form of covered channel, should lead far
enough from any public road to avoid offence. From this point it may be
led by open gutters to the land over which it is to be spread,--or
rather through such a system of surface gutters as will enable us to
deliver it at different parts of the field, according to the
requirements of the crops, and so as to use fresh land at frequent
intervals, leaving that which has been saturated to the purifying
processes of vegetation and atmospheric action.

The gutters having been made, it is easy, by the use of portable
dams,--of thin boiler-iron, like broad shovels,--which may be set in
the course of the flow, to divert the current into any branch channel,
or to stop it at any desired part of this channel. All the gutters
having sufficient descent to lead the sewage rapidly forward, it is
usual to set a dam near the far end of the gutter, and allow the sewage
to overflow and run down over the surface until it has reached as far as
the formation of the ground and the quantity of the liquid will allow it
to spread. This portion having received its due amount of the liquid,
the dam is moved to a higher point, and the overflow is allowed to
spread over a second area. In this way, step by step, we irrigate all
that may be reached by a single gutter. Then the moving of the dam in
the main line turns the water into another gutter, and this is proceeded
with in like manner. In practice it is found best to begin the overflow
at the farthest end of the lowest-lying gutter, working back step by
step until the higher parts of the field are reached. It would be better
that there should be land enough to require the irrigation of any given
area not oftener than once in one or two weeks. The amount required for
a given population cannot be determined by any fixed rule,--so much
depending on the amount of water used _per capita_, and on the
absorptive character of the irrigated soil. In the case of villages, one
acre to each five hundred of the population would generally be found
ample.

There are several instances of the successful use of a much smaller area
than is here indicated, by the use of intermittent downward filtration.
The most noted success in this direction is that at Merthyr-Tydvil in
Wales, a large mining town, where the allowance is only one acre to each
two thousand of the population. There are two filter-beds of light loam
over a gravelly subsoil thoroughly underdrained with tiles at