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could be expanded to measure the saltiness of the oceans, and this, he
said, had implications for the age of the Earth. It was a clever idea, no
doubt formulated on account of his interest in the hydrological cycle.
However, it does have a number of flaws. It would be difficult to find
closed lakes, although some do exist: Halley suggested that the
Caspian Sea and Lake Titicaca could be examined. The largest such
closed system is the Aral Sea in southern Russia, but today this is
shrinking as more water is drawn from it than enters it. Halley's
method did not take into consideration salts washed into the under-
lying soils and sediments on which the lakes sat and therefore
removed from the 'closed' system. The scheme would also take a
long time: it was unlikely that instruments were precise enough at
the time to be able to distinguish salt concentrations in two water
samples taken from the same place ten years apart. Halley knew this
and regretted that the Egyptians or Greeks had not measured the
oceanic salt concentrations two thousand years earlier. Hedging his
bets and being naturally somewhat cautious, given the power of the
Church, Halley noted that his scheme would yield a maximum age for
the Earth, but said that 'the World may be found much older than
some have imagined'.
In 1724, in a further paper delivered before the Royal Society,
Halley tackled the question of the Flood, or Deluge as it was often
called at the time, and its effects on the Earth. Given his interests in
comets, it was natural for him to invoke one to form mountains. When
the Earth was struck by a passing comet, Halley argued, the collision
caused the sea to move landwards, rather like modern-day tsunamis or
giant tidal waves, carrying marine sediments which were deposited in
great piles on the terrestrial surface. As the waters flowed back into
the oceans the sedimentary piles were left behind as mountains.
Halley's ideas on salt-clocks and time were soon forgotten and
only resurrected in 1910 by the American geochronologer George F.
Becker, at a time when it was thought that salt and the oceans held the
key to unlocking the secret of the Earth's true age. This episode is
discussed further in Chapter 12 .
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