l the voids in the layer of stone and furnish a
slight excess. As the screenings are spread they are rolled to work
them into the voids. When these are filled, the surface is sprinkled
thoroughly by means of an ordinary street sprinkling cart and again
rolled. In this way the dust and water are mixed into a mortar which
fills the crevices between the stones. This mortar hardens in a few
days, giving a bond that is weak, but sufficient for the purpose if
the traffic is not too heavy. A broken stone road finished in this way
is called a water-bound macadam, and is ready for traffic in three or
four days after completion.

=Bituminous Surfaces.=--On account of the inadequacy of the
water-bound macadam when subjected to motor traffic and to obviate the
tendency of broken stone surfaces to loosen in dry weather, there has
been developed a method of covering the surface with a bituminous
material such as tar or asphalt. This will be described in detail in a
later chapter.

=Maintenance.=--Even under favorable conditions as regards kind and
amount of traffic the macadam road requires constant maintenance. The
first effect of traffic will be to brush away the fine materials used
for bonding the surface, thus exposing the larger stones in such a way
that they are rather easily loosened and removed from the surface by
wheels and the hoofs of animals. This finer material must be replaced
as fast as it is removed so as to protect the surface. Either stone
dust or clayey sand may be used, but clay if used alone is likely to
be sticky when wet and prove to be worse than the condition it was
expected to correct. In time, ruts and depressions will appear, either
as the gradual effect of wear, which will inevitably effect some
portions of the surface more than others, or on account of subsidence
of the foundation. Uneven places are repaired by first loosening the
stone, then restoring the cross section by adding new material and
tamping or rolling it in place.

If a bituminous coating has been applied, it will eventually peel off
in places and these places must be recoated as soon as practicable.

Eventually the surface will be worn to such an extent that an entirely
new wearing surface must be added. This is done by loosening the
entire surface to a depth of 3 or 4 inches and then adding a new layer
of broken stone. The loosening is sometimes accomplished by means of
heavy spikes inserted in the roller wheels, and at others by mean