1. As to grates, the way of them is well known; however observe, that if they be made of wood, the banks must be set diagonally, like window-bars; for so rubbish stops least against them, and the water passeth freely. And in regard you cannot allow any great distance between them for keeping in the fish, you must help out the room by extending the grate from each side of the cut in the bank where the water is to vent, some considerable space from the bank, and there to meet in a point, forming a triangle upon the bank. Here are many more slits for the water to vent at, than if the grate lay flat upon the bank, covering the passage only. And if need be, there may be doors to slide up and down, made in the grate, to let the water pass more freely; but this endangereth losing the fish. If you will afford iron for these grates, you need only cover the passage of the bank; for the bars need not be so thick, but there will be space enough for the water to vent at.
2. The channels for diverting the water are very useful in this and many other respects; for they give you a perfect command of the water, and you may turn it which way you please, so as to fill or keep dry any of the ponds, and in a wet season are a perfect security. These should be made four feet wide, and on each side of the ponds the loss of ground is not considerable; for wood growing there will make amends for it.
The string of ponds in Hyde-Park are admirably disposed in this respect; for the current of the valley is carried along by the side of all the ponds, and may be let into any of them, or any may be emptied into it; than which, there is not a greater command of water.
However carefully a bank is made, it is probable it will sew a little at first; but this should be no discouragement; for by the settling of the earth, it will continually grow higher, and in a few years, if made with tolerable care, be as firm as a rock.
These are very requisite to the good command of a water, and though very ordinarily used, yet require an experienced carpenter to make and fix them as should be, especially in great waters; and such as have not experience, shall err most grossly in this work. They must be framed so as to stand firm, that the force of any thrust, or a boat’s running against them, may do no prejudice to them: for if they are any thing strained, they are apt to prove leaky; and in so great an height as is needful for deep waters, a small matter will do it, unless they are extraordinarily well abutted.