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._

It may not be out of place to add a word of caution against the practice
of building cellar walls of loose stone, without mortar. They make no
pretense of being water-tight, they offer no resistance to the entrance
of rats, and they soon yield to the pressure of the earth and present
that wobbly, uncertain appearance of cellar walls seen in rural
districts. Nor should the idea that the interior is to be visible and
the exterior invisible blind the builder to the fact that it is far more
important to have the outside smooth. If smooth, there are no projecting
surfaces for water to collect in, no edges fo