ate
rule, but I can assure you of this, that if you go by it you will
never fail of finding a good tide to enable you to enjoy your swim. I
do not say this rule would enable you to construct a respectable
tide-table. A ship-owner who has to creep up the river, and to whom
often the inches of water are material, will require far more accurate
tables than this simple rule could give. But we enter into rather
complicated matters when we attempt to give any really accurate
methods of computation. On these we shall say a few words presently.
What I first want to do, is to impress upon you in a simple way the
fact of the relation between the tide and the moon.

To give another illustration, let us see how the tides at London
Bridge are related to the moon. On Jan. 1st, 1887, it appeared that
the tide was high at 6h. 26m. P.M., and that the moon had crossed the
meridian 56m. previously; on the 8th Jan. the tide was high at 0h.
43m. P.M., and the moon had crossed the meridian 2h. 1m. previously.
Therefore we would have at London Bridge high water following the
moon's transit in somewhere about an hour and a half.

I choose a day at random, for example--the 12th April. The moon
crosses the upper meridian at 3h. 39m. A.M., and the lower meridian at
4h 6m. P.M. Adding an hour and a half to each would give the high
tides at 5h 9m. A.M. and 5h. 36m. P.M.; as a matter of fact, they are
4h. 58m. A.M. and 5h. 20m. P.M.

But these illustrations are sufficient. We find that at London, in a
general way, high water appears at London Bridge about an hour and a
half after the moon has passed the meridian of London. It so happens
that the interval at Dublin is about the same, _i.e._ an hour and a
half; only that in the latter case the high water precedes the moon by
that interval instead of following it. We may employ the same simple
process at other places. Choose two days about a week distant; find on
each occasion the interval between the transit of the moon and the
time of high water, then the mean of these two differences will always
give some notion of the interval between high water and the moon's
transit. If then we take from the _Nautical Almanac_ the time of the
moon's transit, and apply to it the correction proper for the port,
we shall always have a sufficiently good tide-table to guide us in
choosing a suitable time for taking our swim or our walk by the
sea-side; though if you be the captain of a vessel, you will not be so
i