But were there not need of earth for this purpose, it were better to leave the natural soil for the fish to feed upon. I shall give the reason afterwards, and consider the manner of raising and fortifying the bank particularly.
Of the Manner of the making and raising Pond-Heads.
It is obvious, that if you make a dam cross a valley or swamp, where at any time after, the water runs, it will produce a pond; and as the bank or dam is higher at the point or center, which is against the lowest ground, so much is the pond deeper; and if the hills on each side rise steep and quick, the water stopped will cover less ground than if they rise slow.
Now first, for making the bank or head, you must be sure it is tight, and that it do not sew or leak, as it will certainly do, if it be composed of mere earth; therefore a bed or wall of clay, the whole length of the bank, must be carried up with good ramming, from a foot or two below the surface of the ground, to such height as you propose the water shall stand.
If you do not give the bed of clay this foundation, the water lying under a great weight from the depth of it, will work itself underneath, so allow a spit or two at least for it. Then, as you ram the clay, you must be sure that earth be brought to carry the bank up with it, or else the sun will search and crack it, which is of pernicious consequence; so when it is come to its full height, close and cover it with earth immediately, lest the inconvenience happens.
You must allow three feet to the breadth of this bed of clay, and raise it to the height you intend the water shall stand, and lay earth three feet higher; two feet would have served, but that the allowance of one at least must be made for the sinking of the bank; for it will do so notwithstanding the pressing of tumbrels, horses, and men working upon it.
If you project many stews, or other ponds to be sunk right down about the same time, you will have great advantage by the clay you take out of them, which will be much more than is necessary for the bed, and that may fortify the bed, by being pressed down by the tumbrels on each side of it; and so the bank will be very much confirmed, and it will also save breaking of ground within the pond, which is a great advantage in the feed of the fish.