Egypt is the conversion of
pools and marshes into dry land, by a system of transverse dikes, which
compelled the flood-water to deposit its sediment on the banks of the
river instead of carrying it to the sea. The colmate of modern Italy
were thus anticipated in ancient Egypt.]
The fields of Egypt are more regularly watered than those of any other
country bordering on the Mediterranean, except the rice-grounds in
Italy, and perhaps the marcite or winter meadows of Lombardy; but
irrigation is more or less employed throughout almost the entire basin
of that sea, and is everywhere attended with effects which, if less in
degree, are analogous in character, to those resulting from it in Egypt.
There are few things in European husbandry which surprise English or
American observers so much as the extent to which irrigation is employed
in agriculture, and that, too, on soils, and with a temperature, where
their own experience would have led them to suppose it would be
injurious to vegetation rather than beneficial to it. In Switzerland,
for example, grass-grounds on the very borders of glaciers are freely
irrigated, and on the Italian slope of the Alps water is applied to
meadows at heights exceeding 6,000 feet. The summers in Northern Italy,
though longer, are very often not warmer than in the Northern United
States; and in ordinary years, the summer rains are as frequent and as
abundant in the former country as in the latter. [Footnote: The mean
annual precipitation in Lombardy is thirty-six inches, of which nearly
two-thirds fall during the season of irrigation. The rain-fall is about
the same in Piedmont, though the number of days in the year classed as
"rainy" is said to be but twenty-four in the former province while it is
seventy in the latter.--Baird Smith, Italian Irrigation, vol. i., p.
196.
The necessity of irrigation in the great alluivial plain of Northern
Italy is partly explained by the fact that the superficial stratum of
fine earth and vegetable mould is very extensively underlaid by beds of
pebbles and gravel brought down by mountain torrents at a remote epoch.
The water of the surface-soil drains rapdily down into these loose beds,
and passes off by subterranean channels to some unknown point of
discharge; but this circumstance alone is not a sufficient solution. It
is not possible that the habits of vegetables, grown in countries where
irrigation has been immemorially employed, have been so changed that
they require water under conditions of soil and climate where their
congeners, which have not been thus indulgently treated, do not It is a
remarkable fact that during the season of irrigation, when large tracts
of surface are almost constantly saturated with water, there is an
extraordinary dryness in the atmosphere of Lombardy, the hygrometer
standing for days together a few degrees only above zero, while in
winter the instrument indicates extreme humidity of the air, approaching
to total saturation.--Baird Smith, Italian Irrigation, i., p. 189.
There are some atmospheric phenomena in Northern Italy, which an
American finds it hard to reconcile with what he has observed in the
United States. To an American eye, for instance, the sky of Piedmont,
Lombardy, and the northern coast of the Mediterranean, is always whitish
and curdled, and it never has the intensity and fathomless depth of the
blue of his native heavens. And yet the heat of the sun's rays, as
measured by sensation, and, at the same time, the evaporation, are
greater than they would be with the thermometer at the same point in
America. I have frequently felt in Italy, with the mercury below 60
degrees Fahrenheit, and with a mottled and almost opaque sky, a heat of
solar irradiation which I can compare to nothing but the scorching
sensation experienced in America at a temperature twenty degrees higher,
during the intervals between showers, or before a rain, when the clear
blue of the sky seems infinite in depth and transparency. Such
circumstances may create a necessity for irrigation where it would
otherwise be superfluous, if not absolutely injurious.
In speaking of the superior apparent clearness of the SKY in America, I
confine myself to the concave vault of the heavens, and do not mean to
assert that terrestrial objects are generally visible at greater
distances in the United States than in Italy. Indeed, I am rather
disposed to maintain the contrary; for though I know that the lower
strata of the atmosphere in Europe never equal in transparency the air
near the earth in New Mexico, Peru, and Chili, yet I think the accidents
of the coast-line of the Riviera, as, for example, between Nice and La
Spezia, and those of the incomparable Alpine panorama seen from Turin,
are distinguishable at greater distances than they would be in the
United States.] Yet in Piedmont and Lombardy irrigation is bestowed upon
almost every crop, while in our Northern States it is never employed at
all in farming husbandry, or indeed for any purpose except in
kitchen-gardens, and possibly, in rare cases, in some other small branch
of agricultural industry. [Footnote: In our comparatively rainless
Western territory, irrigation is extensively and very beneficially
employed. In the Salt Lake valley and in California, hundreds if not
thousands of miles of irrigation canals have been constructed, and there
is little doubt that artificially watering the soil will soon be largely
resorted to in the older States. See valuable observations on this
subject in Hayden, Preliminary Report on Geological Survey of Wyoming,
1870, pp. 194, 195, 258-261.]
In general, it may be said that irrigation is employed only in the
seasons when the evaporating power of the sun and the capacity of the
air for absorbing humidity are greatest, or, in other words, that the
soil is nowhere artificially watered except when it is so dry that
little moisture would be evaporated from it, and, consequently, every
acre of irrigated ground is so much added to the evaporable surface of
the country. When the supply of water is unlimited, it is allowed, after
serving its purpose on one field, to run into drains, canals, or rivers.
But in most regions where irrigation is regularly employed, it is
necessary to economize the water; after passing over or through one
parcel of ground, it is conducted to another; no more is usually
withdrawn from the canals at anyone point than is absorbed by the soil
it irrigates, or evaporated from it, and, consequently, it is not
restored to liquid circulation, except by infiltration or precipitation.
We are safe, then, in saying that the humidity evaporated from any
artificially watered soil is increased by a quantity bearing a large
proportion to the whole amount distributed over it, for most even of
that which is absorbed by the earth is immediately given out again
either by vegetables or by evaporation; and the hygrometrical and
thermometrical condition of the atmosphere in irrigated countries is
modified proportionally to the extent of the practice.
It is not easy to ascertain precisely either the extent of surface thus
watered, or the amount of water supplied, in any given country, because
these quantities vary with the character of the season; but there are
not many districts in Southern Europe where the management of the
arrangements for irrigation is not one of the most important branches of
agricultural labor. The eminent engineer Lombardini describes the system
of irrigation in Lombardy as, "every day in summer, diffusing over
550,000 hectares [1,375,000 acres] of land 45,000,000 cubic metres
[nearly 600,000,000 cubic yards] of water, which is equal to the entire
volume of the Seine, at an ordinary flood, or a rise of three metres
above the hydrometer at the bridge of La Tournelle at Paris." [Footnote:
Memorie sui progetti per Pestensions dell' Irrigazione, etc., il
Politecniso, for January, 1868, p. 6.]
Niel states the quantity of land irrigated in the former kingdom of
Sardinia, including Savoy, in 1856, at 240,000 hectares, or not much
Ices than 600,000 acres. This is about four-thirteenths of the
cultivable soil of the kingdom. According to the same author, the
irrigated lands in Franco did not exceed 100,000 hectares, or 247,000
acres, while those in Lombardy amounted to 450,000 hectares, more than
1,100,000 acres. [Footnote: Niel, L'Agriculture des Etats Sardes, p.
232. This estimate, it will be observed, is 275,000 acres less than that
of Lombardini.]
In these three states alone, then, there were more than three thousand
square miles of artificially watered land, and if we add the irrigated
soils of the rest of Italy, [Footnote: In 1865 the total quantity of
irrigated lands in the kingdom of Italy was estimated at 1,357,677
hectares, or 2,000,000 acres, of which one-half is supplied with water
by artificial canals. The Canal Cavour adds 250,000 acres to the above
amount. The extent of artificially watered ground in Italy is
consequently equal to the entire area of the States of Delaware and
Rhode Island.--See the official report, Sulle Bonificazione, Risaie, ed
Irrigazioni, 1865, p. 269.] of the Mediterranean islands, of the Spanish
peninsula, of Turkey in Europe and in Asia Minor, of Syria, of Egypt and
the remainder of Northern Africa, we shall see that irrigation increases
the evaporable surface of the Mediterranean basin by a quantity bearing
no inconsiderable proportion to the area naturally covered by water
within it.
Arrangements are concluded, and new plans proposed, for an immense
increase of the lands fertilized by irrigation in France and in Belgium,
as well as in Spain and Italy, and there is every reason to believe that
the artificially watered soil of the latter country will be doubled,
that of France quadrupled, before the end of this century. There can be
no doubt that by these operations man is exercising a powerful influence
on the soil, on vegetable and animal life, and on climate, and hence
that in this, as in many other fields of industry, he is truly a
geographical agency. [Footnote: It belongs rather to agriculture than to
geography to discuss the quality of the crops obtained by irrigation, or
the permanent effects produced by it on the productiveness of the soil.
There is no doubt, however, that all crops which can be raised without
watering are superior in flavor and in nutritive power to those grown by
the aid of irrigation. Garden vegetables, particularly, profusely
watered, are so insipid as to be hardly eatable. Wherever irrigation is
practised, there is an almost irresistible tendency, especially among
ignorant cultivators, to carry it to excess; and in Piedmont and
Lombardy, if the supply of water is abundant, it is so liberally applied
as sometimes not only to injure the quality of the product, but to drown
the plants and diminish the actual weight of the crop. Grass-lands are
perhaps an exception to this remark, as it seems almost impossible to
apply too much water to them, provided it be kept in motion and not
allowed to stagnate on the surface. Protestor Liebig, in his Modern
Agriculture, says: "There is not to be found in chemistry a more
wonderful phenomenon, one which more confounds all human wisdom, than is
presented by the soil of a garden or field. By the simplest experiment,
any one may satisfy himself that rain-water filtered through field or
garden soil does not dissolve out a trace of potash, silicic acid,
ammonia, or phosphoric acid. The soil does not give up to the water one
particle of the food of plants which it contains. The most continuous
rains cannot remove from the field, except mechanically, any of the
essential constituents of ite fertility." "The soil not only retains
firmly all the food of plants which is actually in it, but its power to
preserve all that may be useful to them extends much farther. If rain or
other water holding in solution ammonia, potash, and phosphoric and
silicic acids, be brought in contact with soil, these substances
disappear almost immediately from the solution; the soil withdraws them
from the water. Only such substances are completely withdrawn by the
soil as are indispensable articles of food for plants; all others remain
wholly or in part in solution."
These opinions were confirmed, soon after their promulgation, by the
experimental researches of other chemists, but are now questioned, and
they are not strictly in accordance with the alleged experience of
agriculturists in those parts of Italy where irrigation is most
successfully applied. They believe that the constituents of vegetable
growth are washed out of the soil by excessive and long-continued
watering. They consider it also established as a fact of observation,
that water which has flowed through or over rich ground is more valuable
for irrigation than water from the same source, which has not been
impregnated with fertilizing substances by passing through soils
containing them; and, on the other hand, that water, rich in the
elements of vegetation, parts with them in serving to irrigate a poor
soil, and is therefore less valuable as a fertilizer of lower grounds to
which it may afterward be conducted. See Baird Smith, Italian
Irrigation, i., p. 25; Scott Moncrieff, Irrigation in Southern Europe,
pp. 34, 87, 89; Lombardini, Sulle Inondazioni etc., p. 73; Mangon, Les
Irrigations, p. 48.
The practice of irrigation--except in mountainous countries where
springs and rivulets are numerous--is attended with very serious
economical, social, and political evils. The construction of canals and
their immensely ramified branches, and the grading and scarping of the
ground to be watered, are always expensive operations, and they very
often require an amount of capital which can be commanded only by the
state, by moneyed corporations, or by very wealthy proprietors; the
capacity of the canals must be calculated with reference to the area
intended to be irrigated, and when they and their branches are once
constructed, it in very difficult to extend them, or to accommodate any
of their original arrangements to changes in the condition of the soil,
or in the modes or objects of cultivation; the flow of the water being
limited by the abundance of the source or the capacity of the canals,
the individual proprietor cannot be allowed to withdraw water at will,
according to his own private interest or convenience, but both the time
and the quantity of supply must be regulated by a general system
applicable, as far as may be, to the whole area irrigated by the same
canal, and every cultivator must conform his industry to a plan which
may be quite at variance with his special objects or with his views of
good husbandry. The clashing interests and the jealousies of proprietors
depending on the same means of supply are a source of incessant
contention and litigation, and the caprices or partialities of the
officers who control, or of contractors who farm, the canals, lead not
unfrequently to ruinous injustice towards individual landholders. These
circumstances discourage the division of the soil into small properties,
and there is a constant tendency to the accumulation of large estates of
irrigated land in the hands of great capitalists, and consequently to
the dispossession of the small cultivators, who pass from the condition
of owners of the land to that of hireling tillers.
Though farmers are no longer yeomen, but peasants. Having no interest in
the soil which composes their country, they are virtually expatriated,
and the middle class, which ought to constitute the real physical and
moral strength of the land, ceases to exist as a rural estate, and is
found only among the professional, the mercantile, and the industrial
population of the cities.--See, on the difficulty of regulating
irrigation by law, Negri, Idea su una Legge in materia di Acqua, 1864;
and Agmard, Irrigations du Midi de L'Europe' where curious and
important remarks on the laws and usages of the Spanish Moors and the
Spaniards, in respect to irrigation, will be found. The Moors were so
careful in maintaining the details of their system, that they kept in
publio offices bronze models of their dams and sluices, as guides for
repairs and rebuilding. Some of these models are still preserved.
--Ibidem, pp. 204, 205. For an account of recent irrigation
works in Spain, see Spon, Dictionary of Engineering, article Irrigation.
As near as can be ascertained, the amount of water applied to irrigated
lands is scarcely anywhere less than the total precipitation during the
season of vegetable growth, and in general it much exceeds that
quantity. In grass-grounds and in field-culture it ranges from 27 or 28
to 60 inches, while in smaller crops, tilled by hand-labor, it is
sometimes carried as high as 300 inches. [Footnote: Niel, Agriculture
des Etata Sardes, p. 237. Lombardini's computation just given allows
eighty-one cubic metres per day to the hectare [two hundred and sixty
cubic yards to the acre], which, supposing the season of irrigation to
be one hundred days, in equal to a precipitation of thirty-two inches.
But in Lombardy, water in applied to some crops during a longer period
than one hundred days; and in the marcite it flows over the ground even
in winter. According to Boussingault (Economie Rurale, ii., p. 240),
grass-grounds ought to receive, in Germany, twenty-one centimetres of
water per week, and with less than half that quantity it is not
advisable to incur the expense of supplying it. The ground is irrigated
twenty-five or thirty times, and if the full quantity of twenty-one
centimetres is applied, it receives more than two hundred inches of
water, or six times the total amount of precipitation. Puvis, quoted by
Boussingault, after much research comes to the conclusion that a proper
quantity is twenty centimetres [eight inches] applied twenty-five or
thirty times, which corresponds with the estimate just stated. Puvis
adds--and, as our author thinks, with reason--that this amount might be
doubled without disadvantage.--Ibidem, ii., p. 248, 249. In some parts
of France this quantity is immensely exceeded, and it is very important
to observe, with reference to the employment of irrigation in our
Northern States, that water is most freely supplied in the COLDER
provinces. Thus, in the Vosges, meadows are literally flooded for weeks
together, and while in the department of Vancluse a meadow may receive,
in five waterings of six and a half hours each, twenty-one inches of
wnter, in the Vosges it might be deluged for twenty-four hundred hours
in six applications, the enormous quantity of thirteen hundred feet of
water flowing over it. See the important work of Herve Mangon, Sur
l'emploi des eaux dans les Irrigations, chap. ix. Boussingault observes
that rain-water is vastly more fertilizing than the water of irrigating
canals, and therefore the supply of the latter must be greater. This is
explained partly by the different character of the substances held in
solution or suspension by the waters of the earth and of the sky, partly
by the higher temperature of the latter, and, possibly, partly also by
the mode of application--the rain being finely divided in its fall or by
striking plants on the ground, river-water flowing in a continuous
sheet.
The temperature of the water is thought even more important than its
composition. The sources which irrigate the marcite of Lombardy--meadows
so fertile that less than an acre furnishes grass for a cow the whole
year--are very warm. The ground watered by them never freezes, and a
first crop, for soiling, is cut from it in January or February. The
Canal Cavour--which takes its supply chiefly from the Po at Chivasso,
fourteen or fifteen miles below Turin--furnishes water of much higher
fertilizing power than that derived from the Dora Baltea and the Sesia,
both because it is warmer, and because it transports a more abundant and
a richer sediment than the latter streams, which are fed by Alpine
ice-fields and melting snows, and which flow, for long distances, in
channels ground smooth and bare by ancient glaciers and not now
contributing much vegetable mould or fine slime to their waters.]
The rice-grounds and the marcite of Lombardy are not included in these
estimates of the amount of water applied. [Footnote: About one-seventh
of the water which flows over the marcite is absorbed by the soil of
those meadows or evaporated from their surface, and consequently
six-sevenths of the supply remain for use on ground at lower levels.]
The meteorological effect of irrigation on a large scale, which would
seem prima facie most probable, would be an increase of precipitation in
the region watered. [Footnote: On the pluviometric effect of irrigation,
see Lombardini, Sulle Inondazioni, etc., p. 72, 74; the same author,
Essai Hydrologique sur le Nil, p. 32; Messedaglia, Analisi dell' opera
di Champion, pp. 96, 97, note; and Baird Smith, Italian Irrigation, i.,
pp. 189, 190.
In an article in Aus der Natur, vol. 57, p. 443, it is stated that the
rain on the Isthmus of Suez has increased since the opening of the
canal, and has enlarged the evaporable surface of the country; but this
cannot be accepted as an established fact without further evidence.]
Hitherto scientific observation has recorded no such increase, but in a
question of so purely local a character, we must ascribe very great
importance to a consideration which I have noticed elsewhere, but which,
has been frequently overlooked by meteorologists, namely, that vapors
exhaled in one district may very probably be condensed and precipitated
in another very distant from their source. If then it were proved that
an extension of irrigated soil was not followed by an increase of
rain-fall in the same territory, the probability that the precipitation
was augmented SOMEWHERE would not be in the least diminished.
But though we cannot show that in the irrigated portions of Italy the
summer rain is more abundant than it was before irrigation was
practised--for we know nothing of the meteorological conditions of that
country at so remote a period--the fact that there is a very
considerable precipitation in the summer months in Lombardy is a strong
argument in favor of such increase. In the otherwise similar climate of
Rumelia and of much of Asia Minor, irrigation is indeed practiced, but
in a relatively small proportion. In those provinces there is little or
no summer rain. Is it not highly probable that the difference between
Italy and Turkey in this respect is to be ascribed, in part at least, to
extensive irrigation in the former country, and the want of it in the
latter It is true that, in its accessible strata, the atmosphere of
Lombardy is extremely dry during the period of irrigation, but it
receives an immense quantity of moisture by the evaporation from the
watered soil, and the rapidity with which the aqueous vapor is carried
up to higher regions--where, if not driven elsewhere by the wind, it
would be condensed by the cold into drops of rain or at least visible
clouds--is the reason why it is so little perceptible in the air near
the ground. [Footnote: Is not the mottled appearance of the upper
atmosphere in Italy, which I have already noticed, perhaps due in part
to the condensation of the aqueous vapor exhaled by watered ground ]
But the question of an influence on temperature rests on a different
ground; for though the condensation of vapor may not take place within
days of time and degrees of distance from the hour and the place where
it was exhaled from the surface, a local refrigeration must necessarily
accompany a local evaporation. Hence, though the summer temperature of
Lombardy is high, we are warranted in affirming that it must have been
still higher before the introduction of irrigation, and would again
become so if that practice were discontinued. [Footnote: I do not know
that observations have been made on the thermometric influence of
irrigation, but I have often noticed that, on the irrigated plains of
Piedmont ten miles south of Turin, the morning temperature in summer was
several degrees below that marked at the Observatory in the city.]
The quantity of water artificially withdrawn from running streams for
the purpose of irrigation is such as very sensibly to affect their
volume, and it is, therefore, an important element in the geography of
rivers. Brooks of no trifling current are often wholly diverted from
their natural channels to supply the canals, and their entire mass of
water is completely absorbed or evaporated, so that only such proportion
as is transmitted by infiltration reaches the river they originally fed.
Irrigation, therefore, diminishes great rivers in warm countries by
cutting off their sources of supply as well as by direct abstraction of
water from their main channels. We have just seen that the system of
irrigation in Lombardy deprives the Po of a quantity of water equal to
the total delivery of the Seine at ordinary flood, or, in other words,
of the equivalent of a tributary navigable for hundreds of miles by
vessels of considerable burden.
The new canals executed and projected will greatly increase the loss.
The water required for irrigation in Egypt is less than would be
supposed from the exceeding rapidity of evaporation in that arid
climate; for the soil is thoroughly saturated during the inundation, and
infiltration from the Nile continues to supply a considerable amount of
humidity in the dryest season. Linant Bey computed that, in the Delta,
fifteen and one-third cubic yards per day sufficed to irrigate an acre.
If we suppose water to be applied for one hundred and fifty days during
the season of growth, this would be equivalent to a total precipitation
of about seventeen inches and one-third. Taking the area of actually
cultivated soil in Egypt at the estimate of 4,500,000 acres, and the
average amount of water daily applied in both Upper and Lower Egypt at
twelve hundredths of an inch in depth, we have an abstraction of about
74,000,000 cubic yards, which--the mean daily delivery of the Nile being
in round numbers 320,000,000 cubic yards--is twenty-three per cent of
the average quantity of water contributed to the Mediterranean by that
river. [Footnote: The proportion of the waters of the Nile withdrawn for
irrigation is greater than this calculation makes it. The quantity
required for an acre is less in the Delta than in Upper Egypt, both
because the soil of the Delta, to which Linant Bey's estimate applies,
lies little higher than the surface of the river, and is partly
saturated by infiltration, and because near the sea, in N. L. 30
degrees, evaporation is much less rapid than it is several degrees
southwards and in the vicinity of a parched desert.]
In estimating the effect of this abstraction of water upon the volume of
great rivers, especially in temperate climates and in countries with a
hilly surface, we must remember that all the water thus
withdrawn--except that which is absorbed by vegetation, that which
enters into new inorganic compounds, and that which is carried off by
evaporation--is finally restored to the original current by superficial
flow or by infiltration. It is generally estimated that from one-third
to one-half of the water applied to the fields is absorbed by the earth,
and this, with the deductions just given, is returned to the river by
direct infiltration, or descends through invisible channels to moisten
lower grounds, and thence in part escapes again into the bed of the
river, by similar conduits, or in the form of springs and rivulets.
Interesting observations have lately been made on this subject in France
and important practical results arrived at. It was maintained that
mountain irrigation is not ultimately injurious to that of the plains
below, because lands liberally watered in the spring, when the supply is
abundant, act as reservoirs, storing up by absorption water which
afterwards filters down to lower grounds or escapes into the channel of
the river and keeps up its current in the dry summer months, so as to
compensate for what, during those months, is withdrawn from it for
upland irrigation. Careful investigation showed that though this
proposition is not universally true, it is so in many cases, and there
can be no doubt that the loss in the volume of rivers by the abstraction
of water for irrigation is very considerably less than the measure of
the quantity withdrawn. [Footnote: See Vigan, Etude sur les Irrigations,
Paris, 1867; and Scott Moncrieff, Irrigation in Southern Europe, pp. 89,
90.
The brook Ain Musa, which runs through the ruined city of Petra and
finally disappears in the sands of Wadi el Araba, is a considerable
stream in winter, and the inhabitants of that town were obliged to
excavate a tunnel through the rock near the right bank, just above the
upper entrance of the narrow Sik, to discharge a part of its swollen
current. The sagacity of Dr. Robinson detected the necessity of this
measure, though the tunnel, the mouth of which was hidden by brushwood,
was not discovered till some time after his visit. I even noticed, near
the arch that crosses the Sik, unequivocal remains of a sluice by which
the water was diverted to the tunnel. Immense labor was also expended in
widening the natural channel at several points below the town, to
prevent the damming up and setting back of the water--a fact I believe
not hitherto noticed by travellers.
The Fellahheen above Petra still employ the waters of Ain Musa for
irrigation, and in summer the superficial current is wholly diverted
from its natural channel for that purpose. At this season, the bed of
the brook, which is composed of pebbles, gravel, and sand, is dry in the
Sik and through the town; but the infiltration is such that water is
generally found by digging to a small depth in the channel. Observing
these facts in a visit to Petra in the summer, I was curious to know
whether the subterranean waters escaped again to daylight, and I
followed the ravine below the town for a long distance. Not very far
from the upper entrance of the ravine, arborescent vegetation appeared
upon its bottom, and as soon as the ground was well shaded, a thread of
water burst out. This was joined by others a little lower down, and, at
the distance of a mile from the town, a strong current was formed and
ran down towards Wadi el Araba.
Similar facts are observed in all countries where the superficial
current of water-courses is diverted from their bed for irrigation, but
this case is of special interest because it shows the extent of
absorption and infiltration even in the torrid climate of Arabia. See
Baird Smith, Italian Irrigation, vol. i., pp. 172, 386 and 387.]
Irrigation, as employed for certain special purposes in Europe and
America, is productive of very prejudicial climatic effects. I refer
particularly to the cultivation of rice in the Southern States of the
American Union and in Italy. The climate of the Southern States is in
general not necessarily unhealthy for the white man, but he can scarcely
sleep a single night in the vicinity of the rice-grounds without being
attacked by a dangerous fever. The neighborhood of the rice-fields is
possibly less pestilential in Lombardy and Piedmont than in South
Carolina and Georgia, but still very insalubrious to both man and beast.
"Not only does the population decrease where rice is grown," says
Escourron-Milliago, "but even the flocks are attacked by typhus. In the
rice-grounds the soil is divided into compartments rising in gradual
succession to the level of the irrigating canal, in order that the
water, after having flowed one field, may be drawn off to another, and
thus a single current serve for several compartments, the lowest field,
of course, still being higher than the ditch which at last drains both
it and the adjacent soil. This arrangement gives a certain force of
hydrostatic pressure to the water with which the rice is irrigated, and
the infiltration from these fields is said to extend through neighboring
grounds, sometimes to the distance of not less than a myriametre, or six
English miles, and to be destructive to crops and even trees reached by
it. Land thus affected can no longer be employed for any purpose but
growing rice, and when prepared for that crop, it propagates still
further the evils under which it had itself suffered, and, of course,
the mischief is a growing one." [Footnote: Escourrou-Milliago, D'Italie
a propos de l'Exposition de Paris, 1856, p. 92. According to an article
in the Gazzetto di Torino for the 17th of January, 1869, the deaths from
malarious fever in the Canavese district--which is asserted to have been
altogether free from this disease before the recent introduction of
rice-culture--between the 1st of January and the 15th of October, 1868,
were two thousand two hundred and fifty. The extent of the injurious
influence of this very lucrative branch of rural industry in Italy is
contested by the rice-growers. But see Secondo Laura, Le Risaje, Torino,
1869; Selmi, Il Miasma Palustre, p. 89; and especially Carlo Livi, Della
coltivazione del Riso in Italia, in the Nuova Antologia for July, 1871,
p. 599 et seqq. According to official statistics, the rice-grounds of
Italy, including the islands, amounted in 1866 to 450,000 acres. It is
an interesting fact in relation to geographical and climatic conditions,
that while little rice is cultivated SOUTH of N. L. 44 degrees in Italy,
little is grown in the United States NORTH of 35 degrees. To the
southward of the great alluvial plain of the Po, the surface is in
general too much broken to admit of the formation of level fields of
much extent, and where the ground is suitable, the supply of water is
often insufficient.
The Moors introduced the cultivation of rice into Spain at an early
period of their dominion in that country. The Spaniards sowed rice in
Lombardy and in the Neapolitan territory in the 16th century; but
besides the want of water and of level ground convenient for irrigation,
rice-husbandry has proved so much more pestilential in Southern than in
Northern Italy that it has long been discouraged by the Neapolitan
government.]
Salts deposited by Water of Irrigation.
The attentive traveller in Egypt and Nubia cannot fail to notice many
localities, generally of small extent, where the soil is rendered
infertile by an excess of saline matter in its composition. In many
cases, perhaps in all, these barren spots lie rather above the level
usually flooded by the inundations of the Nile, and yet they exhibit
traces of former cultivation. Observations in India suggest a possible
explanation of this fact. A saline efflorescence called "Reh" and
"Kuller" is gradually invading many of the most fertile districts of
Northern and Western India, and changing them into sterile deserts. It
consists principally of sulphate of soda (Glauber's salts), with varying
proportions of common salt. These salts (which in small quantities are
favorable to fertility of soil) are said to be the gradual result of
concentration by evaporation of river and canal waters, which contain
them in very minute quantities, and with which the lands are either
irrigated or occasionally overflowed. The river inundations in hot
countries usually take place but once in a year, and, though the banks
remain submerged for days or even weeks, the water at that period, being
derived principally from rains and snows, must be less highly charged
with mineral matter than at lower stages, and besides, it is always in
motion. The water of irrigation, on the other hand, is applied for many
months in succession, it is drawn from rivers and canals at the seasons
when the proportion of salts is greatest, and it either sinks into the
superficial soil, carrying with it the saline substances it holds in
solution, or is evaporated from the surface, leaving them upon it. Hence
irrigation must impart to the soil more salts than natural inundation.
The sterilized grounds in Egypt and Nubia lying above the reach of the
floods, as I have said, we may suppose them to have been first
cultivated in that remote antiquity when the Nile valley received its
earliest inhabitants, and when its lower grounds were in the condition
of morasses. They must have been artificially irrigated from the
beginning; they may have been under cultivation many centuries before
the soil at a lower level was invaded by man, and hence it is natural
that they should be more strongly impregnated with saline matter than
fields which are exposed every year, for some weeks, to the action of
running water so nearly pure that it would be more likely to dissolve
salts than to deposit them.
SUBTERRANEAN WATERS.
I have frequently alluded to a branch of physical 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. [Footnote: The great limestone plateau of the Karst
in Carniola is completely honey-combed by caves through which the
drainage of that region is conducted. Rivers of considerable volume pour
into some of these caves and can be traced underground to their exit.
Thus the Recca has been satisfactorily identified with a stream flowing
through the cave of Trebich, and with the Timavo--the Timavus of Virgil
and the ancient geographers--which empties through several mouths into
the Adriatic between Trieste and Aquileia. The city of Trieste is very
insufficiently supplied with fresh water. It has been thought
practicable to supply this want by tunnelling through the wall of the
plateau, which rises abruptly in the rear of the town, until some
subterranean stream is encountered, the current of which can be
conducted to the city. More visionary projectors have gone further, and
imagined that advantage might be taken of the natural tunnels under the
Karst for the passage of roads, railways, and even navigable canals. But
however chimerical these latter schemes may seem, there is every reason
to believe that art might avail itself of these galleries for improving
the imperfect drainage of the champaign country bounded by the Karst,
and that stopping or opening the natural channels might very much modify
the hydrography of an extensive region. See in Aus des Natur, xx., pp.
250-254, 263-266, two interesting articles founded on the researches of
Schmidt.
The cases are certainly not numerous where marine currents are known to
pour continuously into cavities beneath the surface of the earth, but
there is at least one well-authenticated instance of this sort--that of
the mill-stream at Argostoli in the island of Cephalonia. It had been
long observed that the sea-water flowed into several rifts and cavities
in the limestone rocks of the coast, but the phenomenon has excited
little attention until very recently. In 1833, three of the entrances
were closed, and a regular channel, sixteen feet long and three feet
wide, with a fall of three feet, was cut into the mouth of a larger
cavity. The sea-water flowed into this canal, and could be followed
eighteen or twenty feet beyond its inner terminus, when it disappeared
in holes and clefts in the rock.
In 1858 the canal had been enlarged to thewidth of five feet and a half,
and a depth of a foot. The water pours rapidly through the canal into an
irregular depression and forms a pool, the surface of which is three or
four feet below the adjacent soil, and about two and a half or three
feet below the level of the sea. From this pool it escapes through
several holes and clefts in the rock, and has not yet been found to
emerge elsewhere.
There is a tide at Argostoli of about six inches in still weather, but
it is considerably higher with a south wind. I do not find it stated
whether water flows through the canal into the cavity at low tide, but
it distinctly appears that there is no refluent current, as of course
there could not be from a base so much below the sea. Mousson found the
delivery through the canal to be at the rate of 24.88 cubic feet to the
second; at what stage of the tide does not appear. Other mills of the
same sort have been erected, and there appear to be several points on
the coast where the sea flows into the land.
Various hypotheses have been suggested to explain this phenomenon, some
of which assume that the water descends to a great depth beneath the
crust of the earth; but the supposition of a difference of level in the
surface of the sea on the opposite sides of the island, which seems
confirmed by other circumstances, is the most obvious method of
explaining these singular facts. If we suppose the level of the water on
one side of the island to be raised by the action of currents three or
four feet higher than on the other, the existence of cavities and
channels in the rock would easily account for a subterranean current
beneath the island, and the apertures of escape might be so deep or so
small as to elude observation. See Aus der Natur, vol. xix., pp. 129 et
seqq. I have lately been informed by a resident of the Ionian Islands,
who is familiar with the locality, that the sea flows uninterruptedly
into the sub-insular cavities, at all stages of the tide.] 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 [Footnote: "The affluents received by the Seine
below Rouen are so inconsiderable, that the augmentation of the volume
of that river must be ascribed principally to springs rising in its bed.
This is a point of which engineers now take notice, and M. Belgrand, the
able officer charged with the improvement of the navigation of the Seine
between Paris and Rouen, has devoted much attention to it."--Babinet,
Etudes et Lectures, iii., p. 185.
On page 232 of the volume just quoted, the same author observes: "In the
lower part of its course, from the falls of the Oise, the Seine receives
so few important affluents, that evaporation alone would suffice to
exhaust all the water which passes under the bridges of Paris."
This supposes a much greater amount of evaporation than has been usually
computed, but I believe it is well settled that the Seine conveys to the
sea much more water than is discharged into it by all its superficial
branches. Babinet states the evaporation from the surface of water at
Paris to be twice as great as the precipitation.
Belgrand supposes that the floods of the Seine at Paris are not produced
by the superficial flow of the water of precipitation into its channel,
but from the augmented discharge of its remote mountain sources, when
swollen by the rains and melted snows which percolate through the
permeable strata in its upper course.--Annales des Ponts et Chaussees,
1851, vol. i.] or of the ocean, and some remains, though even here not
in forever motionless repose, to fill deep cavities and underground
channels. 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
downwards 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 that sea by the river, "so that five-sixths remained
for evaporation and consumption by the organic world." [Footnote:
Physicalische Geographie, p. 286. It does not appear whether this
inference is Mariotte's or Wittwer's. I suppose it is a conclusion of
the latter.
According to Valles, the Seine discharges into the sea thirty per cent.
of the precipitation in its valley, while the Po delivers into the
Adriatic two-thirds and perhaps even three-quarters of the total
down-fall of its basin. The differences between the tributaries of the
Mississippi in this respect are remarkable, the Missouri discharging
only fifteen per cent., the Yazoo not less than ninety. The explanation
of these facts is found in the geographical and geological character of
the valleys of these rivers. The Missouri flows with a rapid current
through an irregular country, the Yazoo has a very slow flow through a
low, alluvial region which is kept constantly almost saturated by
infiltration.] Maury 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." [Footnote: Physical Geography of the Sea. Tenth edition.
London, 1861, Section 274.] 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. [Footnote: In the low peninsula of
Florida, rivers, which must have their sources in mountains hundreds of
miles distant, pour forth from the earth with a volume sufficient to
permit steamboats to ascend to their basins of eruption. In January,
1857, a submarine fresh-water river burst from the bottom of the sea not
far from the southern extremity of the peninsula, and for a whole month
discharged a current not inferior in volume to the river Mississippi, or
eleven times the mean delivery of the Po, and more than six times that
of the Nile. We can explain this phenomenon only by supposing that the
bed of the sea was suddenly burst up by the hydrostatic upward pressure
of the water in a deep reservoir communicating with some great
subterranean river or receptacle in the mountains of Georgia or of Cuba,
or perhaps even in the valley of the Mississippi.--Thomassy, Essai sur
l'Hydrologie. Late southern journals inform us that the creek under the
Natural Bridge in Virginia has suddenly disappeared, being swallowed up
by newly formed fissures, of unknown depth, in its channel. It does not
appear that an outlet for the waters thus absorbed has been discovered,
and it is not improbable that they are filling some underground cavity
like that which supplied the submarine river just mentioned.]
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. [Footnote: See especially Stoppani, Corso di
Geologia, i., pp. 270 et seqq.] The distribution of these waters has
been minutely studied with reference to a great number of localities,
and though the actual mode and rate of their vertical and horizontal
transmission is still involved in much obscurity, 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, high scientific authorities have testified to the
great practical value of his methods, and the general accuracy of his
predictions. [Footnote: Paramelle, Quellenkunde, mit einem Vorwort von
B. Cotta. 1856.] Hydrographical researches have demonstrated the
existence of subterranean currents and reservoirs in many regions where
superficial geology had not indicated their probable presence. Thus, a
much larger proportion of the precipitation in the valley of the Tiber
suddenly disappears than can be accounted for by evaporation and visible
flow into the channel of the river. Castelli suspected that the excess
was received by underground caverns, and slowly conducted by percolation
to the bed of the Tiber. Lombardini--than whom there is no higher
authority--concludes that the quantity of water gradually discharged
into the river by subterranean conduits, is not less than three-quarters
of the total delivery of its basin. [Footnote: See Lombardini,
Importanza degli studi sulla Statistica da Fiumi, p. 27; also, same
author, Sulle Inondazioni avvenute in Francia, etc., p. 29.] What is
true of the hydrology of the Tiber is doubtless more or less true of
that of other rivers, and the immense value of natural arrangements
which diminish the danger of sudden floods by retaining a large
proportion of the precipitation, and of an excessive reduction of river
currents in the droughts of summer, by slowly conducting into their beds
water accumulated and stored up in subterranean reservoirs in rainy
seasons, is too obvious to require to be dwelt upon. 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 subterranaean 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 almost
impervious to water, by flagstones and cement, for a distance as great
as 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 attain a
considerable age and size 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
often comparatively distant sources, 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. [Footnote: The roots of trees
planted in towns do not depend exclusively on infiltration for their
supply of water, for they receive a certain amount of both moisture and
air through the interstices between the paving-stones; but where wide
surfaces of streets and courts are paved with air and water tight
asphaltum, as in Paris, trees suffer from the diminished supply of these
necessary elements.] 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. [Footnote: See the interesting observations of
Krieck on this subject, Schriften zur allgemeinen Erdkunde, cap. iii.,
Section 6, and especially the passages in Ritter s Erdkunde, vol. i.,
there referred to.
The tenacity with which the parched soil of Egypt retains the supply of
moisture it receives from the Nile is well illustrated by observations
of Girard cited by Lombardini from the Memoires de l'Academie des
Sciences, t. ii., 1817. Girard dug wells at distances of 3,200, 1,800,
and 1,200 metres from the Nile, and after three months of low water in
the river, found water in the most remote well, at 4m. 97, in the next
at 4m. 23, and in that nearest the bank at 3m. 44 above the surface of
the Nile. The fact that the water was highest in the most distant well
appears to show that it was derived from the inundation and not, by
lateral infiltration, from the river. But water is found beneath the
sands at points far above and beyond the reach of the inundations, and
can be accounted for only by subterranean percolation from the Nile. At
high flood, the hydrostatic pressure on the banks, combined with
capillary attribution, sends water to great horizontal distances through
the loose soil; at low water the current is reversed, and the moisture
received from the river is partly returned, and may often be seen oozing
from the banks into the river.--Clot Bey, Apercu sur l'Egypte, i., 128.
Laurent (Memoires sur le Sahara Oriental, pp. 8, 9), in speaking of a
river at El-Faid, "which, like all those of the desert, is, most of the
time, without water," observes, that many wells are dug in the bed of
the river in the dry season, and that the subterranean supply of water
thus reached extends itself laterally, at about the same level, at least
a kilometre from the river, as water is found by digging to the depth of
twelve or fifteen metres at a village situated at that distance from the
bank.
Recent experiments, however, have shown that in the case of rivers
flowing through thickly peopled regions, and especially where the refuse
from industrial establishments is discharged into them, the finely
comminuted material received from sewers and factories sometimes clogs
up the interstices between the particles of sand and gravel which
compose the bed and banks, and the water is consequently confined to the
channel and no longer diffuses itself laterally through the adjacent
soil. This obstruction of course acts in both directions, according to
circumstances. In one case, it prevents the escape of river-water and
tends to maintain a full flow of the current; in another it intercepts
the supply the river might otherwise receive by infiltration from the
land, and thus tends to reduce the volume of the stream. In some
instances pits have been sunk along the banks of large rivers and the
water which filters into them pumped up to supply aqueducts. This method
often succeeds, but where the bed of the stream has been rendered
impervious by the discharge of impurities into it, it cannot be depended
upon.
The tubular wells generally known as the American wells furnish another
proof of the free diffusion and circulation of water through the soil. I
do not know the date of the first employment of these tubes in the
United States, but as early as 1861, the Chevalier Calandra used wooden
tubes for this pose in Piedmont, with complete success. See the
interesting pamphlet, Sulla Estrazione delle Acque Sotterrance, by C.
Calandra. Torino, 1867.
The most remarkable case of infiltration known to me by personal
observation is the occurrence of fresh water in the beach-sand on the
eastern side of the Gulf of Akaba, the eastern arm of the Red Sea. If
you dig a cavity in the beach near the sea-level, it soon fills with
water so fresh as not to be undrinkable, though the sea-water two or
three yards from it contains even more than the average quantity of
salt. It cannot be maintained that this is sea-water freshed by
filtration through a few feet or inches of sand, for salt-water cannot
be deprived of its salt by that process. It can only come from the
highlands of Arabia, and it would seem that there must exist some large
reservoir in the interior to furnish a supply which, in spite of
evaporation, holds out for months after the last rains of winter, and
perhaps even through the year. I observed the fact in the month of June.
See Robinson, Biblical Researches, 1857, vol i., p. 167.
The precipitation in the mountains that border the Red Sea is not known
by pluviometric measurement, but the mass of debris brought down the
ravines by the torrents proves that their volume must be large. The
proportion of surface covered by sand and absorbent earth, in Arabia
Petraea and the neighboring countries, is small, and the mountains drain
themselves rapidly into the wadies or ravines where the torrents are
formed; but the beds of earth and disintegrated rock at the bottom of
the valleys are of so loose and porous texture, that a great quantity of
water is absorbed in saturating them before a visible current is formed
on their surface. In a heavy thunder-storm, accompanied by a deluging
rain, which I witnessed at Mount Sinai in the month of May, a large
stream of water poured, in an almost continuous cascade, down the steep
ravine north of the convent, by which travellers sometimes descend from
the plateau between the two peaks, but after reaching the foot of the
mountain, it flowed but a few yards before it was swallowed up in the
sands.
Fresh-water wells are not unfrequently found upon the borders of ocean
beaches. In the dry summer of 1870, drinkable water was procured in many
places on the coast of Liguria by digging to the depth of a yard in the
beach-sands. Tubular wells reach fresh water at twelve or fifteen feet
below the surface on the sandy plains of Cape Cod. In this latter case,
the supply is more probably derived directly from precipitation than
from lateral infiltration.]
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. [Footnote: Charles Martins, Le Sahara, in Revue des Deux Mondes,
Sept. 1, 1864, p. 619; Stoppani, Corso di Geologia, i., 281; Desor,
Die Sahara, Basel, 1871, pp. 50, 51.] But in general, the sheet 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. [Footnote: It is
conceivable that in shallow subterranean basins superincumbered mineral
strata may rest upon the water and be partly supported by it. In such
case the weight of such strata would be an additional, if not the sole,
cause of the ascent of the water through the tubes of artesian wells.
The ascent of petroleum in the artesian oil-wells in Pennsylvania, and,
in many cases, of salt-water in similar tubes, can hardly be ascribed to
hydrostatic pressure, and there is much difficulty in accounting for the
rise of water in artesian wells in many parts of the African desert on
that principle. Perhaps the elasticity of gases, which probably aids in
forcing up petroleum and saline waters, may be, not unfrequently, an
agency in causing the flow of water in common artesian borings. It is
said that artesian wells lately bored in Chicago, some to the depth of
1,600 feet, raise water to the height of 100 feet above the surface.
What is the source of the pressure ] 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; [Footnote: Many more or less probable conjectures have been
made on this subject but thus far I am not aware that any of the
apprehended results have been actually shown to have happened. In an
article in the Annales des Ponts et Chaussees for July and August, 1839,
p. 131, it was suggested that the sinking of the piers of a bridge at
Tours in France was occasioned by the abstraction of water from the
earth by artesian wells, and the consequent withdrawal of the mechanical
support it had previously given to the strata containing it. A reply to
this article will be found in Viollet, Theorie des Puits Artesiens, p.
217.
In some instances the water has rushed up with a force which seemed to
threaten the inundation of the neighborhood, and even the washing away
of much soil; but in those cases the partial exhaustion of the supply,
or the relief of hydrostatic or elastic pressure, has generally produced
a diminution of theflow in a short time, and I do not know that any
serious evil has everbeen occasioned in this way.
In April, 1866, a case of this sort occurred in boring an artesian well
near the church of St. Agnes at Venice. When the drill reached the depth
of 160 feet, a jet of mud and water was shot up to the height of 130
feet above the surface, and continued to flow with gradually diminishing
force for about eight hours.] 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 bore artesian wells down to this
reservoir, to obtain water for domestic use and irrigation, and there is
evidence that this art was practised in Northern Africa in the Middle
Ages. But it had been lost by the modern Moors, 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. [Footnote: See a very
interesting account of these wells, and of the workmen who clean them
out when obstructed by sand brought up with the water, in Laurent's
memoir on the artesian wells recently bored by the French Government in
the Algerian desert. Mimoire sur le Sahara Oriental, etc., pp. et seqq.
Some of the men remained under water from two minutes to two minutes and
forty seconds. Several officers are quoted as having observed immersions
of three minutes' duration, and M. Berbrugger witnessed ona of six
minutes and five seconds and another of five minutes and fifty-five
seconds. The shortest of these periods is longer than the best
pearl-diver can remain below the surface of salt-water. The wells of the
Sahara are from twenty to eighty metres deep.-Desor, Die Sahara, Basel,
1871, p. 43.
The ancient Egyptians were acquainted with the art of boring artesian
wells. Ayme, a French engineer in the service of the Pacha of Egypt,
found several of these old wells, a few years ago, in the oases. They
differed little from modern artesian wells, but were provided with
pear-shaped valves of stone for closing them when water was not needed.
When freed from the sand and rubbish with which they were choked, they
flowed freely and threw up fish large enough for the table. The fish
were not blind, as cave-fish often are, but were provided with eyes, and
belonged to species common in the Nile. The sand, too, brought up with
them resembled that of the bed of that river. Hence it is probable that
they were carried to the oases by subterranean channels from the
Nile.--Desor, Die Sahara, Basel, 1871, p. 28; Stoppani, Corso di
Geologia, i., p. 281. Barth speaks of common wells in Northern Africa
from 200 to 360 feet deep.--Reisen in Africa, ii., p. 180.
It is certain that artesian wells have been common in China from a very
remote antiquity, and the simple method used by the Chinese--where the
drill is raised and let fall by a rope, instead of a rigid rod--has
lately been employed in Europe with advantage. Some of the Chinese wells
are said to be 3,000 feet deep; that of Neusalzwerk in Silesia is 2,300.
A well was bored at St. Louis, in Missouri, a few years ago, to supply a
sugar refinery, to the depth of 2,199 feet. This was executed by a
private firm in three years, at the expense of only $10,000. Four years
since the boring was recommenced in this well and reached a depth of
3,150 feet, but without a satisfactory result. Another artesian well was
sunk at Columbus, in Ohio, to the depth of 2,500 feet, but without
obtaining the desired supply of water. Perhaps, however, the artesian
well of the greatest depth ever executed until very recently, is that
bored within the last six or seven years, for the use of an Insane
Asylum near St. Louis. This well descends to the depth of three thousand
eight hundred and forty-three feet, but the water which it furnishes is
small in quantity and of a quality that cannot be used for ordinary
domestic purposes. The bore has a diameter of six inches to the depth of
425 feet, and after that it is reduced to four inches. For about three
thousand feet the strata penetrated were of carboniferous and magnesian
limestone alternating with sandstone. The remainder of the well passes
through igneous rock. At St. Louis the Missouri and Mississippi rivers
are not more than twenty miles distant from each other, and it is worthy
of note that the waters of neither of those two rivers appear to have
opened for themselves a considerable subterranean passage through the
rocky strata of the peninsula which separates them.
When in boring an artesian well water is not reached at a moderate
depth, it is not always certain that it will be found by driving the
drill still lower. In certain formations, water diminshes as we descend,
and it seems probable that, except in case of caverns and deep fissures,
the weight of the superincumbent mineral strata so compresses the
underlying ones, at no very great distance below the surface, as to
render them impermeable to water and consequently altogether dry. See
London Quarterly Journal of Science, No. xvii., Jan., 1868, p. 18, 19.
In the silver mines of Nevada water is scarcely found at depths below
1,000 feet, and at 1,200 feet from the surface the earth is quite
dry.--American Annual of Scientific Discovery for 1870, p. 75.
Similar facts are observed in Australia. The Pleasant Creek News writes:
"A singular and unaccountable feature in connection with our deep quartz
mines is being developed daily, which must surprise those well
experienced in mining matters. It is the decrease of water as the
greater depths are reached. In the Magdala shaft at 950 ft. the water
has decreased to a MINIMUM; in the Crown Cross Reef Company's shaft, at
800 ft., notwithstanding the two reefs recently struck, no extra water
has been met with; and in the long drive of the Extended Cross Reef
Company, at a depth of over 800 ft., the water is lighter than it was
nearer the surface."
Boring has been carried to a great depth at Sperenberg near Berlin,
where, in 1871, the drill had descended 5,500 feet below the surface,
passing through a stratum of salt for the last 3,200 feet; but the
drilling was still in progress, the whole thickness of the salt-bed not
having been penetrated.--Aus der Natur, vol. 55, p. 208.
The facts that there are mines extending two miles under the bed of the
sea, which are not particularly subject to inconvenience from water,
that little water was encountered in the Mt. Cenis tunnel, 3500 feet
below the surface, and that at Scarpa, not far from Tivoli, there is an
ancient well 1700 feet deep with but eighteen feet of water, may also be
cited as proofs that water is not universally diffused at great
distances beneath the surface.]
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. [Footnote: "In the anticipation of our success at
Oum-Thiour, everything had been prepared to take advantage of this new
source of wealth without a moment's delay. A division of the tribe of
the Selmia, and their sheikh, Aissa ben Sha, laid the foundation of a
village as soon as the water flowed, and planted twelve hundred
date-palms, renouncing their wandering life to attach themselves to the
soil. In this arid spot, life had taken the place of solitude, and
presented itself, with its smiling images, to the astonished traveller.
Young girls were drawing water at the fountain; the flocks, the great
dromedaries with their slow pace, the horses led by the halter, were
moving to the watering trough; the hounds and the falcons enlivened the
group of party-colored tents, and living voices and animated movement
had succeeded to silence and desolation."--Laurent, Memoires sur le
Sahara, p. 85.
Between 1856 and 1864 the French engineers had bored 83 wells in the
Hodna and the Sahara of the Province of Constantine, yielding, all
together, 9,000 gallons a minute, and irrigating more than 125,000
date-palms. Reclus, La Terre, i., p. 110.] 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. [Footnote: The
variety of hues and tones in the local color of the desert is, I think,
one of the phenomena which most surprise and interest a stranger to
those regions. In England and the United States, rock is so generally
covered with moss or earth, and earth with vegetation, that untravelled
Englishmen and Americans are not very familiar with naked rock as a
conspicuous element of landscape. Hence, in their conception of a bare
cliff or precipice, they hardly ascribe definite color to it, but depict
it to their imagination as wearing a neutral tint not assimilable to any
of the hues with which nature tinges her atmospheric or paints her
organic creations. There are certainly extensive desert ranges, chiefly
limestone formations, where the surface is either white, or has
weathered down to a dull uniformity of tone which can hardly be called
color at all; and there are sand plains and drifting hills of wearisome
monotony of tint. But the chemistry of the air, though it may tame the
glitter of the limestone to a dusky gray, brings out the green and brown
and purple of the igneous rocks, and the white and red and blue and
violet and yellow of the sandstone. Many a cliff in Arabia Petraea is as
manifold in color as the rainbow, and the veins are so variable in
thickness and inclination, so contorted and involved in arrangement, as
to bewilder the eye of the spectator like a disk of party-colored glass
in rapid evolution.
In the narrower wadies the mirage is not common; but on broad expanses,
as at many points between Cairo and Suez, and in Wadi el Araba, it mocks
you with lakes and land-locked bays, studded with inlands and fringed
with trees, all painted with an illusory truth of representation
absolutely indistinguishable from the reality. The checkered earth, too,
is canopied with a heaven as variegated as itself. You see, high up in
the sky, rosy clouds at noonday, colored probably by reflection from the
ruddy mountains, while near the horizon float cumuli of a transparent,
ethereal blue, seemingly balled up out of the clear cerulean substance
of the firmament, and detached from the heavenly vault, not by color or
consistence, but solely by the light and shade of their salient and
retreating outlines.] The most sanguine believer in indefinite human
progress hardly expects that man's cunning will accomplish the universal
fulfilment 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 Lardarel 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." [Footnote: Oeuvres de
Palissy, Des Eaux et Fontaines, p. 157.] 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." [Footnote: Id., p. 166.
Palissy's method has recently been tried with good success in various
parts of France.]
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." [Footnote: Babinet, Etudes et Lectures sur les
Sciences d'Observation, ii., p. 225. Our author precedes his account of
his method with a complaint which most men who indulge in thinking have
occasion to repeat many times in the course of their lives. "I will
explain to my readers the construction of artificial fountains according
to the plan of the famous Bernard de Palissy, who, a hundred and fifty
[three hundred] years ago, came and took away from me, a humble
academician of the nineteenth century, this discovery which I had taken
a great deal of pains to make. It is enough to discourage all invention
when one finds plagiarists in the past as well as in the future!" (P.
224.)] 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 he discharged in a
constant flow or at intervals as convenience may dictate. [Footnote: M.
G. Dumas, La Science des Fontaines, 1857.]
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--or in some countries of that armed peace
which has been called "Platonic war"--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.
Inundations and Torrents.