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Chap. XII. App. Part 6.
No. I. Of Irrigation. |
Sect. I. On [Fears and Dams f and Flood- Hatches or Sluices. '•
One or other of these are commonly- wanted in irrigation. Ifthe stream be sririall, (of only 2 or 3 feet wide), then a dam maybe made with a few sods or turf, which will be sufficient; but if jthe stream be 2 or 3 yards wide, and abundance of fall to the [meadow, then a dim of sods will likewise do ; but it must be :covered over with stones like an arch, otherwise, in floods, the jwater will drive it away. If stones cannot be got, then a pieceof wood may be laid across the stream ; and piles or stakes drove jinto the bottom of the water above the piece of wood,which, rest- !ing against it, wall prevent the water from carrying away thedam made above, and resting on, the stakes. If the water be alarge river, and it be all wanted to be diverted, the wear mustbe made of dressed stone, and well puddled at the back withstiff clay; or if it be wanted sometimes to let the water down thechannel, sluices or hatches in frame work, will answer best. Todescribe these large species of flood-hatches would be tedious, ;and perhaps useless, as no person who is not acquainted with jthem, could execute them to answer the purpose. The kind of |wears used for taking in water to the mills in the North of Scot- iland, built with rough sto'nes, will do, where you have declivity Ienough, without raising the stream above 1 j or 2 feet, as sluicesor hatches are sometimes wanted for the feeders and conductors, i
Sect. II . On the Formation of Floated Meadows.
The rules already given (see Chap. 12. Part 6.), being attend- ;ed to, and the water carried with a proper fall * to the highest |part of the meadow, (for it should never be upon a dead level ifit can be avoided) ; the next object is, to make the conductor or !feeder large enough to receive all the water that the stream con- itains, if there is land enough to use it. If there is a great fall jfrom the wear or dam, to where the meadow is to begin, it may !be made the deeper and narrower ; but if it is nearly level, itmust be made the shallower and wider, as it is only near the topof the stream that the water has any draught. It is of no use tomake the bottom of the conductor deeper than the feeder will
* A proper fall is, from one inch of fall in 20 yards, to one in 50, accord-i»S to the weight and velocity of the water above the mouth of the conductor.