To make an old underground tank water-tight from the outside without entirely reconstructing it is a difficult matter. Any solution or composition applied to the inner face of the walls would he forced off by the outside hydraulic pressure. But a lining of asphalt may be put on, and then, if the tank is very deep, an inner wall of bricks set in cement should be built over tlic asphalt in order to resist the outside pressure of the ground water when the tank is empty. Another remedy that would doubtless be effectual is to excavate the ground for a width of 12 in. to 18 in. outside the walls, and fill the space with puddled clay well consolidated by ramming. If the water comes through the bottom of the tank, lay down a new floor of good Portland cement concrete about 10 in. to 15 in. thick, well consolidated, and with the surface trowelled smooth. If such a floor is laid, the ground water must be kept down by pumping for about thirty-six or forty-eight hours, or until the cement has had time to set properly.