relation of direction of lines of pipe to slope.
Usually the slope of the ground is greater than the 5 inches per 100
feet just referred to, but by laying out the lines of pipe across the
slope instead of with it any grade desired may be obtained. Nor is it
necessary that these lines of draintile be run in straight lines; they
may very properly follow the curving slope, the proper grade being
always carefully maintained.

[Illustration: FIG. 69.--Plan of subsurface irrigation field.]

Common agricultural tiles three inches in diameter and costing about two
cents per running foot are suitable material for these distribution
lines. The sewage enters these distribution lines from a larger pipe,
usually six inches in diameter, and a difficult adjustment is presented
that each branch tile line shall receive its own proportionate share of
the sewage. If only one line of tile is provided, say 200 feet long for
5 members in the family, then all the sewage goes into that line with no
question of distribution arising, but if a number of short parallel
lines must be used, as shown in the sketch (Fig. 69), the difficulty of
subdividing the sewage properly among the different branch lines becomes
very great. For that reason the writer prefers to use not more than two
lines, with the possibility of delivering the sewage alternately in the
one and the other. In this way, the bed not receiving sewage is resting,
while the other bed is acting, and also the outlet for the sewage is
always definitely known. And particularly in the case of these
subsurface tile, the necessity for the intermittent dosing is apparent,
since with small, constant trickling discharges the difficulty of
distribution through the long length of tile is gradually increased, and
usually saturation of the soil occurs from joint to joint, as already
described. Therefore it becomes most necessary, in this case, for the
best results on the soil not merely to alternate the beds receiving
sewage, but also to effect the intermittent discharge onto the beds or
through the pipes although the sewage itself may flow very uniformly in
volume.

_Automatic syphon._

This intermittent discharge is accomplished by constructing on the pipe
line from the house and before it reaches the beds an "automatic
syphon," as it is called, the operation of which may be described as
follows: As the sewage enters the tank containing the syphon and rises
outside the syphon-bell, air is compr