according to the height of the tide; it also indicates the
velocity of the current from time to time. It can be used in
several ways, but as this necessitates the assumption that with
tides of the same height the flow of the currents is absolutely
identical along the coast in the vicinity of the outfall, the
diagram should be checked as far as possible by any
observations that may be taken at other states of tides of the
same heights. Suppose we require to know how far a float will
travel if started at two hours after high water on a 12 ft
tide. From Fig. 10 we see that on a tide of this height the
current turns two hours and a quarter before the tide;
therefore two hours after high water will be four hours and a
quarter after the turn of the current. If the float were
started with the current, we see from Fig. 11 that it would
have travelled three miles in four hours and a quarter; and
subtracting this from four miles, which is its full travel on a
whole tide, we see that it will only cover one mile in the two
hours and a quarter remaining before the current turns to run
back again.

Although sewage discharged into the sea rapidly becomes so
diffused as to lose its identity, still occasionally the
extraneous substances in it, such as wooden matches, banana
skins, etc., may be traced for a considerable distance; so
that, as the sewage continues to be discharged into the sea
moving past the outfall, there is formed what may be described
as a body or column of water having possibilities of sewage
contamination. If the time during which sewage is discharged is
limited to two hours, and starts, say, at the turn of the
current on a 12 ft tide, we see from Fig. 11 that the front of