wind, etc., the majority varying from 6 in to 12 in in height
and from five to fifteen minutes in time. The effect of a
moderately stiff gale is approximately to raise a tide as many
inches as it might be expected to rise in feet under normal
conditions. The Liverpool tide-tables are based on observations
spread over ten years, and even longer periods have been
adopted in other places.

Much valuable information on this subject is contained in the
following books, among others--and the writer is indebted to
the various authors for some of the data contained in this and
subsequent chapters--"The Tides," by G. H. Darwin, 1886;
Baird's Manual of Tidal Observations, 1886; and "Tides and
Waves," by W. H. Wheeler, 1906, together with the articles in
the "Encyclopaedia Britannica" and "Chambers's Encyclopaedia."




Chapter II

Observations of the rise and fall of tides.


The first step in the practical design of the sewage works is
to ascertain the level of high and low water of ordinary spring
and neap tides and of equinoctial tides, as well as the rate of
rise and fall of the various tides. This is done by means of a
tide recording instrument similar to Fig. 4, which represents
one made by Mr. J. H. Steward, of 457, West Strand, London,