[1] Barbezieux to Saint-Mars, May 10, 1694: "J'ai recu la lettre que
vous avez pris la peine de m'ecrire le 29 du mois passe; vous pouvez,
suivant que vous le proposez, faire mettre dans la prison voutee le
valet du prisonnier qui est mort." It may be noted that Barbezieux
had recently told Saint-Mars to designate his prisoners by
circumlocutions in his correspondence, and not by name.
[2] He cites Bingham's _Bastille_, i. 27.
[3] It was the common practice to give pseudonyms to prisoners, and
this is clearly such a case. Mattioli's prison name was Lestang.
[4] Funck-Brentano argues that "un ancien prisonnier qu'il avait a
Pignerol" (du Junca's words) cannot apply to Dauger, because then du
Junca would have added "et a Exiles." But this is decidedly
far-fetched; du Junca would naturally refer specially to Pignerol,
the fortress with which Saint-Mars had been originally and
particularly associated. Funck-Brentano also insists that the
references to the "ancien prisonnier" in 1696 and 1697 must be to
Mattioli, giving _ancien_ the meaning of "late" or "former" (as in
the phrase "ancien ministre"), and regarding it as an expression
pertinent to Mattioli, who had been at Pignerol with Saint-Mars but
not at Exiles, and not to Dauger, who had always been with
Saint-Mars. But when he attempts to force du Junca's phrase "un
ancien prisonnier qu'il avait a Pignerol" into this sense, he is
straining language. The natural interpretation of the word _ancien_
is simply "of old standing," and Barbezieux's use of it, coming after
Louvois's phrase in 1691, clearly points to Dauger being meant.
[5] This identification had been previously suggested by H. Montaudon
in _Revue de la societe des etudes historiques_ for 1888, p. 452, and
by A. le Grain in _L'Intermediaire des chercheurs_ for 1891, col.
227-228.
[6] The view taken by Monsignor Barnes of the phrase "_Ce n'est qu'un
valet_" in Louvois's letter of July 19, is that (reading this part of
the letter as a continuation of what precedes) the mere fact of
Louvois's saying that Dauger is only a valet means that that was just
what he was not! Monsignor Barnes is rather too apt to employ the
method of interpretation by contraries, on the ground that in such
letters the writer always concealed the real facts.
IRON MOUNTAIN, a city and the county-seat of Dickinson county, Michigan,
U.S.A., about 50 m. W. by N. of Escanaba, in the S.W. part of the Upper
Peninsula. Pop. (1900) 9242, of whom 4376 were foreign-born; (1904)
8585; (1910) 9216. It is served by the Chicago & North Western and the
Chicago, Milwaukee & Saint Paul railways. The city is situated about
1160 ft. above sea-level in an iron-mining district, and the mining of
iron ore (especially at the Great Chapin Iron Mine) is its principal
industry. Iron Mountain was settled in 1879, and was chartered in 1889.
IRONSIDES, a nickname given to one of great bravery, strength or
endurance, particularly as exhibited in a soldier. In English history
Ironside or Ironsides first appears as the name of Edmund II., king of
the English. In the Great Rebellion it was first given by Prince Rupert
to Cromwell, after the battle of Marston Moor in 1644 (see S. R.
Gardiner's _History of the Great Civil War_, 1893, vol. ii. p. 1, and
_Mercurius civicus_, September 19-26, 1644, quoted there). From Cromwell
it was transferred to the troopers of his cavalry, those "God-fearing
men," raised and trained by him in an iron discipline, who were the main
instrument of the parliamentary victories in the field. This (see S. R.
Gardiner, _op. cit._ iv. 179) was first given at the raising of the
siege of Pontefract 1648, but did not become general till later.
IRONTON, a city and the county-seat of Lawrence county, Ohio, U.S.A., on
the Ohio river, about 142 m. E.S.E. of Cincinnati. Pop. (1890) 10,939;
(1900) 11,868, of whom 924 were negroes and 714 foreign-born; (1910
census) 13,147. It is served by the Chesapeake and Ohio, the Cincinnati,
Hamilton and Dayton, the Norfolk and Western, and the Detroit, Toledo
and Ironton railways, and by river steamboats. The city is built on a
plain at the base of hills rising from the river bottom and abounding in
iron ore and bituminous coal; fire and pottery clay also occur in the
vicinity. Besides mining, Ironton has important lumber interests,
considerable river traffic, and numerous manufactures, among which are
iron, wire, nails, machinery, stoves, fire-brick, pressed brick,
terra-cotta, cement, carriages and wagons, and furniture. The total
value of its factory product in 1905 was $4,755,304; in 1900,
$5,410,528. The municipality owns and operates its water-works. Ironton
was first settled in 1848, and in 1851 was incorporated.
IRONWOOD, a city of Gogebic county, Michigan, U.S.A., on the Montreal
river, in the N.W. part of the upper peninsula. Pop. (1890) 7745; (1900)
9705, of whom 4615 were foreign-born; (1910 census) 12,821. It is served
by the Chicago and Northwestern and the Wisconsin Central railways. The
city is situated about 1500 ft. above sea-level in the Gogebic
iron-district, and is principally a mining town; some of the largest
iron mines in the United States are within the city limits. Ironwood was
settled in 1884, and was chartered as a city in 1889.
IRON-WOOD, the name applied to several kinds of timber, the produce of
trees from different parts of the tropics, and belonging to very
different natural families. Usually the wood is extremely hard, dense
and dark-coloured, and sinks in water. Several species of _Sideroxylon_
(_Sapotaceae_) yield iron-wood, _Sideroxylon cinereum_ or _Bojerianum_
being the _bois de fer blanc_ of Africa and Mauritius, and the name is
also given to species of _Metrosideros_ (_Myrtaceae_) and _Diospyros_
(_Ebenaceae_).
West Indian iron-wood is the produce of _Colubrina reclinata_ (and _C.
ferruginosa_ (_Rhamnaceae_), and of _Aegiphila martinicensis
Verbenacae_). _Ixora_ (_Siderodendron_) _triflorum_ (_Rubiaceae_) is the
_bois de fer_ of Martinique, and Zanthoxylum _Pterota_ (_Rutaceae_) is
the iron-wood of Jamaica, while _Robinia Ponacoco_ (_Leguminosae_) is
described as the iron-wood of Guiana. The iron-wood of India and Ceylon
is the produce of _Mesua ferrea_ (_Guttiferae_). The iron-wood tree of
Pegu and Arracan is _Xylia dolabriformis_ (_Leguminosae_), described as
the most important timber-tree of Burma after teak, and known as
_pyingado_. The endemic _bois de fer_ of Mauritius, once frequent in the
primeval woods, but now becoming very scarce, is _Stadtmannia
Sideroxylon_ (_Sapindaceae_), while _Cossignya pinnata_ is known as the
_bois de fer de Judas_. In Australia species of _Acacia_, _Casuarina_,
_Eucalyptus_, _Melaleuca_, _Myrtus_, and other genera are known more or
less widely as iron-wood. Tasmanian iron-wood is the produce of
_Notelaea ligustrina_ (_Oleaceae_), and is chiefly used for making
ships' blocks. The iron-wood or lever-wood of North America is the
timber of the American hop hornbeam, _Ostrya virginica_ (_Cupuliferae_).
In Brazil _Apuleia ferrea_ and _Caesalpinia ferrea_ yield a kind of
iron-wood, called, however, the _Pao ferro_ or false iron-wood.
IRON-WORK, as an ornament in medieval architecture, is chiefly confined
to the hinges, &c., of doors and of church chests, &c. Specimens of
Norman iron-work are very rare. Early English specimens are numerous and
very elaborate. In some instances not only do the hinges become a mass
of scroll work, but the surface of the doors is covered by similar
ornaments. In both these periods the design evidently partakes of the
feeling exhibited in the stone or wood carving. In the Decorated period
the scroll work is more graceful, and, like the foliage of the time,
more natural. As styles progressed, there was a greater desire that the
framing of the doors should be richer, and the ledges were chamfered or
raised, then panelled, and at last the doors became a mass of scroll
panelling. This, of course, interfered with the design of the hinges,
the ornamentation of which gradually became unusual. In almost all
styles the smaller and less important doors had merely plain
strap-hinges, terminating in a few bent scrolls, and latterly in
_fleurs-de-lis_. Escutcheon and ring handles, and the other furniture,
partook more or less of the character of the time. On the continent of
Europe the knockers are very elaborate. At all periods doors have been
ornamented with nails having projecting heads, sometimes square,
sometimes polygonal, and sometimes ornamented with roses, &c. The iron
work of windows is generally plain, and the ornament confined to simple
_fleur-de-lis_ heads to the stanchions. For the iron-work of screens
enclosing tombs and chapels see GRILLE; and generally see METAL-WORK.
IRONY (Gr. [Greek: eironeia], from [Greek: eiron], one who says less
than he means, [Greek: eirein], to speak), a form of speech in which the
real meaning is concealed or contradicted by the words used; it is
particularly employed for the purpose of ridicule, mockery or contempt,
frequently taking the form of sarcastic phrase. The word is frequently
used figuratively, especially in such phrases as "the irony of fate," of
an issue or result that seems to contradict the previous state or
condition. The Greek word was particularly used of an under-statement in
the nature of dissimulation. It is especially exemplified in the assumed
ignorance which Socrates adopted as a method of dialectic, the "Socratic
irony" (see SOCRATES). In tragedy, what is called "tragic irony" is a
device for heightening the intensity of a dramatic situation. Its use is
particularly characteristic of the drama of ancient Greece, owing to the
familiarity of the spectators with the legends on which so many of the
plays were based. In this form of irony the words and actions of the
characters belie the real situation, which the spectators fully realize.
It may take several forms; the character speaking may be conscious of
the irony of his words while the rest of the actors may not, or he may
be unconscious and the actors share the knowledge with the spectators,
or the spectators may alone realize irony. The _Oedipus Tyrannus_ of
Sophocles is the classic example of tragic irony at its fullest and
finest.
IROQUOIS, or SIX NATIONS, a celebrated confederation of North American
Indians. The name is that given them by the French. It is suggested that
it was formed of two ceremonial words constantly used by the tribesmen,
meaning "real adders," with the French addition of _ois_. The league was
originally composed of five tribes or nations, viz. Mohawks, Oneidas,
Onondagas, Senecas and Cayugas. The confederation probably took place
towards the close of the 16th century and in 1722 the Tuscaroras were
admitted, the league being then called that of "the Six Nations." At
that time their total number was estimated at 11,650, including 2150
warriors. They were unquestionably the most powerful confederation of
Indians on the continent. Their home was the central and western parts
of New York state. In the American War of Independence they fought on
the English side, and in the repeated battles their power was nearly
destroyed. They are now to the number of 17,000 or more scattered about
on various reservations in New York state, Oklahoma, Wisconsin and
Canada. The _Iroquoian stock_, the larger group of kindred tribes, of
which the five nations were the most powerful, had their early home in
the St Lawrence region. Besides the five nations, the Neutral nation,
Huron, Erie, Conestoga, Nottoway, Meherrin, Tuscarora and Cherokee were
the most important tribes of the stock. The hostility of the Algonquian
tribes seems to have been the cause of the southward migration of the
Iroquoian peoples. In 1535 Jacques Cartier found an Iroquoian tribe in
possession of the land upon which now stand Montreal and Quebec; but
seventy years later it was in the hands of Algonquians.
See L. H. Morgan, _League of the Hodeno Swanee or Iroquois_
(Rochester, N.Y., 1854); _Handbook of American Indians_ (Washington,
1907). Also INDIANS, NORTH AMERICAN.
IRRAWADDY, or IRAWADI, the principal river in the province of Burma,
traversing the centre of the country, and practically running throughout
its entire course in British territory. It is formed by the confluence
of the Mali and N'mai rivers (usually called Mali-kha and N'mai-kha, the
_kha_ being the Kachin word for river) in 25 deg. 45' N. The N'mai is
the eastern branch. The definite position of its source is still
uncertain, and it seems to be made up of a number of considerable
streams, all rising within a short distance of each other in about 28
deg. 30' N. It is shown on some maps as the Lu river of Tibet; but it is
now quite certain that the Tibetan Lu river is the Salween, and that the
N'mai has its source or sources near the southern boundary of Tibet, to
the north-east or east of the source of the Mali. At the confluence the
N'mai is larger than the Mali. The general width of its channel seems to
be 350 or 400 yds. during this part of its course. In the rains this
channel is filled up, but in the cold weather the average breadth is
from 150 to 200 yds. The N'mai is practically unnavigable. The Mali is
the western branch. Like the main river, it is called Nam Kiu by the
Shans. It rises in the hills to the north of the Hkamti country,
probably in about 28 deg. 30' N. Between Hkamti and the country
comparatively close to the confluence little or nothing is known of it,
but it seems to run in a narrow channel through continuous hills. The
highest point on the Mali reached from the south by Major Hobday in 1891
was Ting Sa, a village a little off the river, in 26 deg. 15' N. About 1
m. above the confluence it is 150 yds. wide in January and 17 ft. deep,
with a current of 3(3/4) m. an hour. Steam launches can only ascend from
Myitkyina to the confluence in the height of the rains. Native boats
ascend to Laikaw or Sawan 26 deg. 2' N., all the year around, but can
get no farther at any season. From the confluence the river flows in a
southerly direction as far as Bhamo, then turns west as far as the
confluence of the Kaukkwe stream, a little above Katha, where it again
turns in a southerly direction, and maintains this in its general course
through Upper and Lower Burma, though it is somewhat tortuous
immediately below Mandalay. Just below the confluence of the Mali and
N'mai rivers the Irrawaddy is from 420 to 450 yds. wide and about 30 ft.
deep in January at its deepest point. Here it flows between hills, and
after passing the Manse and Mawkan rapids, reaches plain country and
expands to nearly 500 yds. at Sakap. At Myitkyina it is split into two
channels by Naungtalaw island, the western channel being 600 yds. wide
and the eastern 200. The latter is quite dry in the hot season. At
Kat-kyo, 5 or 6 m. below Myitkyina, the width is 1000 yds., and below
this it varies from 600 yds. to 3/4 m. at different points. Three miles
below Sinbo the third defile is entered by a channel not more than 50
yds. wide, and below this, throughout the defile, it is never wider than
250 yds., and averages about 100. At the "Gates of the Irrawaddy" at
Poshaw two prism-shaped rocks narrow the river to 50 yds., and the water
banks up in the middle with a whirlpool on each side of the raised
pathway. All navigation ceases here in the floods. The defile ends at
Hpatin, and below this the river widens out to a wet-season channel of 2
m., and a breadth in the dry season of about 1 m. At Sinkan, below
Bhamo, the second defile begins. It is not so narrow nor is the current
so strong as in the third defile. The narrowest place is more than 100
yds. wide. The hills are higher, but the defile is much shorter. At
Shwegu the river leaves the hills and becomes a broad stream, flowing
through a wide plain. The first defile is tame compared with the others.
The river merely flows between low hills or high wooded banks. The banks
are covered at this point with dense vegetation, and slope down to the
water's edge. Here and there are places which are almost perpendicular,
but are covered with forest growth. The course of the Irrawaddy after
receiving the waters of the Myit-nge at Sagaing, as far as 17 deg. N.
lat., is exceedingly tortuous; the line of Lower Burma is crossed in 19
deg. 29' 3" N. lat., 95 deg. 15' E. long., the breadth of the river here
being 3/4 m.; about 11 m. lower down it is nearly 3 m. broad. At
Akauk-taung, where a spur of the Arakan hills end in a precipice 300 ft.
high, the river enters the delta, the hills giving place to low alluvial
plains, now protected on the west by embankments. From 17 deg. N. lat.
the Irrawaddy divides and subdivides, converting the lower portion of
its valley into a network of intercommunicating tidal creeks. It reaches
the sea in 15 deg. 50' N. lat. and 95 deg. 8' E. long., by nine
principal mouths. The only ones used by sea-going ships are the Bassein
and Rangoon mouths. The area of the catchment basin of the Irrawaddy is
158,000 sq. m.; its total length from its known source to the sea is
about 1300 m. As far down as Akauk-taung in Henzada district its bed is
rocky, but below this sandy and muddy. It is full of islands and
sandbanks; its waters are extremely muddy, and the mud is carried far
out to sea. The river commences to rise in March; about June it rises
rapidly, and attains its maximum height about September. The total flood
discharge is between four and five hundred million metre tons of 37 cub.
ft. From Mandalay up to Bhamo the river is navigable a distance of
nearly 1000 m. for large steamers all the year round; but small launches
and steamers with weak engines are often unable to get up the second
defile in the months of July, August and September, owing to the strong
current. The Irrawaddy Flotilla Company's steamers go up and down twice
a week all through the rains, and the mails are carried to Bhamo on
intermediate days by a ferry-boat from the railway terminus at Katha.
During the dry season the larger boats are always liable to run on
sandbanks, more especially in November and December, when new channels
are forming after the river has been in flood. From Bhamo up to Sinbo no
steamers can ply during the rains, that is to say, usually from June to
November. From November to June small steamers can pass through the
third defile from Bhamo to Sinbo. Between Sinbo and Myitkyina small
launches can run all the year round. Above Myitkyina small steamers can
reach the confluence at the height of the flood with some difficulty,
but when the water is lower they cannot pass the Mawkan rapid, just
above Mawme, and the navigation of the river above Myitkyina is always
difficult. The journey from Bhamo to Sinbo can be made during the rains
in native boats, but it is always difficult and sometimes dangerous. It
is never done in less than five days and often takes twelve or more. As
a natural source of irrigation the value of the Irrawaddy is enormous,
but the river supplies no artificial systems of irrigation. It is
nowhere bridged, though crossed by two steam ferries to connect the
railway system on either bank. (J. G. Sc.)
IRREDENTISTS, an Italian patriotic and political party, which was of
importance in the last quarter of the 19th century. The name was formed
from the words _Italia Irredenta_--Unredeemed Italy--and the party had
for its avowed object the emancipation of all Italian lands still
subject to foreign rule. The Irredentists took language as the test of
the alleged Italian nationality of the countries they proposed to
emancipate, which were South Tirol (Trentino), Gorz, Istria, Trieste,
Tessino, Nice, Corsica and Malta. The test was applied in the most
arbitrary manner, and in some cases was not applicable at all. Italian
is not universally spoken in South Tirol, Gorz or Istria. Malta has a
dialect of its own though Italian is used for literary and judicial
purposes, while Dalmatia is thoroughly non-Italian though it was once
under the political dominion of the ancient Republic of Venice. The
party was of little note before 1878. In that year it sprang into
prominence because the Italians were disappointed by the result of the
conference at Berlin summoned to make a European settlement after the
Russo-Turkish War of 1877. The Italians had hoped to share in the
plunder of Turkey, but they gained nothing, while Austria was endowed
with the protectorate of Bosnia, and the Herzegovina, the vitally
important hinterland of her possessions on the Adriatic. Under the sting
of this disappointment the cry of Italia Irredenta became for a time
loud and apparently popular. It was in fact directed almost wholly
against Austria, and was also used as a stalking-horse by discontented
parties in Italian domestic politics--the Radicals, Republicans and
Socialists. In addition to the overworked argument from language, the
Irredentists made much of an unfounded claim that the Trentino had been
conquered by Giuseppe Garibaldi during the war of 1866, and they
insisted that the district was an "enclave" in Italian territory which
would give Austria a dangerous advantage in a war of aggression. It
would be equally easy and no less accurate to call the Trentino an
exposed and weak spot of the frontier of Austria. On the 21st of July
1878 a noisy public meeting was held at Rome with Menotti Garibaldi, the
son of the famous Giuseppe, in the chair, and a clamour was raised for
the formation of volunteer battalions to conquer the Trentino. Signor
Cairoli, then prime minister of Italy, treated the agitation with
tolerance. It was, however, mainly superficial, for the mass of the
Italians had no wish to launch on a dangerous policy of adventure
against Austria, and still less to attack France for the sake of Nice
and Corsica, or Great Britain for Malta. The only practical consequences
of the Irredentist agitation outside of Italy were such things as the
assassination plot organized against the emperor Francis Joseph in
Trieste in 1882 by Oberdank, which was detected and punished. When the
Irredentist movement became troublesome to Italy through the activity of
Republicans and Socialists, it was subject to effective police control
by Signor Depretis. It sank into insignificance when the French
occupation of Tunis in 1881 offended the Italians deeply, and their
government entered into those relations with Austria and Germany which
took shape by the formation of the Triple Alliance. In its final stages
it provided a way in which Italians who sympathized with French
republicanism, and who disliked the monarchical governments of Central
Europe, could agitate against their own government. It also manifested
itself in periodical war scares based on affected fears of Austrian
aggression in northern Italy. Within the dominions of Austria
Irredentism has been one form of the complicated language question which
has disturbed every portion of the Austro-Hungarian empire.
See Colonel von Haymerle, _Italicae res_ (Vienna, 1879) for the early
history of the Irredentists.
IRRIGATION (Lat. _in_, and _rigare_, to water or wet), the artificial
application of water to land in order to promote vegetation; it is
therefore the converse of "drainage" (q.v.), which is the artificial
withdrawal of water from lands that are over-saturated. In both cases
the object is to promote vegetation.
I. _General._--Where there is abundance of rainfall, and when it falls
at the required season, there is in general no need for irrigation. But
it often happens that, although there is sufficient rainfall to raise an
inferior crop, there is not enough to raise a more valuable one.
Irrigation is an art that has been practised from very early times. Year
after year fresh discoveries are made that carry back our knowledge of
the early history of Egypt. It is certain that, until the cultivator
availed himself of the natural overflow of the Nile to saturate the
soil, Egypt must have been a desert, and it is a very small step from
that to baling up the water from the river and pouring it over lands
which the natural flood has not touched. The sculptures and paintings of
ancient Egypt bear no trace of anything approaching scientific
irrigation, but they often show the peasant baling up the water at least
as early as 2000 B.C. By means of this simple plan of raising water and
pouring it over the fields thousands of acres are watered every year in
India, and the system has many advantages in the eyes of the peasant.
Though there is great waste of labour, he can apply his labour when he
likes; no permission is required from a government official; no one has
to be bribed. The simplest and earliest form of water-raising machinery
is the pole with a bucket suspended from one end of a crossbeam and a
counterpoise at the other. In India this is known as the _denkli_ or
_paecottah_; in Egypt it is called the _shaduf_. All along the Nile
banks from morning to night may be seen brown-skinned peasants working
these _shadufs_, tier above tier, so as to raise the water 15 or 16 ft.
on to their lands. With a _shaduf_ it is only possible to keep about 4
acres watered, so that a great number of hands are required to irrigate
a large surface. Another method largely used is the shallow basket or
bucket suspended to strings between two men, who thus bail up the water.
A step higher than these is the rude water-wheel, with earthen pots on
an endless chain running round it, worked by one or two bullocks. This
is used everywhere in Egypt, where it is known as the _sakya_. In
Northern India it is termed the _harat_, or Persian wheel. With one such
water-wheel a pair of oxen can raise water any height up to 18 ft., and
keep from 5 to 12 acres irrigated throughout an Egyptian summer. A very
familiar means in India of raising water from wells in places where the
spring level is as much sometimes as 100 ft. below the surface of the
field is the _churras_, or large leather bag, suspended to a rope
passing over a pulley, and raised by a pair of bullocks which go up and
down a slope as long as the depth of the well. All these primitive
contrivances are still in full use throughout India.
It is not improbable that Assyria and Babylon, with their splendid
rivers, the Euphrates and Tigris, may have taken the idea from the Nile,
and that Carthage and Phoenicia as well as Greece and Italy may have
followed the same example. In spite of a certain amount of
investigation, the early history of irrigation in Persia and China
remains imperfectly known. In Spain irrigation may be traced directly to
the Moorish occupation, and almost everywhere throughout Asia and Africa
where the Moslem penetrated is to be found some knowledge of irrigation.
Spain.
India.
Reservoirs are familiar everywhere for the water-supply of towns, but as
the volume necessary, even for a large town, does not go far in
irrigating land, many sites which would do admirably for the former
would not contain water sufficient to be worth applying to the latter
purpose. In the Mediterranean provinces of Spain there are some very
remarkable irrigation dams. The great masonry dam of Alicante on the
river Monegre, which dates from 1579, is situated in a narrow gorge, so
that while 140 ft. high, it is only 190 ft. long at the crest. The
reservoir is said to contain 130 million cub. ft. of water, and to serve
for the irrigation of 9000 acres, but unless it refills several times a
year, it is hardly possible that so much land can be watered in any one
season. The Elche reservoir, in the same province, has a similar dam 55
ft. high. In neither case is there a waste-weir, the surplus water being
allowed to pour over the crest of the dam. South of Elche is the
province of Murcia, watered by the river Segura, on which there is a dam
25 ft. high, said to be 800 years old, and to serve for the irrigation
of 25,000 acres. The Lorca dam in the same neighbourhood irrigates
27,000 acres. In the jungles of Ceylon are to be found remains of
gigantic irrigation dams, and on the neighbouring mainland of Southern
India, throughout the provinces of Madras and Mysore, the country is
covered with irrigation reservoirs, or, as they are locally termed,
tanks. These vary from village ponds to lakes 14 or 15 m. long. Most of
them are of old native construction, but they have been greatly improved
and enlarged within the last half century. The casual traveller in
southern India constantly remarks the ruins of old dams, and the
impression is conveyed that at one time, before British rule prevailed,
the irrigation of the country was much more perfect than it is now. That
idea, however, is mistaken. An irrigation reservoir, like a human being,
has a certain life. Quicker or slower, the water that fills it will wash
in sand and mud, and year by year this process will go on till
ultimately the whole reservoir is filled up. The embankment is raised,
and raised again, but at last it is better to abandon it and make a new
tank elsewhere, for it would never pay to dig out the silt by manual
labour. It may safely be said that at no time in history were there more
tanks in operation than at present. The ruins which are seen are the
ruins of long centuries of tanks that once flourished and became silted
up. But they did not all flourish at once.
In the countries now being considered, the test of an irrigation work is
how it serves in a season of drought and famine. It is evident that if
there is a long cessation of rain, there can be none to fill the
reservoirs. In September 1877 there were very few in all southern India
that were not dry. But even so, they helped to shorten the famine
period; they stored up the rain after it had ceased to fall, and they
caught up and husbanded the first drops when it began again.
Irrigation canals.
Irrigation effected by river-fed canals naturally depends on the regimen
of the rivers. Some rivers vary much in their discharge at different
seasons. In some cases this variation is comparatively little. Sometimes
the flood season recurs regularly at the same time of the year;
sometimes it is uncertain. In some rivers the water is generally pure;
in others it is highly charged with fertilizing alluvium, or, it may be,
with barren silt. In countries nearly rainless, such as Egypt or Sind,
there can be no cultivation without irrigation. Elsewhere the rainfall
may be sufficient for ordinary crops, but not for the more valuable
kinds. In ordinary years in southern India the maize and the millet,
which form so large a portion of the peasants' food, can be raised
without irrigation, but it is required for the more valuable rice or
sugar-cane. Elsewhere in India the rainfall is usually sufficient for
all the cultivation of the district, but about every eleven years comes
a season of drought, during which canal water is so precious as to make
it worth while to construct costly canals merely to serve as a
protection against famine. When a river partakes of the nature of a
torrent, dwindling to a paltry stream at one season and swelling into an
enormous flood at another, it is impossible to construct a system of
irrigation canals without very costly engineering works, sluices, dams,
waste-weirs, &c., so as to give the engineer entire control of the
water. Such may be seen on the canals of Cuttack, derived from the
Mahanadi, a river of which the discharge does not exceed 400 cub. ft.
per second in the dry season, and rises to 1,600,000 cub. ft. per second
in the rainy season.
Very differently situated are the great canals of Lombardy, drawn from
the Ticino and Adda rivers, flowing from the Maggiore and Como lakes.
The severest drought never exhausts these reservoirs, and the heaviest
rain can never convert these rivers into the resistless floods which
they would be but for the moderating influence of the great lakes. The
Ticino and Adda do not rise in floods more than 6 or 7 ft. above their
ordinary level or fall in droughts more than 4 or 5 ft. below it, and
their water is at all seasons very free from silt or mud. Irrigation
cannot be practised in more favourable circumstances than these. The
great lakes of Central Africa, Victoria and Albert Nyanza, and the vast
swamp tract of the Sudan, do for the Nile on a gigantic scale what Lakes
Maggiore and Como do for the rivers Ticino and Adda. But for these great
reservoirs the Nile would decrease in summer to quite an insignificant
stream. India possesses no great lakes from which to draw rivers and
canals, but through the plains of northern India flow rivers which are
fed from the glaciers of the Himalaya; and the Ganges, the Indus, and
their tributaries are thus prevented from diminishing very much in
volume. The greater the heat, the more rapidly melts the ice, and the
larger the quantity of water available for irrigation. The canal system
of northern India is the most perfect the world has yet seen, and
contains works of hydraulic engineering which can be equalled in no
other country. In the deltas of southern India irrigation is only
practised during the monsoon season. The Godaveri, Kistna and Kaveri all
take their rise on the Western Ghats, a region where the rainfall is
never known to fail in the monsoon season. Across the apex of the deltas
are built great weirs (that of the Godaveri being 2(1/2) m. long), at the
ends and centre of which is a system of sluices feeding a network of
canals. For this monsoon irrigation there is always abundance of water,
and so long as the canals and sluices are kept in repair, there is
little trouble in distributing it over the fields. Similar in character
was the ancient irrigation of Egypt practised merely during the Nile
flood--a system which still prevails in part of Upper Egypt. A detailed
description of it will be found below.
Distribution of the water.
Where irrigation is carried on throughout the whole year, even when the
supply of the river is at its lowest, the distribution of the water
becomes a very delicate operation. It is generally considered sufficient
in such cases if during any one crop one-third of the area that can be
commanded is actually supplied with water. This encourages a rotation of
crops and enables the precious liquid to be carried over a larger area
than could be done otherwise. It becomes then the duty of the engineer
in charge to use every effort to get its full value out of every cubic
foot of water. Some crops of course require water much oftener than
others, and much depends on the temperature at the time of irrigation.
During the winter months in northern India magnificent wheat crops can
be produced that have been watered only twice or thrice. But to keep
sugar-cane, or indigo, or cotton alive in summer before the monsoon sets
in in India or the Nile rises in Egypt the field should be watered every
ten days or fortnight, while rice requires a constant supply of water
passing over it.
Experience in these sub-tropical countries shows the absolute necessity
of having, for successful irrigation, also a system of thorough
drainage. It was some time before this was discovered in India, and the
result has been the deterioration of much good land.
In Egypt, prior to the British occupation in 1883, no attempt had been
made to take the water off the land. The first impression of a great
alluvial plain is that it is absolutely flat, with no drainage at all.
Closer examination, however, shows that if the prevailing slopes are not
more than a few inches in the mile, yet they do exist, and scientific
irrigation requires that the canals should be taken along the crests and
drains along the hollows. In the diagram (fig. 1) is shown to the right
of the river a system of canals branching out and afterwards rejoining
one another so as to allow of no means for the water that passes off the
field to escape into the sea. Hence it must either evaporate or sink
into the soil. Now nearly all rivers contain some small percentage of
salt, which forms a distinct ingredient in alluvial plains. The result
of this drainless irrigation is an efflorescence of salt on the surface
of the field. The spring level rises, so that water can be reached by
digging only a few feet, and the land, soured and water-logged, relapses
into barrenness. Of this description was the irrigation of Lower Egypt
previous to 1883. To the left of the diagram is shown (by firm lines) a
system of canals laid out scientifically, and of drains (by dotted
lines) flowing between them. It is the effort of the British engineers
in Egypt to remodel the surface of the fields to this type.
Further information may be found in Sir C. C. Scott-Moncrieff,
_Irrigation in Southern Europe_ (London, 1868); Moncrieff, "Lectures
on Irrigation in Egypt," _Professional Papers of the Corps of Royal
Engineers_, vol. xix. (London, 1893); W. Willcocks, _Egyptian
Irrigation_ (2nd ed., London, 1899).
II. _Water Meadows._--Nowhere in England can it be said that irrigation
is necessary to ordinary agriculture, but it is occasionally employed in
stimulating the growth of grass and meadow herbage in what are known as
water-meadows. These are in some instances of very early origin. On the
Avon in Wiltshire and the Churn in Gloucestershire they may be traced
back to Roman times. This irrigation is not practised in the drought of
summer, but in the coldest and wettest months of the year, the water
employed being warmer than the natural moisture of the soil and proving
a valuable protection against frost.
[Illustration: FIG. 1.--Diagram showing irrigation properly combined
with drainage (_to left_), and laid out regardless of drainage required
later (_to right_).]
Before the systematic conversion of a tract into water-meadows can be
safely determined on, care must be taken to have good drainage, natural
or artificial, a sufficient supply of water, and water of good quality.
It might indeed have been thought that thorough drainage would be
unnecessary, but it must be noted that porous subsoils or efficient
drains do not act merely by carrying away stagnant water which would
otherwise cool the earth, incrust the surface, and retard plant growth.
They cause the soil to perform the office of a filter. Thus the earth
and the roots of grasses absorb the useful matters not only from the
water that passes over it, but from that which passes through it. These
fertilizing materials are found stored up in the soil ready for the use
of the roots of the plants. Stagnation of water is inimical to the
action of the roots, and does away with the advantageous processes of
flowing and percolating currents. Some of the best water-meadows in
England have but a thin soil resting on gravel and flints, this
constituting a most effectual system of natural drainage. The fall of
the water supply must suffice for a fairly rapid current, say 10 in. or
1 ft. in from 100 to 200 yds. If possible the water should be taken so
far above the meadows as to have sufficient fall without damming up the
river. If a dam be absolutely necessary, care must be taken so to build
it as to secure the fields on both sides from possible inundation; and
it should be constructed substantially, for the cost of repairing
accidents to a weak dam is very serious.
Quantity of water.
Even were the objects of irrigation always identical, the conditions
under which it is carried on are so variable as to preclude calculations
of quantity. Mere making up of necessary water in droughty seasons is
one thing, protection against frost is another, while the addition of
soil material is a third. Amongst causes of variation in the quantity of
water needed will be its quality and temperature and rate of flow, the
climate, the season, the soil, the subsoil, the artificial drainage, the
slope, the aspect and the crop. In actual practice the amount of water
varies from 300 gallons per acre in the hour to no less than 28,000
gallons. Where water is used, as in dry and hot countries, simply as
water, less is generally needed than in cold, damp and northerly
climates, where the higher temperature and the action of the water as
manure are of more consequence. But it is necessary to be thoroughly
assured of a good supply of water before laying out a water-meadow.
Except in a few places where unusual dryness of soil and climate
indicate the employment of water, even in small quantity, merely to
avoid the consequences of drought, irrigation works are not to be
commenced upon a large area, if only a part can ever be efficiently
watered. The engineer must not decide upon the plan till he has gauged
at different seasons the stream which has to supply the water, and has
ascertained the rain-collecting area available, and the rainfall of the
district, as well as the proportion of storable to percolating and
evaporating water. Reservoirs for storage, or for equalizing the flow,
are rarely resorted to in England; but they are of absolute necessity in
those countries in which it is just when there is least water that it is
most wanted. It is by no means an injudicious plan before laying out a
system of water-meadows, which is intended to be at all extensive, to
prepare a small trial plot, to aid in determining a number of questions
relating to the nature and quantity of the water, the porosity of the
soil, &c.
Quality of water.
The quality of the water employed for any of the purposes of irrigation
is of much importance. Its dissolved and its suspended matters must both
be taken into account. Clear water is usually preferable for grass land,
thick for arable land. If it is to be used for warping, or in any way
for adding to the solid material of the irrigated land, then the nature
and amount of the suspended material are necessarily of more importance
than the character of the dissolved substances, provided the latter are
not positively injurious. For use on ordinary water-meadows, however,
not only is very clear water often found to be perfectly efficient, but
water having no more than a few grains of dissolved matter per gallon
answers the purposes in view satisfactorily. Water from moors and
peat-bogs or from gravel or ferruginous sandstone is generally of small
utility so far as plant food is concerned. River water, especially that
which has received town sewage, or the drainage of highly manured land,
would naturally be considered most suitable for irrigation, but
excellent results are obtained also with waters which are uncontaminated
with manurial matters, and which contain but 8 or 10 grains per gallon
of the usual dissolved constituents of spring water. Experienced English
irrigators generally commend as suitable for water-meadows those streams
in which fish and waterweeds abound. But the particular plants present
in or near the water-supply afford further indications of quality.
Water-cress, sweet flag, flowering rush, several potamogetons, water
milfoil, water ranunculus, and the reedy sweet watergrass (_Glyceria
aquatica_) rank amongst the criteria of excellence. Less favourable
signs are furnished by such plants as _Arundo Donax_ (in Germany),
_Cicuta virosa_ and _Typha latifolia_, which are found in stagnant and
torpid waters. Water when it has been used for irrigation generally
becomes of less value for the same purpose. This occurs with clear water
as well as with turbid, and obviously arises mainly from the loss of
plant food which occurs when water filters through or trickles over poor
soil. By passing over or through rich soil the water may, however,
actually be enriched, just as clear water passed through a charcoal
filter which has been long used becomes impure. It has been contended
that irrigation water suffers no change in composition by use, since by
evaporation of a part of the pure water the dissolved matters in the
remainder would be so increased as to make up for any matters removed.
But it is forgotten that both the plant and the soil enjoy special
powers of selective absorption, which remove and fix the better
constituents of the water and leave the less valuable.
Seeds for water-meadows.
Of the few leguminous plants which are in any degree suitable for
water-meadows, _Lotus corniculatus major_, _Trifolium hybridum_, and _T.
pratense_ are those which generally flourish best; _T. repens_ is less
successful. Amongst grasses the highest place must be assigned to
ryegrass, especially to the Italian variety, commonly called _Lolium
italicum_. The mixture of seeds for sowing a water-meadow demands much
consideration, and must be modified according to local circumstances of
soil, aspect, climate and drainage. From the peculiar use which is made
of the produce of an irrigated meadow, and from the conditions to which
it is subjected, it is necessary to include in our mixture of seeds some
that produce an early crop, some that give an abundant growth, and some
that impart sweetness and good flavour, while all the kinds sown must be
capable of flourishing on irrigated soil.
The following mixtures of seeds (stated in pounds per acre) have been
recommended for sowing on water-meadows, Messrs Sutton of Reading, after
considerable experience, regarding No. I. as the more suitable:
I. II.| I. II.
|
_Lolium perenne_ 8 12 | _Festuca pratensis_ 0 2
_Lolium italicum_ 0 8 | _Festuca loliacea_ 3 2
_Poa trivialis_ 6 3 | _Anthoxanthum odoratum_ 0 1
_Glyceria fluitans_ 6 2 | _Phleum pratense_ 4 2
_Glyceria aquatica_ 4 1 | _Phalaris arundinacea_ 3 2
_Agrostis alba_ 0 1 | _Lotus corniculatus major_ 3 2
_Agrostis stolonifera_ 6 2 | _Trifolium hybridum_ 0 1
_Alopecurus pratensis_ 0 2 | _Trifolium pratense_ 0 1
_Festuca elatior_ 3 2 |
Changes in irrigated herbage.
In irrigated meadows, though in a less degree than on sewaged land, the
reduction of the amount or even the actual suppression of certain
species of plants is occasionally well marked. Sometimes this action is
exerted upon the finer grasses, but happily also upon some of the less
profitable constituents of the miscellaneous herbage. Thus _Ranunculus
bulbosus_ has been observed to become quite rare after a few years'
watering of a meadow in which it had been most abundant, _R. acris_
rather increasing by the same treatment; _Plantago media_ was
extinguished and _P. lanceolata_ reduced 70%. Amongst the grasses which
may be spared, _Aira caespitosa_, _Briza media_ and _Cynosurus
cristatus_ are generally much reduced by irrigation. Useful grasses
which are increased are _Lolium perenne_ and _Alopecurus pratensis_, and
among those of less value _Avena favescens_, _Dactylis glomerata_ and
_Poa pratensis_.
Methods.
Four ways of irrigating land with water are practised in England: (1)
bedwork irrigation, which is the most efficient although it is also the
most costly method by which currents of water can be applied to level
land; (2) catchwork irrigation, in which the same water is caught and
used repeatedly; (3) subterraneous or rather upward irrigation, in which
the water in the drains is sent upwards through the soil towards the
surface; and (4) warping, in which the water is allowed to stand over a
level field until it has deposited the mud suspended in it.
There are two things to be attended to most carefully in the
construction of a water-meadow on the first or second of these plans.
First, no portion of them whatever should be on a dead level, but every
part should belong to one or other of a series of true inclined planes.
The second point of primary importance is the size and slope of the main
conductor, which brings the water from the river to the meadow. The size
of this depends upon the quantity of water required, but whatever its
size its bottom at its origin should be as low as the bed of the river,
in order that it may carry down as much as possible of the river mud.
Its course should be as straight and as near a true inclined plane as
possible. The stuff taken out of the conductor should be employed in
making up its banks or correcting inequalities in the meadow.
Bedwork.
In bedwork irrigation, which is eminently applicable to level ground,
the ground is thrown into beds or ridges. Here the conductor should be
led along the highest end or side of the meadow in an inclined plane;
should it terminate in the meadow, its end should be made to taper
when there are no feeders, or to terminate in a feeder. The main drain
to carry off the water from the meadow should next be formed. It
should be cut in the lowest part of the ground at the lower end or
side of the meadow. Its dimensions should be capable of carrying off
the whole water used so quickly as to prevent the least stagnation,
and discharge it into the river. The next process is the forming of
the ground intended for a water-meadow into beds or ridges. That
portion of the ground which is to be watered by one conductor should
be made into beds to suit the circumstances of that conductor; that
is, instead of the beds over the meadow being all reduced to one
common level, they should be formed to suit the different swells in
the ground, and, should any of these swells be considerable, it will
be necessary to give each side of them its respective conductor. The
beds should run at or nearly at right angles to the line of the
conductor. The breadth of the beds is regulated by the nature of the
soil and the supply of water. Tenacious soils and subsoils, with a
small supply of water, require beds as narrow as 30 ft. Porous soils
and a large supply of water may have beds of 40 ft. The length of the
beds is regulated by the supply of water and the fall from the
conductor to the main drain. If the beds fall only in one direction
longitudinally, their crowns should be made in the middle; but, should
they fall laterally as well as longitudinally, as is usually the case,
then the crowns should be made towards the upper sides, more or less
according to the lateral slope of the ground. The crowns should rise 1
ft. above the adjoining furrows. The beds thus formed should slope in
an inclined plane from the conductor to the main drain, that the water
may flow equably over them.
The beds are watered by "feeders," that is, channels gradually
tapering to the lower extremities, and their crowns cut down, wherever
these are placed. The depth of the feeders depends on their width, and
the width on their length. A bed 200 yds. in length requires a feeder
of 20 in. in width at its junction with the conductor, and it should
taper gradually to the extremity, which should be 1 ft. in width. The
taper retards the motion of the water, which constantly decreases by
overflow as it proceeds, whilst it continues to fill the feeder to the
brim. The water overflowing from the feeders down the sides of the
beds is received into small drains formed in the furrows between the
beds. These small drains discharge themselves into the main drain, and
are in every respect the reverse of the feeders. The depth of the
small drain at the junction is made about as great as that of the main
drain, and it gradually lessens towards the taper to 6 in. in
tenacious and to less in porous soils. The depth of the feeders is the
same in relation to the conductor. For the more equal distribution of
the water over the surface of the beds from the conductor and feeders,
small masses, such as stones or solid portions of earth or turf
fastened with pins, are placed in them, in order to retard the
momentum which the water may have acquired. These "stops," as they are
termed, are generally placed at regular intervals, or rather they
should be left where any inequality of the current is observed. Heaps
of stones answer very well for stops in the conductor, particularly
immediately below the points of junction with the feeders. The small
or main drains require no stops. The descent of the water in the
feeders will no doubt necessarily increase in rapidity, but the
inclination of the beds and the tapering of the feeders should be so
adjusted as to counteract the increasing rapidity. The distribution of
the water over the whole meadow is regulated by the sluices, which
should be placed at the origin of every conductor. By means of these
sluices any portion of the meadow that is desired can be watered,
whilst the rest remains dry; and alternate watering must be adopted
when there is a scarcity of water. All the sluices should be
substantially built at first with stones and mortar, to prevent the
leakage of water; for, should water from a leak be permitted to find
its way into the meadow, that portion of it will stagnate and produce
coarse grasses. In a well-formed water-meadow it is as necessary to
keep it perfectly dry at one time as it is to place it under water at
another. A small sluice placed in the side of the conductor opposite
to the meadow, and at the upper end of it, will drain away the leakage
that may have escaped from the head sluice.
To obtain a complete water-meadow, the ground will often require to be
broken up and remodelled. This will no doubt be attended with cost;
but it should be considered that the first cost is the least, and
remodelling the only way of having a complete water-meadow which will
continue for years to give satisfaction. To effect a remodelling when
the ground is in stubble, let it be ploughed up, harrowed, and cleaned
as in a summer fallow, the levelling-box employed when required, the
stuff from the conductors and main drains spread abroad, and the beds
ploughed into shape--all operations that can be performed at little
expense. The meadow should be ready by August for sowing with one of
the mixtures of grass-seeds already given. But though this plan is
ultimately better, it is attended with the one great disadvantage that
the soft ground cannot be irrigated for two or three years after it is
sown with grass-seeds. This can only be avoided where the ground is
covered with old turf which will bear to be lifted. On ground in that
state a water-meadow may be most perfectly formed. Let the turf be
taken off with the spade, and laid carefully aside for relaying. Let
the stript ground then be neatly formed with the spade and barrow,
into beds varying in breadth and shape according to the nature of the
soil and the dip of the ground--the feeders from the conductor and the
small drains to the main drain being formed at the same time. Then let
the turf be laid down again and beaten firm, when the meadow will be
complete at once, and ready for irrigation. This is the most beautiful
and most expeditious method of making a complete water-meadow where
the ground is not naturally sufficiently level to begin with.
The water should be let on, and trial made of the work, whenever it is
finished, and the motion of the water regulated by the introduction of
a stop in the conductors and feeders where a change in the motion of
the current is observed, beginning at the upper end of the meadow.
Should the work be finished as directed by August, a good crop of hay
may be reaped in the succeeding summer. There are few pieces of land
where the natural descent of the ground will not admit of the water
being collected a second time, and applied to the irrigation of a
second and lower meadow. In such a case the main drain of a watered
meadow may form the conductor of the one to be watered, or a new
conductor may be formed by a prolongation of the main drain; but
either expedient is only advisable where water is scarce. Where it is
plentiful, it is better to supply the second meadow directly from the
river, or by a continuation of the first main conductor.
Catchwork.
In the ordinary catch work water-meadow, the water is used over and
over again. On the steep sides of valleys the plan is easily and
cheaply carried out, and where the whole course of the water is not
long the peculiar properties which give it value, though lessened, are
not exhausted when it reaches that part of the meadow which it
irrigates last. The design of any piece of catchwork will vary with
local conditions, but generally it may be stated that it consists in
putting each conduit save the first to the double use of a feeder or
distributor and of a drain or collector.
Upward or subterranean.
In upward or subterranean irrigation the water used rises upward
through the soil, and is that which under ordinary circumstances would
be carried off by the drains. The system has received considerable
development in Germany, where the elaborate method invented by
Petersen is recommended by many agricultural authorities. In this
system the well-fitting earthenware drain-pipes are furnished at
intervals with vertical shafts terminating at the surface of the
ground in movable caps. Beneath each cap, and near the upper end of
the shaft, are a number of vertical slits through which the drainage
water which rises passes out into the conduit or trench from which the
irrigating streams originate. In the vertical shaft there is first of
all a grating which intercepts solid matters, and then, lower down, a
central valve which can be opened and closed at pleasure from the top
of the shaft. In the ordinary English system of upward or drainage
irrigation, ditches are dug all round the field. They act the part of
conductors when the land is to be flooded, and of main drains when it
is to be laid dry. The water flows from the ditches as conductors into
built conduits formed at right angles to them in parallel lines
through the fields; it rises upwards in them as high as the surface of
the ground, and again subsides through the soil and the conduits into
the ditches as main drains, and thence it passes at a lower level
either into a stream or other suitable outfall. The ditches may be
filled in one or other of several different ways. The water may be
drainage-water from lands at a higher level; or it may be water from a
neighbouring river; or it may be drainage-water accumulated from a
farm and pumped up to the necessary level. But it may also be the
drainage-water of the field itself. In this case the mouths of the
underground main pipe-drains are stopped up, and the water in them and
the secondary drains thus caused to stand back until it has risen
sufficiently near the surface. Of course it is necessary to build the
mouths of such main drains of very solid masonry, and to construct
efficient sluices for the retention of the water in the drains.
Irrigation of the kind now under discussion may be practised wherever
a command of water can be secured, but the ground must be level. It
has been successfully employed in recently drained morasses, which are
apt to become too dry in summer. It is suitable for stiffish soils
where the subsoil is fairly open, but is less successful in sand. The
water used may be turbid or clear, and it acts, not only for
moistening the soil, but as manure. For if, as is commonly the case,
the water employed be drainage-water from cultivated lands, it is sure
to contain a considerable quantity of nitrates, which, not being
subject to retention by the soil, would otherwise escape. These coming
into contact with the roots of plants during their season of active
growth, are utilized as direct nourishment for the vegetation. It is
necessary in upward or subterranean irrigation to send the water on
and to take it off very gently, in order to avoid the displacement and
loss of the finer particles of the soil which a forcible current would
cause.
Warping.
In warping the suspended solid matters are of importance, not merely
for any value they may have as manure, but also as a material addition
to the ground to be irrigated. The warping which is practised in
England is almost exclusively confined to the overflowing of level
ground within tide mark, and is conducted mostly within the districts
commanded by estuaries or tidal rivers. The best notion of the process
of warping may be gained by sailing up the Trent from the Humber to
Gainsborough. Here the banks of the river were constructed centuries
ago to protect the land within them from the encroachments of the
tide. A great tract of country was thus laid comparatively dry. But
while the wisdom of one age thus succeeded in restricting within
bounds the tidal water of the river, it was left to the greater wisdom
of a succeeding age to improve upon this arrangement by admitting
these muddy waters to lay a fresh coat of rich silt on the exhausted
soils. The process began more than a century ago, but has become a
system in recent times. Large sluices of stone, with strong doors, to
be shut when it is wished to exclude the tide, may be seen on both
banks of the river, and from these great conduits are carried miles
inward through the flat country to the point previously prepared by
embankment over which the muddy waters are allowed to spread. These
main conduits, being very costly, are constructed for the warping of
large adjoining districts, and openings are made at such points as are
then undergoing the operation. The mud is deposited and the waters
return with the falling tide to the bed of the river. Spring-tides are
preferred, and so great is the quantity of mud in these rivers that
from 10 to 15 acres have been known to be covered with silt from 1 to
3 ft. in thickness during one spring of ten or twelve tides. Peat-moss
of the most sterile character has been by this process covered with
soil of the greatest fertility, and swamps which used to be resorted
to for leeches are now, by the effects of warping, converted into firm
and fertile fields. The art is now so well understood that, by careful
attention to the currents, the expert warp farmer can temper his soil
as he pleases. When the tide is first admitted the heavier particles,
which are pure sand, are first deposited; the second deposit is a
mixture of sand and fine mud, which, from its friable texture, forms
the most valuable soil; while lastly the pure mud subsides, containing
the finest particles of all, and forms a rich but very tenacious soil.
The great effort, therefore, of the warp farmer is to get the second
or mixed deposit as equally over the whole surface as he can and to
prevent the deposit of the last. This he does by keeping the water in
constant motion, as the last deposit can only take place when the
water is suffered to be still. Three years may be said to be spent in
the process, one year warping, one year drying and consolidating, and
one year growing the first crop, which is generally seed-hoed in by
hand, as the mud at this time is too soft to admit of horse labour.
The immediate effect, which is highly beneficial, is the deposition of
silt from the tide. To ensure this deposition, it is necessary to
surround the field to be warped with a strong embankment, in order to
retain the water as the tide recedes. The water is admitted by valved
sluices, which open as the tide flows into the field and shut by the
pressure of the confined water when the tide recedes. These sluices
are placed on as low a level as possible to permit the most turbid
water at the bottom of the tide to pass through a channel in the base
of the embankment. The silt deposited after warping is exceedingly
rich and capable of carrying any species of crop. It may be admitted
in so small a quantity as only to act as a manure to arable soil, or
in such a large quantity as to form a new soil. This latter
acquisition is the principal object of warping, and it excites
astonishment to witness how soon a new soil may be formed. From June
to September a soil of 3 ft. in depth may be formed under the
favourable circumstances of a very dry season and long drought. In
winter and in floods warping ceases to be beneficial. In ordinary
circumstances on the Trent and Humber a soil from 6 to 16 in. in depth
may be obtained and inequalities of 3 ft. filled up. But every tide
generally leaves only 1/8 in. of silt, and the field which has only
one sluice can only be warped every other tide. The silt, as deposited
in each tide, does not mix into a uniform mass, but remains in
distinct layers. The water should be made to run completely off and
the ditches should become dry before the influx of the next tide,
otherwise the silt will not incrust and the tide not have the same
effect. Warp soil is of surpassing fertility. The expense of forming
canals, embankments and sluices for warping land is from L10 to L20 an
acre. A sluice of 6 ft. in height and 8 ft. wide will warp from 60 to
80 acres, according to the distance of the field from the river. The
embankments may be from 3 to 7 ft. in height, as the field may stand
in regard to the level of the highest tides. After the new land has
been left for a year or two in seeds and clover, it produces great
crops of wheat and potatoes.
Warping is practised only in Lincolnshire and Yorkshire, on the
estuary of the Humber, and in the neighbourhood of the rivers which
flow into it--the Trent, the Ouse and the Don. The silt and mud
brought down by these rivers is rich in clay and organic matter, and
sometimes when dry contains as much as 1% of nitrogen.
Management and advantages.
Constant care is required if a water-meadow is to yield quite
satisfactory results. The earliness of the feed, its quantity and its
quality will all depend in very great measure upon the proper management
of the irrigation. The points which require constant attention are--the
perfect freedom of all carriers, feeders and drains from every kind of
obstruction, however minute; the state and amount of water in the river
or stream, whether it be sufficient to irrigate the whole area properly
or only a part of it; the length of time the water should be allowed to
remain on the meadow at different periods of the season; the regulation
of the depth of the water, its quantity and its rate of flow, in
accordance with the temperature and the condition of the herbage; the
proper times for the commencing and ending of pasturing and of shutting
up for hay; the mechanical condition of the surface of the ground; the
cutting out of any very large and coarse plants, as docks; and the
improvement of the physical and chemical conditions of the soil by
additions to it of sand, silt, loam, chalk, &c.
Whatever may be the command of water, it is unwise to attempt to
irrigate too large a surface at once. Even with a river supply fairly
constant in level and always abundant, no attempt should be made to
force on a larger volume of water than the feeders can properly
distribute and the drains adequately remove, or one part of the meadow
will be deluged and another stinted. When this inequality of irrigation
once occurs, it is likely to increase from the consequent derangement of
the feeders and drains. And one result on the herbage will be an
irregularity of composition and growth, seriously detrimental to its
food-value. The adjustment of the water by means of the sluices is a
delicate operation when there is little water and also when there is
much; in the latter case the fine earth may be washed away from some
parts of the meadow; in the former case, by attempting too much with a
limited water current, one may permit the languid streams to deposit
their valuable suspended matters instead of carrying them forward to
enrich the soil. The water is not to be allowed to remain too long on
the ground at a time. The soil must get dry at stated intervals in order
that the atmospheric air may come in contact with it and penetrate it.
In this way as the water sinks down through the porous subsoil or into
the subterranean drains oxygen enters and supplies an element which is
needed, not only for the oxidation of organic matters in the earth, but
also for the direct and indirect nutrition of the roots. Without this
occasional drying of the soil the finer grasses and the leguminous
plants will infallibly be lost; while a scum of confervae and other
algae will collect upon the surface and choke the higher forms of
vegetation. The water should be run off thoroughly, for a little
stagnant water lying in places upon the surface does much injury. The
practice of irrigating differs in different places with differences in
the quality of the water, the soil, the drainage, &c. As a general rule,
when the irrigating season begins in November the water may flow for a
fortnight continuously, but subsequent waterings, especially after
December, should be shortened gradually in duration till the first week
in April, when irrigation should cease. It is necessary to be very
careful in irrigating during frosty weather. For, though grass will grow
even under ice, yet if ice be formed under and around the roots of the
grasses the plants may be thrown out by the expansion of the water at
the moment of its conversion into ice. The water should be let off on
the morning of a dry day, and thus the land will be dry enough at night
not to suffer from the frost; or the water may be taken off in the
morning and let on again at night. In spring the newly grown and tender
grass will be easily destroyed by frost if it be not protected by water,
or if the ground be not made thoroughly dry.
Theory.
Although in many cases it is easy to explain the reasons why water
artificially applied to land brings crops or increases their yield, the
theory of our ordinary water-meadow irrigation is rather obscure. For we
are not dealing in these grass lands with a semi-aquatic plant like
rice, nor are we supplying any lack of water in the soil, nor are we
restoring the moisture which the earth cannot retain under a burning
sun. We irrigate chiefly in the colder and wetter half of the year, and
we "saturate" with water the soil in which are growing such plants as
are perfectly content with earth not containing more than one-fifth of
its weight of moisture. We must look in fact to a number of small
advantages and not to any one striking beneficial process in explaining
the aggregate utility of water-meadow irrigation. We attribute the
usefulness of water-meadow irrigation, then, to the following causes:
(1) the temperature of the water being rarely less than 10 deg. Fahr.
above freezing, the severity of frosts in winter is thus obviated, and
the growth, especially of the roots of grasses, is encouraged; (2)
nourishment or plant food is actually brought on to the soil, by which
it is absorbed and retained, both for the immediate and for the future
use of the vegetation, which also itself obtains some nutrient material
directly; (3) solution and redistribution of the plant food already
present in the soil occur mainly through the solvent action of the
carbonic acid gas present in a dissolved state in the irrigation-water;
(4) oxidation of any excess of organic matter in the soil, with
consequent production of useful carbonic acid and nitrogen compounds,
takes place through the dissolved oxygen in the water sent on and
through the soil where the drainage is good; and (5) improvement of the
grasses, and especially of the miscellaneous herbage, of the meadow is
promoted through the encouragement of some at least of the better
species and the extinction or reduction of mosses and of the
innutritious weeds.
To the united agency of the above-named causes may safely be attributed
the benefits arising from the special form of water-irrigation which is
practised in England. Should it be thought that the traces of the more
valuable sorts of plant food (such as compounds of nitrogen, phosphates,
and potash salts) existing in ordinary brook or river water can never
bring an appreciable amount of manurial matter to the soil, or exert an
appreciable effect upon the vegetation, yet the quantity of water used
during the season must be taken into account. If but 3000 gallons hourly
trickle over and through an acre, and if we assume each gallon to
contain no more than one-tenth of a grain of plant food of the three
sorts just named taken together, still the total, during a season
including ninety days of actual irrigation, will not be less than 9 lb.
per acre. It appears, however, that a very large share of the benefits
of water-irrigation is attributable to the mere contact of abundance of
moving water, of an even temperature, with the roots of the grass. The
growth is less checked by early frosts; and whatever advantages to the
vegetation may accrue by occasional excessive warmth in the atmosphere
in the early months of the year are experienced more by the irrigated
than by the ordinary meadow grasses by reason of the abundant
development of roots which the water has encouraged.
III. _Italian Irrigation._--The most highly developed irrigation in the
world is probably that practised in the plains of Piedmont and Lombardy,
where every variety of condition is to be found. The engineering works
are of a very high class, and from long generations of experience the
farmer knows how best to use his water. The principal river of northern
Italy is the Po, which rises to the west of Piedmont and is fed not from
glaciers like the Swiss torrents, but by rain and snow, so that the
water has a somewhat higher temperature, a point to which much
importance is attached for the valuable meadow irrigation known as
_marcite_. This is only practised in winter when there is abundance of
water available, and it much resembles the water-meadow irrigation of
England. The great Cavour canal is drawn from the left bank of the Po a
few miles below Turin, and it is carried right across the drainage of
the country. Its full discharge is 3800 cub. ft. per second, but it is
only from October to May, when the water is least required, that it
carries anything like this amount. For the summer irrigation Italy
depends on the glaciers of the Alps; and the great torrents of the Dora
Baltea and Sesia can be counted on for a volume exceeding 6000 cub. ft.
per second. Lombardy is quite as well off as Piedmont for the means of
irrigation and, as already said, its canals have the advantage that
being drawn from the lakes Maggiore and Como they exercise a moderating
influence on the Ticino and Adda rivers, which is much wanted in the
Dora Baltea. The Naviglio Grande of Lombardy is a very fine work drawn
from the left bank of the Ticino and useful for navigation as well as
irrigation. It discharges between 3000 and 4000 cub. ft. per second, and
probably nowhere is irrigation carried on with less expense. Another
canal, the Villoresi, drawn from the same bank of the Ticino farther
upstream, is capable of carrying 6700 cub. ft. per second. Like the
Cavour canal, the Villoresi is taken across the drainage of the country,
entailing a number of very bold and costly works.
Interesting as these Italian works are, the administration and
distribution of the water is hardly less so. The system is due to the
ability of the great Count Cavour; what he originated in Piedmont has
been also carried out in Lombardy. The Piedmontese company takes over
from the government the control of all the irrigation within a triangle
between the left bank of the Po and the right bank of the Sesia. It
purchases from government about 1250 cub. ft. per second, and has also
obtained the control of all private canals. Altogether it distributes
about 2275 cub. ft. of water and irrigates about 141,000 acres, on which
rice is the most important crop. The association has 14,000 members and
controls nearly 10,000 m. of distributary channels. In each parish is a
council composed of all landowners who irrigate. Each council sends two
deputies to what may be called a water parliament. This assembly elects
three small committees, and with them rests the whole management of the
irrigation. An appeal may be made to the civil courts from the decision
of these committees, but so popular are they that such appeals are never
made. The irrigated area is divided into districts, in each of which is
an overseer and a staff of watchmen to see to the opening and shutting
of the _modules_ (see HYDRAULICS, SS 54 to 56) which deliver the water
into the minor channels. In the November of each year it is decided how
much water is to be given to each parish in the year following, and this
depends largely on the number of acres of each crop proposed to be
watered. In Lombardy the irrigation is conducted on similar principles.
Throughout, the Italian farmer sets a very high example in the loyal way
he submits to regulations which there must be sometimes a strong
temptation to break. A sluice surreptitiously opened during a dark night
and allowed to run for six hours may quite possibly double the value of
his crop, but apparently the law is not often broken.
Characteristics of the Nile Valley and flood.
IV. _Egypt._--The very life of Egypt depends on its irrigation, and,
ancient as this irrigation is, it was never practised on a really
scientific system till after the British occupation. As every one knows,
the valley of the Nile outside of the tropics is practically devoid of
rainfall. Yet it was the produce of this valley that formed the chief
granary of the Roman Empire. Probably nowhere in the world is there so
large a population per square mile depending solely on the produce of
the soil. Probably nowhere is there an agricultural population so
prosperous, and so free from the risks attending seasons of drought or
of flood. This wealth and prosperity are due to two very remarkable
properties of the Nile. First, the regimen of the river is nearly
constant. The season of its rise and its fall, and the height attained
by its waters during the highest flood and at lowest Nile vary to a
comparatively small extent. Year after year the Nile rises at the same
period, it attains its maximum in September and begins to diminish first
rapidly till about the end of December, and then more slowly and more
steadily until the following June. A late rise is not more than about
three weeks behind an early rise. From the lowest to the highest gauge
of water-surface the rise is on an average 25.5 ft. at the First
Cataract. The highest flood is 3.5 ft. above this average, and this
means peril, if not disaster, in Lower Egypt. The lowest flood on record
has risen only to 5.5 ft. below the average, or to 20 ft. above the mean
water-surface of low Nile. Such a feeble Nile flood has occurred only
four times in modern history: in 1877, when it caused widespread famine
and death throughout Upper Egypt, 947,000 acres remained barren, and the
land revenue lost L1,112,000; in 1899 and again in 1902 and 1907, when
by the thorough remodelling of the whole system of canals since 1883 all
famine and disaster were avoided and the loss of revenue was
comparatively slight. In 1907, for instance, when the flood was nearly
as low as in 1877, the area left unwatered was little more than 10% of
the area affected in 1877.
This regularity of flow is the first exceptional excellence of the river
Nile. The second is hardly less valuable, and consists in the remarkable
richness of the alluvium brought down the river year after year during
the flood. The object of the engineer is so to utilize this flood-water
that as little as possible of the alluvium may escape into the sea, and
as much as possible may be deposited on the fields. It is the possession
of these two properties that imparts to the Nile a value quite unique
among rivers, and gives to the farmers of the Nile Valley advantages
over those of any rain-watered land in the world.
Irrigation during high Nile.
Until the 19th century irrigation in Egypt on a large scale was
practised merely during the Nile flood. Along each edge of the river and
following its course has been erected an earthen embankment high enough
not to be topped by the highest floods. In Upper Egypt, the valley of
which rarely exceeds 6 m. in width, a series of cross embankments have
been constructed, abutting at the inner ends on those along the Nile,
and at the outer ends on the ascending sides of the valley. The whole
country has thus been divided into a series of oblongs, surrounded by
embankments on three sides and by the desert slopes on the fourth. These
oblong areas vary from 60,000 to 1500 or 2000 acres in extent.
Throughout all Egypt the Nile is deltaic in character; that is, the
slope of the country in the valley is away from the river and not
towards it. It is easy, then, when the Nile is low, to cut short, deep
canals in the river banks, which fill as the flood rises, and carry the
precious mud-charged water into these great flats. There the water
remains for a month or more, some 3 ft. deep, depositing its mud, and
thence at the end of the flood the almost clear water may either be run
off directly into the receding river, or cuts may be made in the cross
embankments, and it may be allowed to flow from one flat to another and
ultimately into the river. In November the waters have passed off; and
whenever a man can walk over the mud with a pair of bullocks, it is
roughly turned over with a wooden plough, or merely the branch of a
tree, and the wheat or barley crop is immediately sown. So soaked is the
soil after the flood, that the grain germinates, sprouts, and ripens in
April, without a shower of rain or any other watering.
In Lower Egypt this system was somewhat modified, but it was the same in
principle. No other was known in the Nile Valley until the country fell,
early in the 19th century, under the vigorous rule of Mehemet Ali Pasha.
He soon recognized that with such a climate and soil, with a teeming
population, and with the markets of Europe so near they might produce in
Egypt something more profitable than wheat and maize. Cotton and
sugar-cane would fetch far higher prices, but they could only be grown
while the Nile was low, and they required water at all seasons.
Irrigation during low Nile.
The Nile Barrage.
It has already been said that the rise of the Nile is about 25(1/2) ft.,
so that a canal constructed to draw water out of the river while at its
lowest must be 25(1/2) ft. deeper than if it is intended to draw off
only during the highest floods. Mehemet Ali began by deepening the
canals of Lower Egypt by this amount, a gigantic and futile task; for as
they had been laid out on no scientific principles, the deep channels
became filled with mud during the first flood, and all the excavation
had to be done over again, year after year. With a serf population even
this was not impossible; but as the beds of the canals were graded to no
even slope, it did not follow that if water entered the head it would
flow evenly on. As the river daily fell, of course the water in the
canals fell too, and since they were never dug deep enough to draw water
from the very bottom of the river, they occasionally ran dry altogether
in the month of June, when the river was at its lowest, and when, being
the month of greatest heat, water was more than ever necessary for the
cotton crop. Thus large tracts which had been sown, irrigated, weeded
and nurtured for perhaps three months perished in the fourth, while all
the time the precious Nile water was flowing useless to the sea. The
obvious remedy was to throw a weir across each branch of the river to
control the water and force it into canals taken from above it. The task
of constructing this great work was committed to Mougel Bey, a French
engineer of ability, who designed and constructed the great barrage
across the two branches of the Nile at the apex of the delta, about 12
m. north of Cairo (fig. 2). It was built to consist of two bridges--one
over the eastern or Damietta branch of the river having 71 arches, the
other, over the Rosetta branch, having 61 arches, each arch being of 5
metres or 16.4 ft. span. The building was all of stone, the floors of
the arches were inverts. The height of pier from edge of flooring to
spring of arch was 28.7 ft., the spring of the arch being about the
surface-level of maximum flood. The arches were designed to be fitted
with self-acting drop gates; but they were not a success, and were only
put into place on the Rosetta branch. The gates were intended to hold up
the water 4.5 metres, or 14.76 ft., and to divert it into three main
canals--the Behera on the west, the Menufia in the centre and the
Tewfikia on the east. The river was thus to be emptied, and to flow
through a whole network of canals, watering all Lower Egypt. Each
barrage was provided with locks to pass Nile boats 160 by 28 ft. in
area.