_ and _d_ it has fallen correspondingly
low. When they act in opposition to each other, as at the moon's
quarter, there occurs a very low or neap-tide. In _figure 3_ the moon
has raised high tide at _a_ and _b_, but the sun has counteracted its
influence to some extent at _c_ and _d_, thus producing neap-tides,
which neither rise so high nor fall so low as do other tides. Tides
attain various elevations in different parts of the world, partly owing
to local influences. In the Bristol Channel the tide rises to nearly
sixty feet, while in the Mediterranean it is extremely small, owing to
the landlocked nature of that sea preventing the tidal wave from having
its full effect. Up some gulfs and estuaries the tides sweep with the
violence of a torrent, and any one caught by them on the shore would be
overtaken and drowned before he could gain the dry land. In the open
sea they rise and fall to an elevation of little more than three or four
feet.

The value of the tides is unspeakable. They sweep from our shores
pollution of every kind, purify our rivers and estuaries, and are
productive of freshness and health all round the world.

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The gentlemen here referred to are agreed as to the fact of systematic
arrangement of currents, though they differ in regard to some of the
causes thereof and other matters.

CHAPTER FOUR.

THE GULF STREAM--ITS NATURE--CAUSE--ILLUSTRATION--EFFECT OF SMALL POWERS
UNITED--ADVENTURES OF A PARTICLE OF WATER--EFFECT OF GULF STREAM ON
CLIMATE--ITS COURSE--INFLUENCE ON NAVIGATION--SARGASSO SEA--SCIENTIFIC
EFFORTS OF PRESENT DAY--WIND AND CURRENT CHARTS--EFFECTS ON COMMERCE--
CAUSE OF STORMS--INFLUENCE OF GULF STREAM ON MARINE ANIMALS.

Of the varied motions of the sea, the most important, perhaps, as well
as the most wonderful, is the Gulf Stream. This mighty current has been
likened by Maury to a "river in the ocean. In the severest droughts it
never fails, and in the mightiest floods it never overflows. Its banks
and its bottom are of cold water, while its current is of warm. It
takes its rise in the Gulf of Mexico (hence its name), and empties into
the arctic seas. Its current is more rapid than the Mississippi or the
Amazon, and its volume more than a thousand times greater."

This great current is of the most beautiful indigo-blue colour as far
out as the Carolina coasts; and its waters are so distinctly