Where water can be had at nearly the level of the premises, such as from rivers, canals, &c., if it is not thought prudent to erect elevated tanks, the water may be conducted under the surface by large cast-iron pipes, with openings at such distances as may seem advisable for introducing the suction-pipes (Fig. 6). This plan should not be adopted where the level of the water is more than 12 feet below the surface of the ground, as although a fire-engine will, if perfectly tight, draw from a much greater depth than 14 feet (2 feet being allowed for the height of the engine), still a very trifling leakage will render it useless for the time, at such a depth. The worst mode of supplying engines with water is by covered sunk tanks; they are generally too small, and unless very numerous, confine the engines to one or two particular spots, obliging the firemen to increase the length of the hose which materially diminishes the effect of the fire-engine. If the tank is supplied by mains from a reservoir, it would be much better to save the expense of the tank, and to place plugs or firecocks on the water-pipe. Another evil in sunk tanks is, that the firemen can seldom guess what quantity of water they may depend upon, and they may thus be induced to attempt to stop a fire, at a point they would not have thought of if they had known correctly the quantity of water in store. Where sunk tanks are already constructed, they may be rendered more available by a partial use of the method shown in Fig. 6. _Memoranda of Experiments tried on the mains and service pipes of the Southwark Water Company, between 4 and 9_ A.M. _of the 31st January, 1844. The wind blowing fresh from N.N.W._ The pressure at the water-works at Battersea was kept at 120 feet during the experiments, and every service pipe or other outlet was kept shut. _1st Experiment._--Six standcocks, with one length of 2-1/2 inches riveted leather hose 40 feet long, and one copper branch 4 feet to 5 feet long, with a jet 7/8 inch in diameter on each, were placed in six plugs on a main 7 inches diameter, in Union-street, between High-Street, Borough, and Gravel-lane, Southwark, at distances of about 120 yards apart. The water was brought from the head at Battersea, by 4250 yards of iron pipes 20 inches diameter, 550 yards of 15 inches diameter, and 500 yards of 9 inches diameter. 1st. One standcock was opened, which gave a jet of 50 feet in height, and delivered 100 gallons per minute. With four lengths of hose the jet was 40 feet high, and the delivery 92 gallons per minute. When the branch and jet were taken off with one length of hose the delivery was 260 gallons per minute. 2nd. The second standcock was then opened, and the jet from the first was 45 feet high. 3rd. The third standcock was opened, and the jet from the first 40 feet high. 4th. The fourth standcock being opened, the first gave a jet of 35 feet high. 5th. The fifth being opened, the first gave a jet of 30 feet high.