visitor will spend most of the day out of doors, and therefore
cause less than the average quantity of water to be used for
house-cleansing purposes, in addition to which the bulk of the
soiled linen will not be washed in the town. An allowance of 10
gallons per head per day for the resident visitor and 5 gallons
per head per day for the trippers will usually be found a
sufficient provision.

It is, of course, well known that the flow of sewage varies
from day to day as well as from hour to hour, and while there
is no necessity to consider the daily variation--calculations
being based on the flow of the maximum day--the hourly
variation plays a most important part where storage of the
sewage for any length of time is an integral part of the
scheme. There are many important factors governing this
variation, and even if the most elaborate calculations are made
they are liable to be upset at any time by the unexpected
discharge of large quantities of trade wastes. With a small
population the hourly fluctuation in the quantity of sewage
flowing into the sewers is very great, but it reduces as the
population increases, owing to the diversity of the occupations
and habits of the inhabitants. In all cases where the
residential portions of the district are straggling, and the
outfall works are situated at a long distance from the centre
of the town, the flow becomes steadier, and the inequalities
are not so prominently marked at the outlet end of the sewer.
The rate of flow increases more or less gradually to the
maximum about midday, and falls off in the afternoon in the
same gradual manner. The following table, based on numerous
gaugings, represents approximately the hourly variations in the