and dense soils. The reason is manifest. In a
gravel soil, the sewage is at first rapidly absorbed, so that as the
sewage goes down the pipe line the first joints take up the water and
deliver it to the soil, where it disappears, and probably no flow
reaches the end of the line at all. This means that the soil surrounding
the first joints does the work which the entire pipe line was intended
to do and thus becomes overworked. When overworked, the soil always
refuses to do anything, so that when the succeeding joints take up the
sewage and in their turn become overworked, the line is useless. If, on
the other hand, the grade had been steep enough to carry the sewage down
the pipe line gradually so as to secure a uniform distribution, then the
same or approximately the same amount of sewage would be taken out of
the pipe at each joint, securing a long life for the system. In loamy
soil, on the contrary, there is not the same absorption at the joints,
and so on a steep grade there is the tendency for all the sewage to
follow down the pipe line to the lower end and there escape to clog the
soil and thus spoil the system. As a general average, it may be said
that the proper grade for such a subsurface distribution pipe line in a
fairly good sandy loam should be 5 inches in 100 feet; less than this as
the loam becomes clay and more as the loam becomes gravel.

The other essential point for the successful operation of this method
of distribution is to provide a proper length of pipe for the number of
persons contributing sewage. The soil itself will absorb about the same
amount as when the sewage is spread over the surface, so that a family
of ten persons would require, as before, an area about 70 feet square.
The pipe lines may be laid in different sections, provided the different
lines of pipe are not nearer together than 10 feet. On an area 70 feet
square there would be, therefore, 7 lines of pipe each 70 feet long, or
490 lineal feet of pipe in all, or 49 feet per person. The writer
generally allows 40 feet in well-cultivated soil as a reasonable length
of pipe for each person in the family. If the soil is sandy, this may be
reduced one half, but need not be increased under any conditions, since
a soil requiring a greater length of pipe than 40 feet per person would
be so dense as to be unfit for use. To properly arrange the lines of
pipe on a sloping ground requires careful study of the inclination of
the ground and of the