2
Bruce A. Warren
descriptive apparatus of water masses and their origins; and (5) the beginnings of
physics.
1. SURFACE CURRENTS
Knowledge of surface currents came from mariners’ observations of displacement
imparted to ships. In the ninth century, ca. 846, Ibn Khurrad¯ adhbih (1865, p. 293)
cited sailors’ information about the semiannual reversal of the zonal currents in the
northern Indian Ocean; later Arab geographers repeated his report (Warren, 1987).
However, they never had any sense of horizontal circulatory gyres, and conceived of
this phenomenon as a sort of annual tide, with high and low water alternating between
east and west. They may have been influenced in making this misinterpretation by
their physics teacher, Aristotle (1931, Book II, Chap. 1), who believed that the natural
direction of “flow” for water was downward, and therefore, at its lowest point, in the
ocean, it would merely be “swinging to and fro.”
Further known discoveries of major ocean currents did not occur until the
European voyages of exploration. In 1498 Columbus (1933, p. 38) found strong
westward flow along the north coast of Venezuela (the Caribbean Current?), and
supposed it to be part of a general westward movement of ocean water following
the heavens—a medieval and Renaissance idea perhaps elaborated from Aristotle’s
spheres rotating about a stationary earth. The Portuguese rounding southern Africa
at the end of the fifteenth century encountered the Agulhas Current, and Ponce de
Leon came upon the Gulf Stream in 1513. Reports by Portuguese pilots (Mota, 1972)
show that by the second quarter of the sixteenth century they had learned (somehow)
that the (South) Equatorial Current ran all the way from the Gulf of Guinea to the
Antilles. While Japanese writers and mapmakers from the seventeenth century onward
had identified segments of the Kuroshio as it passed through the island chains south
of Japan, nineteenth-century European cartographers seem to have been the first to
recognize it as a long, continuous stream (Kawai, 1998).
These were strong currents, met mostly close to shore, and so were detectible
with primitive methods of navigation. To disclose the predominantly zonal currents
in the open ocean far from land, however, required means to determine longitude
well, and both the method of lunar distances and accurate, sturdy chronometers did
not become available until the late eighteenth century. Even then the accuracy was
not great, only about half a degree. [The accuracy of celestial determinations of latitude was of order ten minutes of arc (and highly variable) in the sixteenth century
(various comparisons are given by Morison, 1971); it improved to somewhat better
than one minute in good observing conditions by mid-twentieth century, according to a retired sea-captain friend of mine.] With just a few fixes usually taken per
day (none at all under cloudy skies, such as hang over the Gulf Stream), and with
dead-reckoned estimates of displacement through the water of uncertain reliability,
Bruce A. Warren
descriptive apparatus of water masses and their origins; and (5) the beginnings of
physics.
1. SURFACE CURRENTS
Knowledge of surface currents came from mariners’ observations of displacement
imparted to ships. In the ninth century, ca. 846, Ibn Khurrad¯ adhbih (1865, p. 293)
cited sailors’ information about the semiannual reversal of the zonal currents in the
northern Indian Ocean; later Arab geographers repeated his report (Warren, 1987).
However, they never had any sense of horizontal circulatory gyres, and conceived of
this phenomenon as a sort of annual tide, with high and low water alternating between
east and west. They may have been influenced in making this misinterpretation by
their physics teacher, Aristotle (1931, Book II, Chap. 1), who believed that the natural
direction of “flow” for water was downward, and therefore, at its lowest point, in the
ocean, it would merely be “swinging to and fro.”
Further known discoveries of major ocean currents did not occur until the
European voyages of exploration. In 1498 Columbus (1933, p. 38) found strong
westward flow along the north coast of Venezuela (the Caribbean Current?), and
supposed it to be part of a general westward movement of ocean water following
the heavens—a medieval and Renaissance idea perhaps elaborated from Aristotle’s
spheres rotating about a stationary earth. The Portuguese rounding southern Africa
at the end of the fifteenth century encountered the Agulhas Current, and Ponce de
Leon came upon the Gulf Stream in 1513. Reports by Portuguese pilots (Mota, 1972)
show that by the second quarter of the sixteenth century they had learned (somehow)
that the (South) Equatorial Current ran all the way from the Gulf of Guinea to the
Antilles. While Japanese writers and mapmakers from the seventeenth century onward
had identified segments of the Kuroshio as it passed through the island chains south
of Japan, nineteenth-century European cartographers seem to have been the first to
recognize it as a long, continuous stream (Kawai, 1998).
These were strong currents, met mostly close to shore, and so were detectible
with primitive methods of navigation. To disclose the predominantly zonal currents
in the open ocean far from land, however, required means to determine longitude
well, and both the method of lunar distances and accurate, sturdy chronometers did
not become available until the late eighteenth century. Even then the accuracy was
not great, only about half a degree. [The accuracy of celestial determinations of latitude was of order ten minutes of arc (and highly variable) in the sixteenth century
(various comparisons are given by Morison, 1971); it improved to somewhat better
than one minute in good observing conditions by mid-twentieth century, according to a retired sea-captain friend of mine.] With just a few fixes usually taken per
day (none at all under cloudy skies, such as hang over the Gulf Stream), and with
dead-reckoned estimates of displacement through the water of uncertain reliability,