this body of water will have reached a point five-eighths of a
mile away when the discharge ceases; so that there will be a
virtual column of water of a total length of five-eighths of a
mile, in which is contained all that remains of the noxious
matters, travelling through the sea along the course of the
current. We see, further, that at a distance of three miles
away this column would only take thirty minutes to pass a given
point. The extent of this column of water will vary
considerably according to the tide and the time of discharge;
for instance, on a 22 ft tide, if the discharge starts one hour
after the turn of the current and continues for two hours, as
in the previous example, it will form a column four miles long,
whereas if it started two hours after the current, and
continued for the same length of time, the column would be six
miles and a half long, but the percentage of sewage in the
water would be infinitesimal.

[Illustration: Hours after turn of current FIG. 11]

In some cases it may be essential that the sewage should be
borne past a certain point before the current turns in order to
ensure that it shall not be brought back on the return tide to
the shore near the starting point. In other words, the sewage
travelling along the line of a branch current must reach the
junction on the line of the main current by a certain time in
order to catch the connection. Assuming the period of discharge
will be two hours, and that the point which it is necessary to
clear is situated three miles and a half from the outfall, the
permissible time to discharge the sewage according to the
height of the tide can be obtained from Fig. 11. Taking the 22