e. But
when we confine our attention to the tides at a single port, the
problem becomes at once a very intelligible one. Indeed, the
investigation of the tides is an easy subject, if we are contented
with a reasonably approximate solution; should, however, it be
necessary to discuss fully the tides at any port, the theory of the
method necessary for doing so is available, and a most interesting and
beautiful theory it certainly is.

Let us then speak for a few moments about the methods by which we can
study the tides at a particular port. The principle on which it is
based is a very simple one.

It is the month of August, the 18th, we shall suppose, and we are
going to enjoy a delicious swim in the sea. We desire, of course, to
secure a high tide for the purpose of doing so, and we call an almanac
to help us. I refer to the Thom's Dublin Directory, where I find the
tide to be high at 10h. 14m. on the morning of the 18th of August.
That will then be the time to go down to the baths at Howth or
Kingstown.

But what I am now going to discourse to you about is not the delights
of sea-bathing, it is rather a different inquiry. I want to ask, How
did the people who prepared that almanac know years beforehand, that
on that particular day the tide would be high at that particular hour?
How do they predict for every day the hour of high water? and how
comes it to pass that these predictions are invariably correct?

We first refer to that wonderful book, the _Nautical Almanac_. In that
volume the movements of the moon are set forth with full detail; and
among other particulars we can learn on page iv of every month the
mean time of the moon's meridian passage. It appears that on the day
in question the moon crossed the meridian at 11h. 23m. Thus we see
there was high water at Dublin at 10h. 14m., and 1h. 9m. later, that
is, at 11h. 23m., the moon crossed the meridian.

Let us take another instance. There is a high tide at 3.40 P.M. on the
25th August, and again the infallible _Nautical Almanac_ tells us that
the moon crossed the meridian at 5h. 44m., that is, at 2h. 4m. after
the high water.

In the first case the moon followed the tide in about an hour, and in
the second case the moon followed in about two hours. Now if we are to
be satisfied with a very rough tide rule for Dublin, we may say
generally that there is always a high tide an hour and a half before
the moon crosses the meridian. This would not be a very accur