THE BRITISH ENCYCLOPEDIA: IRRIGATION. 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. 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 merous channels, but even warp whole tracts of low land; whereby they insuge immense returns. The Afri Cans, in some parts, follow the Hindostanee plan; but raise their water chiefly from the rivers, or obtain their supplics of that invaluable element from natural reservoirs, VOL. IV. The Milanese territory exhibits the greatest expanse of irrigation known in Europe. In that country are to be seen noble cauals running in every direction, totally exempted from local prejudice, 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 B 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 Hine, 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 at tempt being made to retain streams which, from elevated situations, glide with some velocity through deep vallies, whose very 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 intelligi. ble; 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, though 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 Itold 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 da mage. 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 impregnated 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 stream, 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. Bat 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 acre. The crop is usually two tous, in all seasons: in dry ones it is not subject to the ordinary risk of being burnt up; and, 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 cattle, 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 whenever, 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 cut 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 cautious 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; bat where the stream takes another course and the descent is trifling, some artificial means must be resorted to. Perhaps so more simple or efficacious plan can hit upon, |