out; and so expeditious is the machine, that the tides of a port for
an entire year can be completely worked out in a couple of hours.

While the student or the philosopher who seeks to render any account
of the tide on dynamical grounds is greatly embarrassed by the
difficulties introduced by friction, we, for our present purpose in
the study of the great romance of modern science opened up to us by
the theory of the tides, have to welcome friction as the agent which
gives to the tides their significance from our point of view.

There is the greatest difference between the height of the rise and
fall of the tide at different localities. Out in mid-ocean, for
instance, an island like St. Helena is washed by a tide only about
three feet in range; an enclosed sea like the Caspian is subject to no
appreciable tides whatever, while the Mediterranean, notwithstanding
its connection with the Atlantic, is still only subject to very
inconsiderable tides, varying from one foot to a few feet. The
statement that water always finds its own level must be received, like
many another proposition in nature, with a considerable degree of
qualification. Long ere one tide could have found its way through the
Straits of Gibraltar in sufficient volume to have appreciably affected
the level of the great inland sea, its effects would have been
obliterated by succeeding tides. On the other hand, there are certain
localities which expose a funnel-shape opening to the sea; into these
the great tidal wave rushes, and as it passes onwards towards the
narrow part, the waters become piled up so as to produce tidal
phenomena of abnormal proportions. Thus, in our own islands, we have
in the Bristol Channel a wide mouth into which a great tide enters,
and as it hurries up the Severn it produces the extraordinary
phenomenon of the Bore. The Bristol Channel also concentrates the
great wave which gives Chepstow and Cardiff a tidal range of
thirty-seven or thirty-eight feet at springs, and forces the sea up
the river Avon so as to give Bristol a wonderful tide. There is hardly
any more interesting spot in our islands for the observation of tides
than is found on Clifton Suspension Bridge. From that beautiful
structure you look down on a poor and not very attractive stream,
which two hours later becomes transformed into a river of ample
volume, down which great ships are navigated. But of all places in the
world, the most colossal tidal phenomena are those