 The next major step in aviation measurement came from the
Greeks about 2500 years ago. The Greeks sought rules for the way
number-ideas seemed to work. They applied a reasoning process
to build on known facts to reach a conclusion. They knew it as
deduction. Some flyers call it, albeit incorrectly, the origin
of the term Dead Reckoning. Actually it is a deductive system
of navigation. The first ship's time was kept with sand glass
and the speed was determined by counting oar strokes during a
sand glass sequence. Distance over water could be deduced by the
number of strokes in a given time period.
A sailing ship's speed over a nautical a mile was, historically,
measured by means of a knotted (knots) rope tied to a log.
A sand filled timing glass would be used to measure the time from
leaving the log dead (much as a dead man might appear) in the
water (dead reckoning) and the number of evenly spaced
knots passed along the rope. All of this would be recorded in
the logbook.
I contend that dead reckoning is a not a corruption of deduced
reckoning, the term derived from the navigational practice
of starting from a point (Buoy) that was dead in the water.
From this point the direction and time would be used to deduce
location along the route as it crossed longitudinal lines.
Since the chronometer was yet to be invented, sailors had no way
to determine longitude except by this dead reckoning. Within crude
limits, speed and compass indications could be used to determine
estimated distance and estimated longitude. Magellan in 1519 had
access to charts, globe, theodolites, quadrants, compasses, magnetic
needles, hour glasses, and timepieces. He was unable to determine
exact longitude.
By the 18th Century a chronometer (first weighed over 36 pounds)
was used to get longitude. A chronometer differs from a clock
or watch because it has a temperature adjustment for greater accuracy.
Captain Cook in 1768 had three such clocks for his voyage. In
1779 he sailed with 4 chronometers and a nautical almanac which
enabled him to determine longitude. The very first effort to make
a calculator was financed by the British to make the making of
the nautical almanac easier. The original design was completed
in 1991 and found to work accurately. Interesting to speculate
where the world would be had it been completed in the 1700s.
This ability was treated as a military secret by England to
the extent that Mr. Harrison was a very old man before his family
was rewarded by the English government. This single invention
enabled England to become the master of the world seas for several
hundred years. Knowledge is power. If you have not yet read a
small pocketbook "Longitude", you should.
Written by Gene Whitt
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