But the paved streets are not absolutely Impervious,
only relatively so, and the hard-earth yards, while
allowing some rain to run off, also retain some, so that
the percentages given above are only of the areas to
be considered. It remains to determine what propor-
tion runs off from the four classes. The loss of water
by absorption and evaporation from roofs is generally
so small in heavy rains that it may be neglected, so
that the roof-surface may be taken as truly impervious.
As to the percentages furnished from pavements and
sidewalks the amount varies with the quality of the
pavement; and while no record of exact experiments
was available, it was estimated that from a well-paved
stone or asphalt pavement 80 per cent of the rain ran
off. From well-kept macadam or gravel roads from
30 to 50 per cent of the rain was obtained, and, inter-
polating for other pavements, for second-class side-
walks and stone pavements the discharge would be 60
per cent; for the best macadam, 50 per cent; and for
inferior macadam and gravel roads not more than 40
per cent would reach the sewers during a hard storm.
The proportion to be expected from the hard-earth
surfaces of streets and yards is evidently subject to
great variation, but it was assumed that it would be
20 per cent of the rain falling.
RELATION OF DENSITY TO PERCENTAGE.
6l



Correcting Table VI by these percentages of dis-
charge, and assuming, as indicated by the Rochester
studies, that the quantities of the different classes of
pavement were divided as given, in proportion to the
different densities, we obtain the following table:

Table VII.
fb'.;


TowL
Surfsce.
US Fully Impenious.
as


PcTsans


Cl«8.
Cla».
aS..
CL.*M.
Cluss.
dL.
