152 FEAGMENTS OF SCIENCE. 



which plunged bodies rises 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 capil- 

 lary 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-Kaihan, because he was the first of 

 the race of men to construct a table of specific gravi- 

 ties/ 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 obliga- 

 tions to the Mahommedans.'* 



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 in- 

 vented, according to which an opinion might be held 

 'theologically/ and the opposite opinion 'philosophic- 

 ally.'! 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 af- 



* ' Intellectual Development of Europe,' p. 359. 

 f ' Lange,' 2nd edit., pp. 181, 182. 



