results obtained and the proper interpretation of them, by
estimating, and mentally eliminating, any disturbing
influences, such as wind, etc. Care must also be taken in
noting the height of the tide and the relative positions of the
sun, moon, and earth at the time of making the observations,
and in estimating from such information the extent to which the
tides and currents may vary at other times when those bodies
are differently situated.

It is obvious that if the levels of the sewers and other
circumstances are such that the sewage can safely be discharged
at low water, and the works are to be constructed accordingly,
it is most important to have accurate information as to the
level of the highest low water which may occur in any ordinary
circumstances. If the level of a single low water, given by a
casual observation, is adopted without consideration of the
governing conditions, it may easily be that the tide in
question is a low one, that may not be repeated for several
years, and the result would be that, instead of having a free
outlet at low water, the pipe would generally be submerged, and
its discharging capacity very greatly reduced.

The run of the currents will probably differ at each of the
points under consideration, so that if one point were selected
the best result would be obtained by discharging the sewage at
high water and at another point at low water, whereas at a
third point the results would show that to discharge there
would not be satisfactory at any stage of the tide unless the
sewage were first partially or even wholly purified. If these
results are considered in conjunction with the levels of the