Instead of an open space, nowadays, it is more customary to thoroughly plaster the outside of the cellar wall, and then paint it with a tar paint put on hot, which will adhere fairly well to the cement or masonry. Asphalt cannot be very readily used for this purpose unless it is an asphalt oil with but little bitumen paste. A paving asphalt, for example, even applied hot, does not adhere to the masonry, but slides down the walls as fast as it is applied. A successful method, however, of using such asphalt is to build the cellar wall in two parts, separated about half an inch, and filling in the intervening space with liquid asphalt. In this way, the asphalt is held in position, and is an absolute prevention of dampness. Another method used successfully in the construction of one of the large railroad stations in Boston consists in painting the outside of the wall with tar and then pressing into the hot tar several layers of tar paper, the separate sheets overlapping in a special coating of tar. These sheets are thus made continuous around the building and under the basement so that no water can enter the building. [Illustration: FIG. 8.--Water-tight wall.] A cross-section of one of the depressed tracks entering the Boston Station is shown in Fig. 8. The heavy black line represents ten thicknesses of tar paper, each one thoroughly painted with a thick paint of hot tar. It should be noticed that this water-tight coating is inclosed between masonry walls, so that the coating cannot be injured. It is possible theoretically by these methods to build an underground cellar so truly water-tight that it could be set down in a lake, where it might float like a boat and not leak a drop, and there may be some locations that require such construction, such as a low river valley or an old salt marsh or a city flat, where no adequate drainage is provided. But practically such construction will always be found expensive, and is, in most cases, unnecessary and ineffective, as already indicated, and where the percolating water cannot be tolerated, involves the installation of some kind of pump to throw out the water that will inevitably, in larger or small quantities, pass through the best water-proofing. It is, therefore, the part of wisdom to place reliance on draining the water away from the house rather than on water-proofing the cellar wall. _Dry masonry for cellar walls._