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 declivi ties, 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-termed 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 planes to be watered, an abundant and certain supply may be obtained. Thus in fig. 8, the upper level, A, and fall, II, 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 Citr into the lower level of still water, E, from which it would not be possible to raise the watee to the superior parts of the inclined plane, C, D, without the aid of expensive machi nery. This section will,we trust, prove com pletely satisfactory, by showing how ne cessary it is to look back to superior levels, often within reach.
Under the head ofcompound irrigation, we consider the various changes of direc tion, attended with an intermixture of the several modes laid down for simple irri gation. In the former, we occasionally find the water caught several times by the same stream, which, being obstructed at its several turns by weirs, sluices, &c,, enables us to abbreviate the succession of ridges. This is a matter of great import ance, because it renders a less body of wa ter, 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 he un. derstood.
With respect to the season for watering land, so many varieties prevail, in con sequence of soil, and of locality, that we can only observe, in general terms, that where lands are to be inundated com pletely, 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 saturat• ed 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 ofr 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 wa ter while the air is frosty ; but so soon the weather opens, the ground ought to be moderately moistened. The sun's power should guide us to the frequency and quantity of water; nor should its quali ty 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 gravel ly 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 sub ject of irrigation, under the conviction of its extreme importance t the reader may, under the head of AGRICULTURE, find a few additional remarks, which were given with the view to bringing all matters relating to farming under one general head, while we reserved this mechanical part to be separately treated, under its proper designation.