Differences in level, produced by high tides, cause currents which vary
in force and direction with the condition of the tide, producing, in
some cases, dangerous whirlpools. The famous Maelstrom, off the coast of
Norway, is but a tidal current, which rushes with great violence between
two of the Lofoden Islands, causing a whirling motion in the water which
is reversed at each ebb and flow of the tide.
Such is, also, the famous whirlpool of Charybdis, in the Straight of
Messina, and many others of less note. The powerful currents of Hell
Gate, in the passage from Long Island Sound to New York Bay, are due to
a similar cause, high water occurring at different hours in the bay and
in the west end of the sound.