456 FRAGMENTS OF SCIENCE. 
Jove and the Most High God." Draper traces still further 
than Whewell the Arab elements in our scientific terms. 
He gives examples of what Arabian men of science accom- 
plished, dwelling particularly on A]hazen, who was the 
first to correct the Platonic notion that rays of light are 
emitted by the eye. Alhazen discovered atmospheric 
refraction, and showed that we see the snn and the moon 
after they have set. He explained the enlargement of the 
sun and moon, and the shortening of the vertical diameters 
of both these bodies when near the horizon. He was aware 
that the atmosphere decreases in density with increase of 
elevation, and actually fixed its height at fifty-eight and 
one-half miles. In the " Book of the Balance of Wisdom/' 
he sets forth the connection between the weight of the 
atmosphere 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 center of gravity, and applied it to the investigation of 
balances and steelyards. He recognized 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 determi- 
nations 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 his- 
toric 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." * 
The strain upon the mind during the stationary period 
toward 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 
*" Intellectual Development of Europe," p. 359 
