at the commencement of the tide, while the fresh water on the
surface is running in the opposite direction. After a time the
salt water becomes diffused in the fresh, so that the density
of the water in a river decreases as the distance from the sea
increases. The disposal of sewage discharged into a river is
due primarily to the mixing action which is taking place;
inasmuch as the tidal current which is the transporting agent
rarely flows more rapidly than from two to four miles per hour,
or, say, twelve to fifteen miles per tide. The extent to which
the suspended matter is carried back again up stream when the
current turns depends upon the quantity of upland water which
has flowed into the upper tidal part of the river during the
ebb tide, as this water occupies a certain amount of space,
according to the depth and width of the river, and thus
prevents the sea water flowing back to the position it occupied
on the previous tide, and carrying with it the matter in
suspension. The permanent seaward movement of sewage discharged
into the Thames at Barking when there is only a small quantity
of upland water is at the rate of about one mile per day,
taking thirty days to travel the thirty-one miles to the sea,
while at the mouth of the river the rate does not exceed one-
third of a mile per day.




CHAPTER IV.

SELECTION OF SITE FOR OUTFALL SEWER.