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THE

BRITISH ENCYCLOPEDIA.

IRRIGATION.

[RRIGATION is the art of conducting water at pleasure over levels or inclined planes, in such manner that the whole may receive the benefit of partial immersion; whereby the surface may be duly supplied with moisture, and the vegetable productions intended to be encouraged, should be enabled to put forth abundantly, and to yield a good crop. Irrigation is with us rather a novel practice, but was well understood by the ancients, and has been in use among the Chinese up to the earliest dates of their records. In Hindostan, the whole of the rubbee, or small-grain crop, is artificially watered; the grain being deposited in October, while the ground remains moist, after the heavy rains which had fallen for months previously to the operations of tillage; so that the seed speedily germinates. But the perfect drought attendant on the five successive months, would infallibly destroy the promising verdure, were it not that the peasants divide their lands into small squares, about four or five feet each way, between each pair of which a small channel, made by banking the soil, pro tempore, in a very simple manner, conducts the little stream supplied from numerous wells made expressly for the occasion. When the ear, or blossom, has shot forth, watering is discontinued. The Chinese proceed on the grand scale; they not only divide their fields by numerous channels, but even warp whole tracts of low land; whereby they insure immense returns. The Afri cans, in some parts, follow the Hindostanee plan; but raise their water chiefly from the rivers, or obtain their supplies of that invaluable element from natural reservoirs, VOL. IV.

formed by the hollows among hills. In every part of Asia, but especially in the Mysore country, formerly under the dominion of the late Sultan Tippoo, the retention of water, for the purposes of irrigation, is a matter of such importance as to be entirely under the auspices and controul of the government. Tippoo caused banks, or, as they are called in India, bunds, to be made between the bases of hills, so as to intercept the copious streams, which, during the rainy seasons, flow from the hilly country. An example worthy of imitation! Thus immense bodies of water might be collected in many parts of the United Kingdoms, whence mills and various machinery might be worked, without causing any waste of valuable land; the soil, in situations appropriate to such purposes, being for the most part poor, and unfit for tillage.

The Milanese territory exhibits the greatest expanse of irrigation known in Europe. In that country are to be seen noble canals running in every direction, totally exempted from local préjudice, private pique, or self-interest. All are under the authority and protection of government, which lets out the water to the various occupiers of meadows, at a fixed rate, according to the quantity supplied. Sometimes these canals are farmed out, by putting up the several sluices to auction; in other instances the canals go with the lands.

Whatever may be the manner in which their water is dispersed, its due preservation is an object of general solicitude, on account of the benefits which individuals derive from its use; while the government, both from

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that motive, and the support of the revenue produced by farming of the canals, do not allow the smallest despoliation to pass unpunished. We are assured, by the best authorities, that the whole of the pasture lands in the Milanese exhibit uncommon fertility; and that the canals are so very extensive, and the branches from them so numerous, that few need complain of a want of water for irrigation. These works are known to be of no modern date; some have existed for centuries, chiefly appertaining to monasteries; their waters being let out by measure to fertilize their adjacent lands. The great canal, known by the designation of Vecchiabbia, was in a flourishing state early in the eleventh century, beyond which we do not know what might have been its age. In 1220, the great canal of Adda, which waters the plains of Lodi, was finish ed; in 1305, the canal of Treveglio, which communicated with four others of very ancient workmanship, was completed; and in 1460, the canal of Martesano, extending thirty-two English miles: in this aqueduct, besides the main branch, of thirty-five feet in width, there were made nineteen scaricatori, or lesser canals, which served, when the waters rose very high, to draw off the surplus, so as to prevent injury to the main line, and to prevent inundation along its course: when the latter returned to a more tranquil state, the scaricatori, which were not so deep as the main line, served to supply it with what remained of their contents.

It is worthy our notice, that although the Italian aqueducts have, to our certain know. ledge, been duly supported for upwards of eight centuries, by a race of people far beneath us in the more noble sciences, in wealth, in population, and in many other circumstances in which we pride ourselves; yet that Britain cannot boast of one aqueduct, made exclusively with the important view to improve her agriculture; though it would be as easy to shew a thousand situations where such canals would double the value of the lands adjoining, as it would be to prove that such value would be doubled. It is, indeed, only in a few counties, that irrigation is carried on to any extent; though we may in various places see partial adoptions of this most beneficial practice: yet we daily observe situations naturally of fering this advantage, without the smallest attempt being made to retain streams which, from elevated situations, glide with some velocity through deep vallies, whose very

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borders, perhaps, are verdant, but whose more retired parts would be doubled or trebled in value, by the influence of that element, which is allowed to pass by unheeded, to be lost in some marsh, or eventually in the ocean ! It is true, that, in some parts, irrigation is not understood; and, that it is not always practicable to obtain proper assistance; whence many, who would wil. lingly water their meadows, are prevented from taking advantage of streams capable of effecting the intention. For the benefit of such persons, in particular, as well as of our readers in general, we shall endeavour to simplify, even this simple process, in such a manner as may prove perfectly intelligible; and, by shewing with what ease irrigation may be carried on, induce a portion of our landholders to attempt, even without professional aid, or the tuition of experienced persons, that retention and gradual distribution of waters whose sources are sufficiently elevated, which may favour such a slight and temporary inundation, as may give vigour and freshness both to the soil and to its produce.

We shall divide this subject into two distinct heads, viz. simple, and compound irrigation; observing that the former may be practical in various modes separately, as will be shewn, and that they may be blended so as to come under the second term. We shall also, by way of preparation, give the reader an insight into some modes of cutting off, or of supplying water, from sources of different heights, and under different circumstances: by this means, with a moderate portion of judgment, the novice in this art may speedily acquire sufficient of the principles to answer his own purposes, at least, if not to form a correct opinion of most of the cases which may come under his observation.

The greatest difficulty we generally experience, is from the water lieing below the level of the lands over which it is to be conducted. In many instances, the springs whence streams are fed, lie very deep; and, thongh copious, for want of a sufficient inclination of their beds, move very slowly. In other parts, jealousy of improvement, personal enmity, the owner being a minor, or insane, and the property in the hands of trustees, or the estate being in Chancery, mortgaged, &c. perhaps debars the possibility of taking advantage of some peculiarly favourable fall, from which the water might be conducted with perfect facility and effect, over inclined planes, which, by their

sterility, seem to reproach the owner with neglect !

In treating this subject, we must suppose the speculator to be a free agent, not shackled by such an unhappy neighbour hood; and content ourselves with cautioning him not to injure the property of others, such as mills, bleaching grounds below the lands, &c. &c., by drawing off that water on which their very existence depends: a want of attention to this particular, has ruined many a deserving and enterprising individual, and converted a blessing into a serious mischief!

Where the stream is rapid, the bed has usually a very marked declivity, such as admits of throwing the water over the lands, and of withdrawing them when they have flowed, in every part, to a sufficient height. The first step towards this, is to hold it up by means of a dam or weir, laid across the stream, (if its breadth admit, and that it be not navigable), so that, in the first place, the level may be raised as circumstances may admit. In this, it will be necessary to guard against injury to the property of other persons, above the dam; for the raising a head of water, by means of a dam, might subject lands, which before were perfectly dry, to be inundated; and, even though such should actually prove beneficial thereto, the owners might recover in a court of law, under various pleas of damage.

The water should, if practicable, be raised to one foot, at least, above the level of the highest land to be irrigated; because that depth may be then kept as a surplus, in case of long-continued drought; being let in upon the first drain, by a very small penstock, made only to the depth of the first level. The water, when abundant, may flow both into the upper level, and over the weir, so as to make a fall. When the water is not wanted over the land, the penstock may be shut up altogether. It is to be remarked, that authors of eminence in this branch differ in opinion, though some suppose water to be more richly impreg. nated with vegetable sustenance, in proportion as it is taken nearer to the spring; provided the water be clear. The lands over which it is made to flow, will be benefited in exact proportion as they may be near to the first level, which will always receive the most obvious benefit. In foul streams, the result is usually found to be in an inverse ratio; the water being richer, in proportion as it is more remote from its

source; but the first level will still receive the greatest portion of the benefit. Where rivers are very muddy, and of any magnitude, it is common to allow their flowing, to the depth of many feet, over low lands; so that, when kept stationary for a few hours, the fecula and sediment may be deposited; as is often the case, to the depth of many inches during a single tide; and give a new stratum of the finest soil. See WARPING.

These points must be well understood, because they form a very prominent feature in the practice of irrigation, and will be found highly worthy the notice of all who lay their lands down with that intention. But we must observe that many soils laying contiguous to streams, and well situated for irrigation are naturally so rich, as not to depend on any deposit from the waters for their annual produce: such require but moderate watering, and in some instances, more to be sheltered during the winter by complete inundation, than by refreshing flows. Where such prevail, the water ought to be admitted only when clear, and then from the very surface; in contra-distinction to poor, or dry soils, which want heart as well as moisture. The fact is, that by means of an artificial supply of water, the grass will shoot out far more early, which is an object of the utmost im portance to most farmers and graziers; and the crop will be much heavier than on lands not so watered. But the hay from watered meadows is frequently coarse, and not much relished by the more delicate classes of cattle. However, store cattle, which indeed scarcely ever refuse whatever is offered, will consume it with avidity. Another objection to hay from watered meadows, is that being sometimes gritty in consequence of the sediment deposited by muddy water, it is in a measure injurious to the teeth of those animals, by which it is eaten. But the great importance of an early bite, for at least a month, in general, before other pastures are sufficiently forward to receive cattle, is of itself such a consideration as outweighs every objection, and causes watered meadows to yield double the rent given before they were subjected to irrigation. In many places the grass of watered meadows from the fifteenth of March to the fifteenth of May, lets from twenty to twenty-five shillings per

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not only proves highly serviceable to the farmer himself, but to his neighbours; who thus obtain a supply of hay, when their own meadows have failed.

When land has been long watered, its qualities are meliorated considerably; but this is not the work of a day; and when the adjoining lands abound with coarse herbage, with water grasses especially, the crops will too frequently suffer by such vicinity. It will, at first view, appear strange, but it is nevertheless true, that swampy lands become firmer when regularly watered. In their natural state the water oozes upwards, and loosens the soil; but after the proper levels are found, and the catch drains are laid, so as to draw off the surplus water, the moisture is drawn downwards, and the finer parts get into the insterstices, so as to compact the whole, and give a firm footing, where before even a sheep would have been bogged. We must, however, state, that though some watered meadows will bear castle, it is by no means adviseable to let any thing heavier than a sheep feed upon them: the latter do little injury to the ridges, and by their close bite, as well as by their excellent manure, cause the grass to tiller forth, so as to form a close mat upon the soil. Whereas when large animals are allowed to tramp on the ridges, the borders of the drains are in general injured; and when ever, as will happen, the prints of their feet are left, the soil will become quaggy, and retain little pools which infallibly sour the grass, and negative the intention of watering. Hence clay soils are extremely difficult to improve by this operation; nor can such be reclaimed but by a very expensive course of draining, manuring, and breaking into a crumbly state: certainly clay soils may be formed into ridges, and grass may be made to grow upon them; but they will not produce sweet herbage; their surfaces will crack, their crops will be precarious, and their seasons for feeding must depend entirely on the dryness of the weather. Hence we may, in general terms, consider clay soils to be unfit for irrigation; the expence being great, and the money being more likely to yield a greater profit by other means; while their crops and pasturage are, in various points, of an inferior value.

But to proceed: the secondary drain, which supplies the whole of a field through which it passes, should be interrupted at every fall of four inches at farthest, by small sluices, or penstocks, and have small

branch-drains cnt to the right and left, in such manner as may cause the water to branch out into the whole expanse of its level. The turf cut from the surface of each drain, ought to be placed, face downwards, between it and the land it is to overflow; being made firm and level, by beating with the flat of a spade. As the penstocks are situated just below the lines of the branchdrains above described, they keep up the water, so as to fill, and to cause their overflowing into the next inferior talus or slope, as shewn in fig. 1 and 2, where A is the main drain, taken from the water-head or river, B; the drain C, C, C, C, shews the secondary drain, which, being on a declivity, would carry off all the water, were it not kept up at the places where the catchdrains, or branches D, D, D, D, proceed laterally from it, by the sluices E, E, E, E. By this means, any particular level, either 1,2,3,4, may be irrigated at pleasure, without wetting the others; the water being kept on by the sluice above, and carried away by the sluice appertaining to each level respectively. Or, if other meads at some distance are to be watered, the secondary channel, having all its sluices open, will convey it to them without interruption, when all its sluices are opened.

It is evident, that in this manner the whole of the water is carried down to the lowest level: hence, it becomes a matter of no small importance to ascertain, that the whole shall either be absorbed or be carried off; so as not to injure the last level, which might otherwise be subjected to very considerable injury, were the inundation to be too long supported. The judicious computer will be cautions not to allow so much to remain, as may rot his grass in lieu of causing it to vegetate vigorously. This, in some situations, presents a very serious difficulty; for if the water is debarred free access to the lowest levels, they will be less fruitful than the others, which, exclusive of the great fecundity derived from first receiving the fluid, receive absolutely a larger portion of moisture. The greatest care is therefore requisite, to insure that the tail, or spent-water, shall be carried off. Where the declivity is considerable, and that the stream, or any other water-course, offers itself to receive such tail-water, at a due level beneath, there is no difficulty; but where the stream takes another course, and the descent is trifling, some artificial means must be resorted to. Perhaps no more simple or efficacious plan can be hit upon,

than that of forming a fish-pond, of a suitable extent and depth, to receive the tailwater; whereby the apprehended damage may be avoided, and a useful store be created.

We shall shew what we may term a truly ingenious device, whereby water may be laid upon lands that are above the level of the stream: it consists merely of an air-vessel, A, fig. 3, into which the water descends forcibly from the stream, B, and by compressing the air in the upper part, C, is it self forced to ascend through the conduct ing pipe, D, with such force as to rise to a level, E, far above that at which it formerly stood. This is the principle of the common fire-engine, which, we are all sensible, can, when exerted, throw water to a great height. By such means, the tail-water may also be forced up to such a level as may canse it to return into the stream.

Where the stream runs through the lands that are watered, and that its declivity is moderate, it will sometimes be found difficult to restore the tail-water to its level. To effect this with as little expence as possible, wooden pipes should be laid from the lowest level of the land along the bank of the stream, but carried horizontally on a bank, to such extent as may suffice to convey the tail-water to the surface of the surface. This, however, is not applicable to all situations; for where the stream is very slow, its declivity would be very trifling. Where that happens, the air vessel will be found a good plan, provided the height to which the water is to be returned, be not considerable. In many situations, a waterwheel might answer well; observing, that in deep, slow waters, that are broad, and under the speculator's own management, it will be best to throw a weir across, and then to let the whole body of the stream rush through a narrow slip, so as to turn a wheel placed immediately in the line of the water's run. By this device, the current may be made to pass that particular spot with sufficient velocity to turn a wheel; whereby water might either be raised out of the river, to supply a main drain, or the tail-water might be restored to the stream: in either case, one or more pumps would be necessary. (See fig. 4.)

The second mode of laying water over the land, is by means of ridges, whose centres are occupied by small horizontal drains, out of which the water, furnished by the main drain, is allowed to flow to the depth of about an inch down each side of the

pitch. These ridges should be from four to six feet measurement for each face; the drain being about a foot broad, and four inches deep; thus the whole breadth of a pitch, declining each way equally, might occupy a base of about ten feet at the utmost. The declivities ought not to exceed an inch to the foot; in loose soils, not more than half an inch; else the finer parts will be washed away, and the drains, formed by the junctions of the ridges, will be filled up, whereby the water will be detained, and prevented from passing into the next level. Fig. 5, shews the profile, or section of a range of ridges on the same level, and fig. 6, displays an inclined plane, whereon ridges are formed in regular succession, the catch-drains being a little higher than the branch-drains of the next lower level, so that the latter may be filled from the former: the water thus gradually descending, until the whole is gradually absorbed by the successive ridges; or the surplus is carried off by a large catch-drain, made to direct it into some other succession of ridges, as seen in the ground-plan, fig. 7.

The reader will perceive, that the levels may lay in any direction, according to the cast of the land; and, that where water can be had at a due height, all the land below it may be watered. It matters not if a deep valley lay between two declivities, to be watered by the same spring. A pipe, of suitable diameter, being made to descend one face, and to rise up the other, will couvey the stream with facility to any part; so as to re-assume the level on the opposite side. For further insight into that circumstance, see FLUIDS, HYDRAULICS, and HY

DROSTATICS.

It often happens, that small rivers have a very winding course among little hills, banks, rocky masses, &c., and that they suddenly lose many feet of their altitude, owing to a fall, or steep declivity; white the lower parts of the stream, being more expanded, and the water being kept up by another impediment, perhaps a few bu dred yards lower, offer a seemingly invincible impediment to the conducting it over the finely-formed planes, which present themselves on either bank. Here the diffi culty is far less than at first sight is supposed; since, by making an outlet from the superior level of the stream, through the bank which separates it from the planes to be watered, an abundant and certain supply may be obtained. Thus in fig. 8, the upper level, A, and fall, B, are shewn, and

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