2208. Ponds, or large basins (fig. 663.), are reservoirs formed in excavations, either in soils retentive of water, or rendered so by the use of clay. This clay is tempered, or made compact and tenacious, by working it so as to exclude the larger globules of air and water, and intimately to unite all its parts with as much moisture as leaves it plastic. The bottom and sloping sides of the excavation being smoothed and made firm, this tempered clay or puddle is to be spread evenly over it, from margin to margin, about a foot thick, and well compacted by beating. To preserve it from injury by the pressure of feet, or other accidents, it should be covered with gravel, in thickness according to the supposed liability to accidents. If cattle are to enter it, 18 in. of coarse gravel, or stones covered with 6 in. of fine gravel, will not be too much. Sometimes these basins are lined with pavement, tiles, or even lead; and the last material is the best, where complete dryness is an object around the margin.
