The extent of a pond must depend upon circumstances, but it should never be less than five or six acres, and as much more as is convenient; in shape it should be rather long, as regards the front of the mansion, than broad, and its outline should either be varied, or broken by planting trees and shrubs in small groups on its banks. Where ponds are intended solely for fish, without regard to their appearance in the landscape, their banks should be quite straight, and their shape square or oblong. The pond, whether ornamental or not, may be divided into two parts by an embankment in the centre, which is kept about two feet below the general surface of the water when the pond is full. The use of this is, that, when the pond is to be run dry, either for clearing it of its mud or catching the larger fish, only one half need be emptied at a time; and the other half may serve as a place to put the store fish in while the first division is being cleared out; and, when that is quite clean and the water let into it again, the fish may be transferred to it while the second half is being cleared. Of course, according to this arrangement, there must be a wooden sluice for letting off the water in each division. The beauty of a pond depends principally upon its being well supplied with water, and this constant supply can only be insured during the summer months, by having a brook or rivulet running through the pond. Where the pond is of sufficient size, there should be an island or two for the swans and other aquatic birds to make their nests on. Where several kinds of fish are desired, different soils may be laid at the bottom of the pond, to suit the fish which breed in it. Thus, trout will never thrive unless the pond in which they are kept has a gravelly bottom. Carp (of which there are two kinds, the common, and the Prussian which is much smaller than the other,) and tench require loam, with abundance of aquatic weeds; and eels will only thrive in mud. It is very seldom practicable to keep all kinds of fish in the same pond, as some kinds destroy other kinds; but providing them with different bottoms is the most effectual way of keeping them, as they will remain in separate communities in the part of the pond that suits them best.