Moving the Waters - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 91

energy to Oceanus at the surface. Later, Aristotle dismissed the idea of
subterranean caverns, but did not come much closer to establishing
how ocean waters moved. Perhaps it was something to do with the
way the rivers flowed into them, he speculated. 63
Aristotle's interpretation of the world, together with the dogma of
the Church, dominated—or perhaps stifled—Western scientific enquiry
for much of the next millennium. When these philosophical shackles
were eventually loosened, it was the practical observations of sailors
that yielded clues as to the movement of the ocean waters. For
instance, in the first transatlantic journeys it became clear that the
journey across from North America to Europe was easier and shorter
than the return voyage. Going westwards, the early ships were often
pushing against a current that was trying to drive them back home.
The journey could take more than two weeks longer in this direction
than when coming back from America, although some learned to
take other routes to avoid the delaying factor. Then there was the
strange way in which the ocean temperature changed from shallow to
deep waters. Around the equator the surface waters often reach a
comfortable 30 degrees Celsius or so. However, when buckets or large
bottles were tossed overboard on the end of a rope, allowed to sink to
100 metres or so, and hauled back, the deeper waters were shown to
be much colder, the temperature having plummeted to 10 degrees
Celsius or less. These early sailors had found a temperature structure
that we now call the thermocline. The first record of this seems to
have been made by a slave-trading ship in 1751. The captain, one Henry
Ellis, was surprised 64 to find that below the Sun-warmed surface layer
of water—even in the tropics—the water is icy cold, and the cold
extends right to the ocean floor. He was pleased, though: in that
burning climate, it allowed him to chill his wine to an agreeable
temperature.