enever a place is reached by two channels of different length,
its tides are sure to be peculiar, and probably small.

Another cause of small tide is the way the wave surges to and fro in a
channel. The tidal wave surging up the English Channel, for instance,
gets largely reflected by the constriction at Dover, and so a crest
surges back again, as we may see waves reflected in a long trough or
tilted bath. The result is that Southampton has two high tides rapidly
succeeding one another, and for three hours the high-water level varies
but slightly--a fact of evident convenience to the port.

Places on a nodal line, so to speak, about the middle of the length of
the channel, have a minimum of rise and fall, though the water rushes
past them first violently up towards Dover, where the rise is
considerable, and then back again towards the ocean. At Portland, for
instance, the total rise and fall is very small: it is practically on a
node. Yarmouth, again, is near a less marked node in the North Sea,
where stationary waves likewise surge to and fro, and accordingly the
tidal rise and fall at Yarmouth is only about five feet (varying from
four and a half to six), whereas at London it is twenty or thirty feet,
and at Flamborough Head or Leith it is from twelve to sixteen feet.

It is generally supposed that water never flows up-hill, but in these
cases of oscillation it flows up-hill for three hours together. The
water is rushing up the English Channel towards Dover long after it is
highest at the Dover end; it goes on piling itself up, until its
momentum is checked by the pressure, and then it surges back. It
behaves, in fact, very like the bob of a pendulum, which rises against
gravity at every quarter swing.

To get a very large tide, the place ought to be directly accessible by a
long sweep of a channel to the open ocean, and if it is situate on a
gradually converging opening, the ebb and flow may be enormous. The
Severn is the best example of this on the British Isles; but the largest
tides in the world are found, I believe, in the Bay of Fundy, on the
coast of North America, where they sometimes rise one hundred and twenty
feet. Excessive or extra tides may be produced occasionally in any place
by the propelling force of a high wind driving the water towards the
shore; also by a low barometer, _i.e._ by a local decrease in the
pressure of the air.

Well, now, leaving these topographical details concerning tides, whic