scharge passengers and cargo without further trouble, and with no need
for docks or gates. However, rivers like the St. Lawrence and the Hudson
are the natural property of a gigantic continent; and we in England may
be well contented with the possession of such tidal estuaries as the
Mersey, the Thames, and the Humber. That by pertinacious dredging the
citizens of Glasgow manage to get large ships right up their small
river, the Clyde, to the quays of the town, is a remarkable fact, and
redounds very highly to their credit.

We will now proceed to consider the connection existing between the
horizontal rush of water and its vertical elevation, and ask, Which is
cause and which is effect? Does the elevation of the ocean cause the
tidal flow, or does the tidal flow cause the elevation? The answer is
twofold: both statements are in some sense true. The prime cause of the
tide is undoubtedly a vertical elevation of the ocean, a tidal wave or
hump produced by the attraction of the moon. This hump as it passes the
various channels opening into the ocean raises their level, and causes
water to flow up them. But this simple oceanic tide, although the cause
of all tide, is itself but a small affair. It seldom rises above six or
seven feet, and tides on islands in mid-ocean have about this value or
less. But the tides on our coasts are far greater than this--they rise
twenty or thirty feet, or even fifty feet occasionally, at some places,
as at Bristol. Why is this? The horizontal motion of the water gives it
such an impetus or momentum that its motion far transcends that of the
original impulse given to it, just as a push given to a pendulum may
cause it to swing over a much greater arc than that through which the
force acts. The inrushing water flowing up the English Channel or the
Bristol Channel or St. George's Channel has such an impetus that it
propels itself some twenty or thirty feet high before it has exhausted
its momentum and begins to descend. In the Bristol Channel the gradual
narrowing of the opening so much assists this action that the tides
often rise forty feet, occasionally fifty feet, and rush still further
up the Severn in a precipitous and extraordinary hill of water called
"the bore."

Some places are subject to considerable rise and fall of water with very
little horizontal flow; others possess strong tidal races, but very
little elevation and depression. The effect observed at any given place
entirely dep