LECTURES AND ESS A YS 



jealousy of physicists as to the accep- 

 tance of so-called a priori truths. Aris- 

 totle's errors of detail, as shown by 

 Eucken and Lange, were grave and 

 numerous. He affirmed that only in 

 man we had the beating of the heart, 

 that the left side of the body was colder 

 than the right, that men have more teeth 

 than women, and that there is an empty 

 space at the back of every man's head. 



There is one essential quality in physical 

 conceptions which was entirely wanting 

 in those of Aristotle and his followers 

 a capability of being placed as coherent 

 pictures before the mind. The Germans 

 express the act of picturing by the word 

 vorstellen, and the picture they call 

 a Vorstellung. We have no word in 

 English which comes nearer to our 

 requirements than Imagination ; and, 

 taken with its proper limitations, the 

 word answers very well. But it is tainted 

 by its associations, and therefore objec- 

 tionable to some minds. Compare, with 

 reference to this capacity of mental 

 presentation, the case of the Aristotelian, 

 who refers the ascent of water in a pump 

 to Nature's abhorrence of a vacuum, 

 with that of Pascal when he proposed 

 to solve the question of atmospheric 

 pressure by the ascent of the Puy de 

 Dome. In the one case the terms of 

 the explanation refuse to fall into place 

 as a physical image ; in the other the 

 image is distinct, the descent and rise 

 of the barometer being clearly figured 

 beforehand as the balancing of two 

 varying and opposing pressures. 



\ DURING the drought of the Middle 

 ) Ages in Christendom, the Arabian in- 

 tellect, as forcibly shown by Draper, was 

 active. With the intrusion of the Moors 

 into Spain, order, learning, and refine- 

 ment took the place of their opposites. 

 When smitten with disease, the Christian 

 peasant resorted to a shrine, the Moorish 

 one to an instructed phys*ician. The 

 Arabs encouraged translations from the 

 Greek philosophers, but not from the 



Greek poets. They turned in disgust 

 "from the lewdness of our classical 

 mythology, and denounced as an un- 

 pardonable blasphemy all connection 

 between the impure Olympian Jove and 

 the Most High God." Draper traces 

 still farther than Whewell the Arab 

 elements in our scientific terms. He 

 gives examples of what Arabian men of 

 science accomplished, dwelling particu- 

 larly on Alhazen, 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 sun 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 58^ miles. In the Book of 

 tfie 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 differ- 

 ently 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 steel- 

 yards. 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 deter- 

 minations 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 



