phere and its
increasing density. He shows that a body will weigh differently in a
rare and dense atmosphere, and he considers the force with which
plunged bodies rise through heavier media. He understood the doctrine
of the centre of gravity, and applied it to the investigation of
balances and steelyards. He recognised gravity as a. force, though
he fell into the error of assuming it to diminish simply as the
distance, and of making it purely terrestrial. He knew the relation
between the velocities, spaces, and times of falling bodies, and had
distinct ideas of capillary attraction. He improved the hydrometer.
The determinations of the densities of bodies, as given by Alhazen,
approach very closely to our own. 'I join,' says Draper, 'in the
pious prayer of Alhazen, that in the day of judgment the All-Merciful
will take pity on the soul of Abur-Raihan, because he was the first of
the race of men to construct a table of specific gravities.' If all
this be historic truth (and I have entire confidence in Dr. Draper),
well may he 'deplore the systematic manner in which the literature of
Europe has, contrived to put out of sight our scientific obligations
to the Mahommedans.' [Footnote: Intellectual Development of Europe,'
p. 359.]

The strain upon the mind during the stationary period towards
ultra-terrestrial things, to the neglect of problems close at hand,
was sure to provoke reaction. But the reaction was gradual; for the
ground was dangerous, and a power was at hand competent to crush the
critic who went too far. To elude this power, and still allow
opportunity for the expression of opinion, the doctrine of 'two-fold
truth' was invented, according to which an opinion might be held
'theologically,' and the opposite opinion 'philosophically.'
[Footnote: 'Lange,' 2nd edit. pp. 181, 182.] Thus, in the thirteenth
century, the creation of the world in six days, and the
unchangeableness of the individual soul, which had been so distinctly
affirmed by St. Thomas Aquinas, were both denied philosophically, but
admitted to be true as articles of the Catholic faith. When
Protagoras uttered the maxim which brought upon him so much
vituperation, that 'opposite assertions are equally true,' he simply
meant to affirm men's differences to be so great, that what was
subjectively true to the one might be subjectively untrue to the
other. The great Sophist never meant to play fast and loose with the
truth by saying that one