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· BRITISH ENCYCLOPEDIA.
IRRIGATION. TRRIGATION is the art of conducting formed by the hollows among hills. In 1 water at pleasure over levels or inclined every part of Asia, but especially in the planes, in such manner that the whole may Mysore country, formerly under the domireceive the benefit of partial immersion; nion of the late Sultan Tippoo, the retenwhereby the surface may be duly supplied tiou of water, for the purposes of irrigation, with moisture, and the vegetable produc. is a matter of such importance as to be entions intended to be encouraged, should be tirely under the auspices and controul of enabled to put forth abundantly, and to the government. Tippoo caused banks, or, yield a good crop. Irrigation is with us ra- as they are called in India, bunds, to be ther a povel practice, but was well under- made between the bases of hills, so as stood by the ancients, and has been in use to intercept the copious streams, which, among the Chinese up to the earliest dates during the rainy seasons, flow from the of their records. In Hindostan, the whole hilly country. An example worthy of imi. of the rubbee, or small-grain crop, is artifi. tation! Thus immense bodies of water cially watered; the grain being deposited might be collected in many parts of the in October, while the ground remains United Kingdoms, whence mills apd va. moist, after the heavy rains which had fallen rious machinery might be worked, without for months previously to the operations causing any waste of valuable land; the of tillage ; so that the seed speedily germi- soil, in situations appropriate to such purdates. But the perfect drought attendant poses, being for the most part poor, and un. on the five successive months, would infalli. fit for tillage. bly destroy the promising verdure, were it The Milanese territory exhibits the greatnot that the peasants divide their lands into est expanse of irrigation known in Europe. small squares, about four or five feet each In that country are to be seen noble canals way, between each pair of which a small running in every direction, totally exempt. channel, made by banking the soil, pro tem. ed from local préjudice, private pique, or pore, in a very simple inanner, conducts the self-interest. All are under the authority little stream supplied from numerous wells and protection of government, which leis made expressly for the occasion. When out the water to the various occupiers of the ear, or blossom, bas shot forth, water- meadows, at a fixed rate, according to the ing is discontinued. The Chinese proceed quantity supplied. Sometimes these canals on the grand scale; they not only divide are farmed out, by putting up the several their fields by numerous channels, but even sluices to auction; in other instances the warp whole tracts of low land; whereby canals go with the lands. . they insure immense returns. The Afri Whatever may be the manner in which cans, in some parts, follow the Hindostanee their water is dispersed, its due preserva. plan; but raise their water chiefly from the tion is an object of general solicitude, on acrivers, or obtain their supplies of that inva- count of the benefits which individuals derive luable element from natural reservoirs, from its use; while the goverpinent, both from
that motive, and the support of the revenue borders, perhaps, are verdant, but whose produced by farming of the canals, do not more retired parts would be doubled or treallow the smallest despoliation to pass un. bled in value, by the influence of that elepunished. We are assured, by the best au- ment, which is allowed to pass by unheedthorities, that the whole of the pasture ed, to be lost in some marsh, or eventually lands in the Milanese exhibit uncommon in the ocean! It is true, that, in some parts, fertility; and that the canals are so very exirrigation is not understood; and, that it is tensive, and the branches from them so nu- not always practicable to obtain proper merous, that few need complain of a want assistance; whence many, who would wil. of water for irrigation. These works are lingly water their meadows, are prevented known to be of no anodern date; some have from taking advantage of streams capable existed for centuries, chiefly appertaining of effecting the intention. For the benefit to monasteries ; their waters being let out of such persons, in particular, as well as of by measure to fertilize their adjacent lands. our readers in general, we shall endeavour The great canal, known by the designation to simplify, even this simple process, in such of Vecchiabbia, was in a flourishing state a manner as may prove perfectly intelligi. early in the eleventh century, beyond which ble; and, by shewing with what ease irriwe do not know what might have been its gation may be carried on, induce a portion age. In 1920, the great canal of Adda, of our landholders to attempt, even withwhich waters the plains of Lodi, was finish out professional aid, or the tuition of expeed; in 1305, the canal of Treveglio, which rienced persons, that retention and gradual communicated with four others of very an- distribution of waters whose sources are cient workmanship, was completed; and in sufficiently elevated, which may favour 1460, the canal of Martesano, extending such a slight and temporary inundation, as thirty-two English miles : in this aqueduct, may give vigour and freshness both to the besides the main branch, of thirty-five feet soil and to its produce. in width, there were made nineteen scari- We shall divide this subject into two discatori, or lesser canals, which served, when tinct heads, viz. simple, and compound irthe waters rose very high, to draw off the rigation; observing that the former may be surplus, so as to prevent injury to the main practical in various modes separately, as line, and to prevent inundation along its will be slewn, and that they may be blendcourse: when the latter returned to a ed so as to come under the second term. more tranquil state, the scaricatori, which We shall also, by way of preparation, give were not so deep as the main line, served the reader an insight into some modes of to supply it with what remained of their cutting off, or of supplying water, from contents.
sources of different heights, and under difIt is worthy our notice, that although the ferent circumstances : by this means, with Italian aqueducts have, to our certain know. a moderate portion of judgment, the novice ledge, been duly supported for upwards of in this art may speedily acquire sufficient of eight centuries, by a race of people far be. the principles to answer his own purposes, neath ns in the more noble sciences, in at least, if not to form a correct opinion of wealth, in population, and in many other most of the cases which may come under circumstances in which we pride ourselves; his observation. yet that Britain cannot boast of one aque- The greatest difficulty we generally ex. duct, made exclusively with the important perience, is from the water lieing below view to improve her agriculture; though it the level of the lands over which it is to be would be as easy to shew a thousand situa. conducted. In many instances, the springs tions where such canals would double the whence streams are fed, lie very deep; value of the lands adjoining, as it would be and, thongh copious, for want of a sufficient to prove that such value would be doubled. inclination of their beds, move very slowly.
It is, indeed, only in a few counties, that In other parts, jealousy of improvement, irrigation is carried on to any extent; personal enmity, the owner being a minor, though we may in various places see partial or insane, and the property in the hands of adoptions of this most beneficial practice: trustees, or the estate being in Chancery, yet we daily observe situations naturally of mortgaged, &c. perhaps debars the possibi. fering this advantage, without the smallest at- lity of taking advantage of some peculiarly tempt being made to retain streams which, favourable fall, from which the water might from elevated situations, glide with some · be conducted with perfect facility and efvelocity through deep vallies, whose very fect, over inclined planes, which, by their sterility, seem to reproach the owner with source; but the first level will still receive neglect !
the greatest portion of the benefit. Where In treating this subject, we must suppose rivers are very muddy, and of any magntthe speculator to be a free agent, not tude, it is common to allow their flowing, shackled by such an unhappy neighbour to the depth of many feet, over low lands; hood; and content ourselves with caution. So that, when kept stationary for a few ing him not to injure the property of others, hours, the fecula and sediment may be de. such as mills, bleaching grounds below the posited; as is often the case, to the depth lands, &c. &c., by drawing off that water of many inches during a single tide; and on which their very existence depends : a 'give a new stratum of the finest soil. See want of attention to this particular, has ru- WARPING. ined many a deserving and enterprising in These points must be well understood, dividual, and converted a blessing into a se- because they form a very prominent fearious mischief!
ture in the practice of irrigation, and will Where the stream is rapid, the bed has be found highly worthy the notice of all usually a very marked declivity, such as ad. who lay their lands down with that intenmits of throwing the water over the lands, tion. But we must observe that many and of withdrawing them when they have soils laying contiguous to streams, and well flowed, in every part, to a sufficient height. situated for irrigation are naturally so rich, The first step towards this, is to hold it up as not to depend on any deposit from the by means of a dam or weir, laid across the waters for their annual produce : such restream, (if its breadth admit, and that it be quire but moderate watering, and in some not navigable), so that, in the first place, instances, more to be sheltered during the the level may be raised as circumstances winter by complete inundation, than by may admit. In this, it will be necessary refreshing flows. Where such prevail, the to guard against injury to the property of water ought to be admitted only when other persons, above the dam; for the rais- clear, and then from the very surface; in ing a head of water, by means of a dam, contra-distinction to poor, or dry soils, might subject lands, which before were per- which want heart as well as moisture. The fectly dry, to be inundated; and, even fact is, that by means of an artificial supply though such should actually prove benefi- of water, the grass will shoot out far more cial thereto, the owners might recover in a early, which is an object of the utmost imcoart of law, under various pleas of da- portance to most farmers and graziers; and mage.
the crop will be much leavier than on The water should, if practicable, be rais- lands not so watered. But the bay from ed to one foot, at least, above the level of watered meadows is frequently coarse, and the highest land to be irrigated; because not much relished by the more delicate that depth may be then kept as a surplus, classes of cattle. However, store cattle, in case of long-continued drought; being which indeed scarcely ever refuse whatlet in upon the first drain, by a very small ever is offered, will consume it with avidity. penstuck, made only to the depth of the Another objection to hay from watered first level. The water, when abundant, meadows, is that being sometimes gritty may flow both into the upper level, and in consequence of the sediment deposited over the weir, so as to make a fall. When by muddy water, it is in a measure inthe water is not wanted over the land, the jurious to the teeth of those animals, by penstock may be shut up altogether. It is which it is eaten. But the great importto be remarked, that authors of eminence ance of an early bite, for at least a month, in this branch differ in opinion, though some in general, before other pastures are suffisuppose water to be more richly impreg. ciently forward to receive cattle, is of itself nated with vegetable sustenance, in pro- such a consideration as out veighis every portion as it is taken nearer to the spring; objection, and causes watered meadows to provided the water be clear. The lands yield double the rent given before they over which it is made to flow, will be bene. were subjected to irrigation. In many fited in exact proportion as they may be places the grass of watered meadows from near to the first level, which will always re- the tifteenth of March to the fifteenth of May, ceive the most obvious benefit. In foul lets from twenty to twenty-five shillings per streams, the result is usually found to be in acre. The crop is usually two tons, in all an inverse ratio ; the water being ricber, in seasons; in dry ones it is not subject to proportion as it is more remote from its the ordinary risk of being burnt up; and, not only proves highly serviceable to the branch-drains ont to the right and left, in farmer himself, but to his neighbours; who such manner as may canse the water to thius obtain a supply of hay, when their own branch out into the whole expanse of its lemeadows have failed..
vel. The turf cut from the surface of each When land has been long watered, drain, ought to be placed, face downwards, its qualities are meliorated considerably; between it and the land it is to overflow; but this is not the work of a day; and when being made firm and level, by beating with the adjoining lands abound with coarse the flat of a spade. As the penstocks are herbage, with water grasses especially, the situated just below the lines of the branchcrops will too frequently suffer by such drains above described, they keep up the vicinity. It will, at first view, appear water, so as to fill, and to cause their over. strange, but it is nevertheless true, that flowing into the next inferior talus or slope, swampy lands become firmer when regular- as shewn in fig. 1 and 2, where A is the ly watered. In their natural state the main drain, taken from the water-head or water oozes upwards, and loosens the soil ; river, B; the drain C, C, C, C, shews the but after the proper levels are found, and secondary drain, which, being on a declithe catch drains are laid, so as to draw off vity, wonld carry off all the water, were it the surplus water, the moisture is drawn not kept up at the places where the catch. downwards, and the finer parts get into the drains, or branches D, D, D, D, proceed lainsterstices, so as to compact the whole, terally from it, by the sluices E, E, E, E. and give a firm footing, where before even By this means, any particular level, either a sheep would have been bogged. We 1, 2, 3, 4, may be irrigated at pleasure, withmost, however, state, that thongh some ont wetting the others; the water being watered meadows will bear cattle, it is by kept on by the sluice above, and carried no means adviseable to let any thing heavier away by the sluice appertaining to each lethan a sheep feed upon them: the latter vel respectively. Or, if other meads at do little injury to the ridges, and by their some distance are to be watered, the se. close bite, as well as by their excellent condary channel, having all its slnices open, manure, cause the grass to tiller forth, so will convey it to them without interruption, as to form a close mat upon the soil. when all its sluices are opened. Whereas when large animals are allowed It is evident, that in this manner the to tramp on the ridges, the borders of the whole of the water is carried down to the lowdrains are in general injured; and when est level : hence, it becomes a matter of no ever, as will happen, the prints of their feet small importance to ascertain, that the are left, the soil will become qnaggy, and whole shall either be absorbed or be carried retain little pools which infallibly sour the off; so as not to injure the last level, which grass, and negative the intention of water. might otherwise be snbjected to very coning. Hence clay soils are extremely diffi- siderable injury, were the inundation to be cult to improve by this operation; nor can too long supported. The judicious comsuch be reclaimed but by a very expensive puter will be cautions not to allow so much course of draining, manuring, and breaking to remain, as may rot his grass in lieu of into a crumbly state: certainly clay soils causing it to vegetate vigoroasly. This, in may be formed into ridges, and grass may some situations, presents a very serious diffi. be made to grow upon them; but they will culty; for if the water is debarred free acnot produce sweet herbage; their surfaces cess to the lowest levels, they will be less will crack, their crops will be precarious, fruitful than the others, which, exclusive of and their seasons for feeding most depend the great fecundity derived from first reentirely on the dryness of the weather. ceiving the fuid, receive absolutely a larger Hence we may, in general terms, consider portion of moisture. The greatest care is clay soils to be unfit for irrigation; the ex. therefore requisite, to insure that the tail, or pence being great, and the money being spent-water, shall be carried off. Where more likely to yield a greater profit by other the declivity is considerable, and that the means; while their crops and pasturage stream, or any other water course, offers are, in various points, of an inferior value. itself to receive such tail-water, at a due le
But to proceed: the secondary drain, vel bencath, there is no difficulty; but which supplies the whole of a field through wiere the stream takes another course, and which it passes, shonld be interrupted at the descent is trilling, some artificial means every fall of four inches at farthest, by must be resorted to. Perhaps no more small sluices, or penstocks, and have small simple or efficacious plan can be hit upon, than that of forming a fish-pond, of a suita- pitch. These ridges should be from four ble extent and depth, to receive the tail. to six feet measurement for each face; the water; whereby the apprehended damage drain being abont a foot broad, and four may be avoided, and a useful store be cre- inches deep; thus the whole breadth of a ated.
pitch, declining each way equally, might We shall shew what we may term a truly occnpy a base of about ten feet at the ntingenious device, whereby water may be most. The declivities ought not to exceed laid upon lands that are above the level of an inch to the foot; in loose soils, not more the stream: it consists merely of an air-ves- than bialf an inch ; else the finer parts will sel, A, fig. 3, into which the water descends be washed away, and the drains, formed by forcibly from the stream, B, and by com. the junctions of the ridges, will be filled up, pressing the air in the upper part, c, is it. whereby the water will be detained, and self forced to ascend through the conduct. prevented from passing into the next level. ing pipe, D, with such force as to rise to a Fig. 5, shews the profile, or section of a level, E, far above that at which it formerly range of ridges on the same level, and stood. This is the principle of the com- tig. 6, displays an inclined plane, whereon mon fire-engine, which, we are all sensible, ridges are formed in regular succession, the can, when exerted, throw water to a great catch-drains being a little higher than the height. By such means, the tail-water may brancla-drains of the next lower level, also be forced up to such a level as may so that the latter may be filled from the canse it to return into the stream.
former : the water thus gradually descendWhere the stream runs through the landsing, until the whole is gradually absorbed that are watered, and that its declivity is by the successive ridges; or the surplus is moderate, it will sometimes be found diffi- carried off by a large catch-erain, made to cult to restore the tail-water to its level. direct it into some other succession of To effect this with as little expence as pos- ridges, as seen in the ground-plan, fig. 7. sible, wooden pipes should be laid from the The reader will perceive, that the levels lowest level of the land along the bank of may lay in any direction, according to the the stream, but carried horizontally on a cast of the land; and, that where water bank, to such extent as may suffice to con- can be had at a due height, all the land bevey the tail-water to the surface of the sur. low it may be watered. It matters not if a face. This, however, is not applicable to deep valley lay between two declivities, to all situations ; for where the stream is very be watered by the same spring. A pipe, slow, its declivity would be very trifling. of suitable diameter, being made to descend Where that happens, the air vessel will be one face, and to rise up the otiver, will coufound a good plan, provided the height to vey the stream with facility to any part ; so which the water is to be returned, be not as to re-assume the level on the opposite considerable. In many situations, a water- side. For further insight into that circumwheel might auswer well; observing, that stance, see FLUIDS, HYDRAULICS, and Hy. in deep, slow waters, that are broad, and DROSTATICS. under the speculator's owo manageinent, it It often happens, that smal rivers have will be best to throw a weir across, and a very winding course among little lills, then to let the whole body of the stream banks, rocky masses, &c., and that they rush through a narrow slip, so as to turn a suddenly lose many feet of their altitude, wheel placed immediately in the line of the owing to a fall, or steep declivity, while water's run. By this device, the current the lower parts of the stream, being more may be made to pass that particular spot expanded, and the water being kept up by with sufficient velocity to turn a wheel; another impediment, perhaps a few bwiwhereby water might either be raised out dred yards lower, offer a seemingly invinci. of the river, to supply a main drain, or the ble impediment to the conducting it over tail-water might be restored to the stream: the finely-formed planes, which present in either case, one or more pumps would themselves on either bank. Here the diffibe necessary. (See fig. 4.)
culty is far less than at first sight is supThe second mode of laying water over posed; since, by making an outlet from tie the land, is by means of ridges, whose cen- superior level of the stream, through the tres are occupied by small horizontal drains, bank which separates it from the planes to out of which the water, furnisbed by the be watered, an abundant and certain supmain drain, is allowed to flow to the depth ply may be obtained. Thus in fig. 8, the of about an inch down each side of the upper level, A, and fall, B, are shewn, and