[Forticsl-progetto] Iology_ for 1868. If the problem discussed in my addr

Tue Sep 21 02:20:03 CEST 2010

R their vigorous growth a temperature which must
not fall below 68 degrees Fahrenheit all the year round, and this
temperature is only to
be found within the distance on each side of
the equator which has been mentioned, or thereabouts. But even within
the coral zone this degree of warmth is not everywhere to be had. On
the west coast of America, and on the corresponding coast of Africa,
currents of cold water from the
icy regions which surround the South Pole set northward, and it appears
to be due to their cooling influence that the sea in these regions is
free from the reef builders. Again, the coral polypes cannot live in
water which is rendered brackish by floods from the land, or which is
perturbed by mud from the same source, and hence it is that they cease
to exist opposite the mouths of rivers, which damage them

in both these ways. Such is the general distribution of the
reef-building
corals, but there
are some very interesting and singular circumstances
to be observed in the conformation of the reefs, when we consider them
individually. The reefs, in fact, are
of three different kinds; some of them stretch out from the shore,
almost like a prolongation of the beach, covered
only by shallow water, and in the case of an island, surrounding it
like a fringe of no considerable breadth. These are termed "fringing
reefs." Others are separated by a channel which may attain

a width of many
miles, and a depth of twenty or thirty fathoms or more, from the
nearest land; and when this land is an island, the reef surrounds it
like a low wall, and the sea between the reef and the land is,

as it were, a moat
inside this wall. Such reefs as these are called "encircling" when
they surround an island; and "barrier" reefs, when they stretch
parallel

with the coast of a continent. In both these cases there is ordinary
dry land inside the reef, and separated
from it only by a narrower or a wider, a shallower or a deeper, space
of sea, which is called a "lagoon," or "inner passage."

But there is a third kind of reef, of very common
occurrence in the Pacific and Indian Oceans, which goes by the name of
an "Atoll." This is, to all intents and purposes, an encircling reef,
without anything to
encircle; or, in other
words, without an island in the middle of its
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