Figs. 453 to 457 are sketches of a medium-sized cast-iron water-tank, to hold about 7800 gallons. The size may be varied both in length, width, and depth, without in any great measure altering the type. The lower portion, or tank-house, may be of stone, brick, wood, or iron framework, and may be utilized as a pump-room, store, or lamp-room. In the sketch given a row of cast-iron girders are placed across the top of the walls of the tank-house, to carry the tank, the plate-joints of the latter being made to coincide with the centre lines of the girders. The lower and upper edges of the tank-plates are shown curved in section, the former for appearance and facility of cleaning, and the latter to check the tendency of the water rippling or splashing 301 over the sides when disturbed during high winds. The large pipe, A, is securely bolted at the bottom of the tank, and forms a shield or funnel through which the supply pipe, B, passes upwards into the tank. C is an overflow, or waste pipe, to carry away any surplus which may find its way into the tank after the water has risen to its fixed maximum height. All the contact surfaces of the cast-iron tank-plates must be accurately chipped or planed, and fitted to ensure water-tight joints. Stay-rods must be placed at frequent intervals, connecting the vertical or outer plates to the horizontal or floor plates. When required to hold more than 20,000 gallons, it is better to make the tank in two parts, by placing a permanent plate partition across the middle, in reality making two separate tanks, which can be connected or disconnected at will. The double tank arrangement gives additional strength, and possesses the advantage that the one tank can be emptied and cleaned out while the other remains in service. Water-tanks constructed of wrought-iron or steel plates are usually made circular in form, with vertical sides. The floor-plates must be either carried on small girders, as in the cast-iron tank, or be strengthened internally with angle-irons, tee-irons, and tie-rods. The rivetting must be well done, all joints sound and watertight. This class of tank must be kept well painted, or oxidization will take place very rapidly. The arrangement of inlet, waste-pipe, and delivery pipe may be the same as for the cast-iron tank. Although frequently seen abroad, these circular wrought-iron tanks are not often adopted at home. By many the appearance of the circular tank is considered inferior to one of neat rectangular shape, and the form of the round tower does not lend itself so conveniently for use as a pump-room or store.