dy, 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 importance 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 tons, 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 interstices, 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 advisable 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 expense 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 shown in fig. 1 and 2, where A is the main drain, taken from the waterhead or river, B; the drain C, C, C, C, shows the secondary train, which, being on a declivity, would carry off all the water, were it not kept up at the places where the catch-drains, 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 ab

sorbed 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; 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 tail-water; whereby the apprehended damage may be avoided, and a useful store be created.

We shall show 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 itself forced to ascend through the conducting 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 cause it to return into the stream.

Where the stream runs through the lands that are watered, and that its de

clivity is moderate, it will sometimes be

found difficult to restore the tail-water to its level. To effect this with as little ex

pense 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 level 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 restor

ed 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, shows 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 convey 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 HYDROSTATICS.

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; while the lower parts of the stream, being more expanded, and the water being kept up by another impediment, perhaps a few hundred yards lower, offer a seemingly invincible impediment to the conducting it over the finely-formed planes, which present themselves on either bank. Here the difficulty 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 plains to be watered, an abundant and certain supply may be obtained. Thus in fig. 8, the upper level, A, and fall, B, are shown, and the place pointed out where a cut, C, should be made, whereby the whole of the inclined plane, D, might be irrigated to the greatest advantage; the surplus-water, draining off into the lower level of still water E, from which VOL. VII.

it would not be possible to raise the water to the superior parts of the inclined plane, C, D, without the aid of expensive machinery. This section will, we trust, prove completely satisfactory, by showing how necessary it is to look back to superior levels, often within reach.

Under the head of compound irrigation, we consider the various changes of direc tion, attended with an intermixture of the several modes laid down for simple irrigation. In the former, we occasionally find the water caught several times by the same stream, which, being obstructed at its several turns of weirs, sluices, &c. enables us to abbreviate the succession of ridges. This is a matter of great importance, because it renders a less body of water, in the branch drains of the first level, equal to every purpose, and obviates the mischief that sometimes attends upon a numerous succession of levels, when the quantity of water required for the whole is forced through the first, in which, by its weight and volume, the roots of the grass are denudated, and the finer parts of the soil completely washed away. It is far better to give the stream a second, or even a third, turn through the land, than to allow all the water, necessary to moisten six or seven successive levels, to pass through the first. A reference to fig. 9, will give some idea of this mode; by the courses of the dotted lines, and arrows, the various descents may be understood.

With respect to the season for watering land, so many varieties prevail, in consequence of soil, and of locality, that we can only observe, in general terms, that where lands are to be inundated completely, by letting the water assume an unlimited range, and to expand over all parts which come under its level, such places require, during the winter season, to be kept well covered, that the frost may not attack the plants while saturated with moisture: if that were to happen, the whole would be destroyed; whereas, by a periodical inundation, the grass is sheltered from frost; and, by drawing off the water as the spring advances, and at intervals of about ten days, when the weather is fair, such grass will shoot out vigorously, and afford a very early bite for cattle, at that season when green food is both valuable and scarce. The same principle may be followed, though the practice is different, in places watered by drains. In such, the greatest care ought to be taken to avoid throwing on the water while the air is frosty; but so soon as

B

the weather opens, the ground ought to be moderately moistened. The sun's power should guide us as to the frequency and quantity of water; nor should its quality be overlooked: water from warm soils will produce effects widely different from the streams flowing out of clay lands, or such as are impregnated with iron, &c.; the best water usually rises out of gravelly or chalky lands. It is better to throw the water on early in the day, during cold weather, in order that the grass may dry well, and the danger apprehended from frosty nights be obviated; but in summer, the watering should take place late in the evening, whereby the ground will be cold, without danger of scorching the plants.

We have dwelt thus long on the subject of irrigation, under the conviction of its extreme importance: the reader may, under the head of AGRICULTURE, find a few additional remarks, which were given with the view to bring all matters relating to farming under one general head, while we reserved this mechanical part to be separately treated, under its proper designation.

IRRITABILITY, in physiology, is the property peculiar to the muscles, by which they contract upon the application of certain stimuli, without a consciousness of action. Haller and other physiologists denominate that part of the human body irritable, which becomes shorter by being touched: very irritable, if it contracts upon a slight touch. They call that a sensible part of the human body, which, upon being touched, transmits the impression of it to the mind: on the contrary, they call that insensible, which, being burnt, torn, cut, &c. occasions no sign of pain or convulsion, nor any sort of change in the situation of the body. It is inferred that the epidermis is insensible; that the true skin is the most sensible part of the body; that the fat and cellular membranes are insensible; and the muscular flesh sensible, the sensibility of which he ascribes rather to the nerves than the flesh itself. The tendons, having no nerves distributed among them, are deemed insensible. Irritability then is the distinguishing characteristic between the muscular and cellular fibres. Irritability differs from sensibility, and is not proportioned to it: the intestines are less sensible than the stomach, but more irritable; the heart is very irritable, though it has but a small degree of sensation. The laws of irritability, according to Dr. Crichton, are: 1. After every action in an irritable part, a state of rest,

or cessation from motion, must take place, before the irritable part can be again incited to action. If by an act of volition we throw any of our muscles into action, that action can only be continued for a certain space of time; the muscle becomes relaxed, notwithstanding all our endeavours to the contrary, and remains a certain time in that relaxed state, before it can be again thrown into action. 2. Each irritable part has a certain portion or quantity of the principle of irritability which is natural to it, part of which it loses during action, or from the application of stimuli. 3. By a process, wholly unknown to us, it regains this lost quantity during its repose or state of rest. In order to express the different quantities of irritability in any part, we say that it is

either more or less redundant, or more or less defective. It becomes redundant in a part, when the stimuli which are calculated to act on that part are length of time, because then no action withdrawn, or withheld for a certain

4.

can take place; while, on the other hand, the application of stimuli causes it to be exhausted, or to be deficient, not only by exciting action, but by some secret influence, the nature of which has not yet been detected; for it is a circumstance extremely deserving of attention, that an irritable part or body may be suddenly deprived of its irritability by powerful stimuli, and yet no apparent cause of muscular or vascular action takes place at the time. Thus a certain quantity of spirits taken at once into the stomach kills almost as instantaneously as lightning does ; the same thing may be observed of some poisons, as opium, laurel-water, the juice of some poisonous vegetables, &c. Each irritable part has stimuli which are peculiar to it; and which are intended to support its natural action: thus blood, which is the stimulus proper to the heart and arteries, if by any accident it gets into the stomach, produces sickness or vomiting. 5. Each irritable part differs from the rest in regard to the quantity of irritability which it possesses. This law explains to us the reason of the great diversity which we observe in the action of various irritable parts: thus the muscles of voluntary motion can remain a long time in a state of action, and if it be continued as long as possible, another considerable portion of time is required before they regain the irritability they lost; but the heart and arteries have a more short and sudden action, and their state of rest is equally so. The circular muscles of the intestines have also a quick ac

tion and short rest. 6. All stimuli produce action in proportion to their irritating powers. As a person approaches his hand to the fire, the action of all the vessels in the skin is increased, and it glows with heat; if the hand be approached still nearer, the action is increased to such an unusual degree as to occasion redness and pain; and if it be continued too long, real inflammation takes place; but if this heat be continued, the part at least loses its irritability, and a sphacelus or gangrene ensues. 7. The action of every stimulus is in an inverse ratio to the frequency of its application. A small quantity of spirits taken into the stomach increases the action of its muscular coat, and also of its various vessels, so that digestion is thereby facilitated. If the same quantity, however, be taken frequently, it loses its effect. In order to produce the same effect as at first, a larger quantity is necessary; and hence the origin of dram-drinking. 8. The more the irritability of a part is accumulated, the more that part is disposed to be acted upon. It is on this account that the activity of all animals, while in perfect health, is much livelier in the morning than at any other time of the day; for during the night the irritability of the whole frame, and especially that of the muscles destined for labour, viz. the muscles of voluntary action, is re-accumulated. The same law explains why digestion goes on more rapidly the first hour after food is swallowed than at any other time; and it also accounts for the great danger that accrues to a famished person upon first taking in food. 9. If the stimuli which keep up the action of any irritable body be withdrawn for too great a length of time, that process on which the formation of the principle depends is gradually diminished, and at last entirely destroyed. When the irritability of the system is too quickly exhausted by heat, as is the case in certain warm climates, the application of cold invigorates the frame, because cold is a mere diminution of the overplus of that stimulus which was causing the rapid consumption of the principle. Under such, or similar circumstances, therefore, cold is a tonic remedy; but if in a climate naturally cold, a person were to go into a cold bath, and not soon return into a warmer atmosphere, it would destroy life, just in the same manner as many poor people, who have no comfortable dwellings, are often destroyed from being too long exposed to the cold in winter. Upon the first application of cold the irritability is accumulated, and the vascular system therefore is disposed

to great action; but after a certain time all action is so much diminished, that the process, whatever it be, on which the formation of the irritable principle depends, is entirely lost. See Dr. Crichton on Mental Derangement for more on this subject.

ISATIS, in botany, a genus of the Tetradynamia Siliculosa class and order. Natural order of Siliquosæ or Cruciformes. Cruciferæ, Jussieu. Essential character; silicle lanceolate, one-celled, one-seeded, deciduous, bivalve; valves nivicular. There are five species, of which I. tinctoria, dyer's woad, is a biennial plant, with a fusiform, fibrous root; stem upright, round and smooth, woody at bottom, branched at top; stem leaves from two to three inches long, and scarcely half an inch in breadth; flowers small, terminating the stem and branches in a close raceme; both corolla and calyx yellow; petals notched at the end; seed vessels on slender peduncles, hanging down, chesnut coloured or dark brown, shining when ripe, of an oblong elliptic form, compressed at top and on the sides into a sharp edge, swelling like a convex lens in the middle; cotyledons ovate, fleshy, plano convex; radicle subcylindrical, bent in upwards. It is a native of most parts of Europe. Woad is much used by dyers for its blue colour; it is the basis of black and many other colours.

ISCHÆMUM, in botany, a genus of the Polygamia Monoecia class and order. Natural order of Gramina, or Grasses. Gramineæ, Jussieu. Essential character; hermaphrodite calyx; glume two flowered; corolla two valved; stamens three; styles three; seed one: male, calyx and corolla as in the other; stamens three. There are eight species.

ISERINE, in mineralogy, a species of the Menachine genus: it is of an ironblack, inclining a little to the brownishblack; it occurs in small, obtuse, angular grains, and in rolled pieces, with a rough glimmering surface. Internally it is glistering, and its lustre is semi-metallic. Specific gravity 4.5. Before the blowpipe, it melts into a blackish-brown coloured glass, which is slightly attracted by the magnet. It is composed of Oxide of menachine

Loss

-iron

uran

59.1

30.1

10.2

99.4

6

100.0