of the Mediterranean, and the boldest maritime nation was able to
sail round Africa and find the way to India by sea. Then came voyages
to the northern waters of Europe, and far back in the Middle Ages
enterprising seamen crossed from Norway to Iceland and Greenland and
the north-eastern part of North America. They sailed straight across
the North Atlantic, and were thus the true discoverers of that ocean.

Even in antiquity the Greek geographers had assumed that the greater
part of the globe was covered by sea, but it was not till the beginning
of the modern age that any at all accurate idea arose of the extent of
the earth's great masses of water. The knowledge of the ocean advanced
with more rapid steps than ever before. At first this knowledge
only extended to the surface, the comparative area of oceans, their
principal currents, and the general distribution of temperature. In
the middle of the last century Maury collected all that was known,
and drew charts of the currents and winds for the assistance of
navigation. This was the beginning of the scientific study of the
oceanic waters; at that time the conditions below the surface were
still little known. A few investigations, some of them valuable, had
been made of the sea fauna, even at great depths, but very little
had been done towards investigating the physical conditions. It
was seen, however, that there was here a great field for research,
and that there were great and important problems to be solved; and
then, half a century ago, the great scientific expeditions began,
which have brought an entire new world to our knowledge.

It is only forty years since the Challenger sailed on the first
great exploration of the oceans. Although during these forty years
a quantity of oceanographical observations has been collected with a
constant improvement of methods, it is, nevertheless, clear that our