2209. Tanks, or cisterns (fig. 664.) are generally excavations in the earth, lined with masonry, and sometimes raised 2 ft. or 3 ft. above it. This masonry is always built with mortar which sets or hardens under water, as the Dorking and other sorts of lime, gypsum, and any lime mixed with oxide of iron, in the form of what is called Roman cement, or Puzzolana earth. (Davy's Elements of Agr, Chem., lect. vii.) To protect this, the bottom of the cistern or basin is sometimes covered with 6 in. or 8 in. of clay. Sometimes the bottom of the excavation for a pond or tank is naturally a retentive clay, while the sides are of porous earth. In this case, the simplest way is to raise a wall, or vertical stratum of puddle (fig. 665.), from the horizontal stratum of clay, to within a few inches of the surface of the ground.