the ba

lance.

Theory of tre of gravity. He applies it to the investigation of balances and steelyards, showing the relations between the centre of gravity and the centre of suspension-when those instruments will set and when they will vibrate. He recognizes gravity as a force; asserts that it diminishes with the distance; but falls into the mistake that the diminution is as the distance, and

Gravity;

capillary

meter.

Tables of specific gravities.

not as its square. He considers gravity as terrestrial, and fails to perceive that it is universal,—that was reserved for Newton. He knows correctly the relation between the veloattraction; cities, spaces, and times of falling bodies, and has very distinct the hydro- ideas of capillary attraction. He improves the construction of that old Alexandrian invention, the hydrometer,—the instrument which, in a letter to his fair but pagan friend Hypatia, the good Bishop of Ptolemais, Synesius, six hundred years before, requests her to have made for him in Alexandria, as he wishes to try the wines he is using, his health being a little delicate. The determinations of the densities of bodies, as given by Alhazen, approach very closely to our own; in the case of mercury they are even more exact than some of those of the last century. I join, as, doubtless, all natural philosophers will do, in the pious prayer of Alhazen, that, in the day of judgment, the All-Merciful will take pity on the soul of Abur-Raihân, because he was the first of the race of men to construct a table of specific gravities; and I will add Albazen's name thereto, for he was the first to trace the curvilinear path of a ray of light through the air. Though more than seven centuries part him from our times, the physiologists of this age may accept him as their compeer, since he received and defended the doctrine, now forcing its way, of the progressive The theory developement of animal forms. He upheld the affirmation of ment of those who said that man, in his progress, passes through a definite succession of states; not, however, "that he was once a bull, and was then changed to an ass, and afterward into a horse, and after that into an ape, and finally became a man.” This, he says, is only a misrepresentation by "common people" of what is really meant. The "common people" who withstood Alhazen have representatives among us, themselves the only example in the Fauna of the world of that non-develope

of develope

organisms.

ment which they so loudly affirm. At the best they are only passing through some of the earlier forms of that series of transmutations to which the devout Mohammedan in the above quotation alludes.

La- The pendu

lum clock.

Ebn Junis.

The Arabians, with all this physical knowledge, do not appear to have been in possession of the thermometer, though they knew the great importance of temperature measures, employing the areometer for that purpose. They had detected the variation in density of liquids by heat, but not the variation in volume. In their measures of time they were more successful; they had several kinds of clepsydras. A balance clepsydra is described in the work from which I am quoting. But it was their great astronomer, Ebn Junis, who accomplished the most valuable of all chronometric improvements. He first applied the pendulum to the measure of time. place, in the fifth note to his 'Système du Monde,' avails himself of the observations of this philosopher, with those of Albategnius and other Arabians, as incontestable proof of the diminution of the eccentricity of the earth's orbit. He states, more- Astronomiover, that the observation of Ebn Junis of the obliquity of the cal works of ecliptic, properly corrected for parallax and refraction, gives for the year A.D. 1000 a result closely approaching to the theoretical. He also mentions another observation of Ebn Junis, October 31, A.D. 1007, as of much importance in reference to the great inequalities of Jupiter and Saturn. I have already The Arabic remarked that, in the writings of this great Arabian, the Arabic numerals and our common arithmetical processes are currently used. From Africa and Spain they passed into Italy, finding ready acceptance among commercial men, who recognized at once their value, and, as William of Malmesbury says, being a wonderful relief to the "sweating calculators ;" an epithet of which the correctness will soon appear to any one who will try to do a common problem in multiplication or division by the aid of the old Roman numerals. It is said that Gerbert-Pope Sylvester-was the first to introduce a knowledge of them into Europe; he had learned them at the Mohammedan university of Cordova. It is in allusion to the cipher, which, placed after any of the other digits, increases

numerals.

Arabian philosophy.

The writ

ings of

by tenfold their value, that in a letter to his patron, the Emperor Otho III., with humility he playfully but truly says, "I am like the last of all the numbers."

The disuse of the Roman by the introduction of the Arabic numerals foreshadowed the result of a far more importanta political-contest between those rival names. But, before showing how the Arabian intellect pressed upon Rome, and the convulsive struggles of desperation which Rome made to resist it, I must for a moment consider the former under another point of view, and speak of Saracen philosophy. And here Algazzali shall be my guide. He was born A.D. 1058.

Let us hear him speak for himself. He is relating his atAlgazzali. tempt to detach himself from the opinions which he had imbibed in his childhood: "I said to myself, 'My aim is simply to know the truth of things; consequently, it is indispensable for me to ascertain what is knowledge.' Now it was evident to me that certain knowledge must be that which explains the object to be known in such a manner that no doubt can remain, so that in future all error and conjecture respecting it must be impossible. Not only would the understanding then need no efforts to be convinced of certitude, but security against error is in such close connection with knowledge, that, even were an apparent proof of falsehood to be brought forward, it would cause no doubt, because no suspicion of error would The certi- be possible. Thus, when I have acknowledged ten to be more knowledge. than three, if any one were to say, 'On the contrary, three is more than ten, and, to prove the truth of my assertion, I will change this rod into a serpent,' and if he were to change it, my conviction of his error would remain unshaken. His manœuvre would only produce in me admiration for his ability. I should not doubt my own knowledge.

tude of

Unreliability of the

senses.

"Then was I convinced that knowledge which I did not possess in this manner, and respecting which I had not this certainty, could inspire me with neither confidence nor assurance; and no knowledge without assurance deserves the name of knowledge.

"Having examined the state of my own knowledge, I found it divested of all that could be said to have these qualities, un

less perceptions of the senses and irrefragable principles were to be considered such. I then said to myself, 'Now, having fallen into this despair, the only hope of acquiring incontestable convictions is by the perceptions of the senses and by necessary truths.' Their evidence seemed to me to be indubitable. I began however to examine the objects of sensation and speculation, to see if they possibly could admit of doubt. Then doubts crowded upon me in such numbers that my incertitude became complete. Whence results the confidence I have in sensible things? The strongest of all our senses is sight; and yet, looking at a shadow, and perceiving it to be fixed and immovable, we judge it to be deprived of movement; nevertheless, experience teaches us that, when we return to the same place an hour after, the shadow is displaced, for it does not vanish suddenly, but gradually, little by little, so as never to be at rest. If we look at the stars, they seem to be as small as money-pieces; but mathematical proofs convince us that they are larger than the earth. These and other things are judged by the senses, but rejected by reason as false. I abandoned the senses, therefore, having seen all my confidence in their truth shaken.

66 6

Perhaps,' said I, 'there is no assurance but in the notions of reason, that is to say, first principles, as that ten is more than three; the same thing cannot have been created and yet have existed from all eternity; to exist and not to exist at the same time is impossible.'

66

Upon this the senses replied, 'What assurance have you Unreliability of that confidence in reason is not of the same nature as reason. your your confidence in us? When you relied on us, reason stepped in and gave us the lie; had not reason been there, you would have continued to rely on us. Well, may there not exist some other judge superior to reason, who, if he appeared, would refute the judgments of reason in the same way that reason refuted us? The non-appearance of such a judge is no proof of his non-existence.'

of dreams.

"I strove in vain to answer the objection, and my difficulties The nature increased when I came to reflect on sleep. I said to myself, 'During sleep, you give to visions a reality and consistence,

VOL. II.

F

Intellectual despair.

and you have no suspicion of their untruth. On awakening, you are made aware that they are nothing but visions. What assurance have you that all you feel and know when you are awake does actually exist? It is all true as respects your condition at that moment; but it is nevertheless possible that another condition should present itself which should be to your awakened state that which to your awakened state is now to you sleep; so that, as respects this higher condition, your waking is but sleep.'

It would not be possible to find in any European work a clearer statement of the scepticism to which philosophy leads us than what is thus given by this Arabian. Indeed, it is not possible to put the argument in a more effective way. His perspicuity is in singular contrast with the obscurity of many metaphysical writers.

66

Reflecting on my situation, I found myself bound to this world by a thousand ties, temptations assailing me on all sides. I then examined my actions. The best were those relating to instruction and education, and even there I saw myself given up to unimportant sciences, all useless in another world. Reflecting on the aim of my teaching, I found it was not pure in the sight of the Lord. I saw that all my efforts were directed toward the acquisition of glory to myself. Having, therefore, distributed my wealth, I left Bagdad and retired into Syria, where I remained for two years in solitary struggle with my soul, combating my passions, and exercising myself in the purification of my heart and in preparation for the other world."

This is a very beautiful picture of the mental struggles and the actions of a truthful and earnest man. In all this the Christian philosopher can sympathize with the devout Mohammedan. After all, they are not very far apart. Algazzali is not the only one to whom such thoughts have occurred, but he has found words to tell his experience better than any other Algazzali's man. And what is the conclusion at which he arrives? The ages of man. life of man, he says, is marked by three stages: "the first, or infantile stage, is that of pure sensation; the second, which begins at the age of seven, is that of understanding; the third is that of reason, by means of which the intellect perceives the