mprudent as to enter port without taking counsel of the accurate
tide-tables, for which we are indebted to the Admiralty.

Every one who visits the sea-side, or who lives at a sea-port, should
know this constant for the tides, which affect him and his movements
so materially. If he will discover it from his own experience, so much
the better.

The first point to be ascertained is the time of high water. Do not
take this from any local table; you ought to observe it for yourself.
You will go to the pier head, or, better still, to some place where
the rise and fall of the mere waves of the sea will not embarrass you
in your work. You must note by your watch the time when the tide is
highest. An accurate way of doing this will be to have a scale on
which you can measure the height at intervals of five minutes about
the time of high water. You will then be able to conclude the time at
which the tide was actually at its highest point; but even if no
great accuracy be obtainable, you can still get much interesting
information, for you will without much difficulty be right within ten
minutes or a quarter of an hour.

The correction for the port is properly called the "establishment,"
this being the average time of high water on the days of full and
change of the moon at the particular port in question.

We can considerably amend the elementary notion of the tides which the
former method has given us, if we adopt the plan described by Dr.
Whewell in the first four editions of the _Admiralty Manual of
Scientific Inquiry_. We speak of the interval between the transit of
the moon and the time of high water as the luni-tidal interval. Of
course at full and change this is the same thing as the establishment,
but for other phases of the moon the establishment must receive a
correction before being used as the luni-tidal interval. The
correction is given by the following table--

Hour of Moon's transit after Sun:

0| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11
-+----+----+----+----+----+----+----+----+----+----+----
0|-20m|-30m|-50m|-60m|-60m|-60m|-40m|-10m|+10m|+20m|+10m

Correction of establishment to find luni-tidal interval:

Thus at a port where the establishment was 3h. 25m., let us suppose
that the transit of the moon took place at 6 P.M.; then we correct the
establishment by -60m., and find the luni-tidal interval to be 2h.
25m., and accordingly the high water takes place at 8h. 25m. P.M.

But even this met