essed between the water surface
inside the bell and the water left inside the syphon-leg. With greater
and greater height of water outside, this compression inside becomes
greater and forces the water in the syphon-leg lower and lower. Finally,
the water sinks so low as to allow the compressed air to escape suddenly
around this bend, instantly relieving the compression, and the water
outside rushing in to fill up the space occupied by the air starts the
syphon (see Fig. 70).

[Illustration: FIG. 70.--Section of "Miller" syphon.]

This syphon, in size suitable for a single house, costs about $12
delivered, and will always be available to secure an intermittent dosing
of the bed or pipe line. Usually the chamber in which this syphon is
placed holds about one hour's flow, so that it may be estimated that
this syphon will discharge on the bed every sixty minutes. The exact
interval of time is not essential nor, perhaps, important, although it
may be noted that the coarser the material,--that is, the nearer uniform
all the sand particles are to the largest size passing the ten-mesh
size,--the smaller must be the dose applied, but the more frequently
must the application be made. This has been very thoroughly studied in
Massachusetts, and the views of experts on this subject may be found in
the report of that Board.

Such an intermittent discharge may be made and often is made by a hand
valve leading out from this chamber in institutions or in private houses
where some one constantly is available for the purpose. Thus it becomes
the duty of the man in charge every hour or perhaps three times a day to
pull the valve and allow the sewage to discharge (see Fig. 71). An
overflow pipe should always be provided, so that if he forgets to pull
the valve, the sewage will still find its way into the system rather
than out on the ground.

[Illustration: FIG. 71.--Plan and section of a septic tank with valve.]

_Sedimentation._

As a matter of economy of operation, it has been found desirable to take
out from the sewage before the treatment already described as much of
the solid matter as may be reasonably done, and for this purpose
sedimentation is made use of. Most of the solids in sewage are slightly
heavier than water, so that if they be allowed to stand in the water for
a short length of time, they will settle to the bottom of the tank and
allow the liquid above to pass on, considerably clarified. It has been
found worth whil