

IRRIGATION. 



each way equally, might occupy a base 

 of about ten feet at the utmost. The de- 

 clivities 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 pi-evented from passing into the next 

 level. Fig. 5, bhows due 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 

 higlver than the branch drains of the next 

 lower level, so that the latter may be fill- 

 ed from the former : the water thus gra- 

 dually descending, until the whole is gra- 

 dually 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 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 Tew 

 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 planes 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, Ji, from which 



VOL. IV 



it would not be possible to raise the watei- 

 to the superior parts of the inclined plane, 

 C, D, without the aid of expensive machi- 

 nery. This section vvill,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 of compound 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 ot' 

 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 



r through the first. A reference to 

 9, will give some idea of this mode ; 

 by the courses of the dotted liaes, and 

 arrows, the various descents may be 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 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 th 

 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 frost y ; but so oon a* 

 B 



