et beneath the surface of the sea; a clear proof that
the current exceeds that depth. On arriving near the Azores, the stream
widens, and overflows, as it were, forming a large expanse of warm water
in the centre of the North Atlantic, over a space of 200 or 300 miles
from north to south, and having a temperature of from 8 degrees to 10
degrees Fahr. above the surrounding ocean. The whole area, covered by
the Gulf water, is estimated by Rennell at 2000 miles in length, and, at
a mean, 350 miles in breadth; an area more extensive than that of the
Mediterranean. The warm water has been sometimes known to reach the Bay
of Biscay, still retaining five degrees of temperature above that of the
adjoining ocean; and a branch of the Gulf current occasionally drifts
fruits, plants, and wood, the produce of America and the West Indies, to
the shores of Ireland and the Hebrides.

From the above statements we may understand why Rennell has
characterized some of the principal currents as oceanic rivers, which he
describes as being from 50 to 250 miles in breadth, and having a
rapidity exceeding that of the largest navigable rivers of the
continents, and so deep as to be sometimes obstructed, and occasionally
turned aside, by banks, the tops of which do not rise within forty,
fifty, or even one hundred fathoms of the surface of the sea.[384]

_Greatest velocity of currents._--The ordinary velocity of the principal
currents of the ocean is from one to three miles per hour; but when the
boundary lands converge, large bodies of water are driven gradually into
a narrow space, and then wanting lateral room, are compelled to raise
their level. Whenever this occurs their velocity is much increased. The
current which runs through the Race of Alderney, between the island of
that name and the main land, has a velocity of about eight English miles
an hour. Captain Hewett found that in the Pentland Firth, the stream, in
ordinary spring tides, runs ten miles and a half an hour, and about
thirteen miles during violent storms. The greatest velocity of the tidal
current through the "Shoots" or New Passage, in the Bristol Channel, is
fourteen English miles an hour; and Captain King observed, in his survey
of the Straits of Magellan, that the tide ran at the same rate through
the "First Narrows," and about eight geographical miles an hour, in
other parts of those straits.

_Causes of currents._--That movements of no inconsiderable magnitude
should be