place is the period of time between new or full moon and the
occurrence of spring tides at that place. The range of a tide
is the height between high and low water of that tide, and the
rise of a tide is the height between high water of that tide
and the mean low water level of spring tides. It follows,
therefore, that for spring tides the range and rise are
synonymous terms, but at neap tides the range is the total
height between high and low water, while the rise is the
difference between high water of the neap tide and the mean low
water level of spring tides. Neither the total time occupied by
the flood and ebb tides nor the rate of the rise and fall are
equal, except in the open sea, where there are fewer disturbing
conditions. In restricted areas of water the ebb lasts longer
than the flood.

Although the published tide-tables give much detailed
information, it only applies to certain representative ports,
and even then it is only correct in calm weather and with a
very steady wind, so that in the majority of cases the engineer
must take his own observations to obtain the necessary local
information to guide him in the design of the works. It is
impracticable for these observations to be continued over the
lengthy period necessary to obtain the fullest and most
accurate results, but, premising a general knowledge of the
natural phenomena which affect the tides, as briefly described
herein, he will be able to gauge the effect of the various
disturbing causes, and interpret the records he obtains so as
to arrive at a tolerably accurate estimate of what may be
expected under any particular circumstances. Generally about 25
per cent. of the tides in a year are directly affected by the