CHAPTER III. Arabian Days

ARABIAN DAYS

TO the east of the Red Sea there is a land called Arabia. Part of it is a desert over which a wind, called the simoom, blows its suffocating blasts. Another part is fertile mountain land, where the red anemone blooms. The third part adjoins the Indian Ocean and the Gulf of Persia. This part is rich and green, and planted with date-palms and coffee shrubs.

From Arabia comes the Arab horse, small and grey, but beautiful to behold. It can go for forty-eight hours without a drink. The Arabs ride it without a saddle or stirrup, and only guide it by the pressure of the knee and a kind word. Its greased hoofs traverse the Arabian desert from end to end.

Sometimes the Arabs ride on a white ass, mounted on a side saddle. They also ride the camel with a single hump, and from it good milk and wool are obtained. It can travel eighty miles per day and goes for a week without water; but it never learns to know or love its master. Jackals, panthers, and hyenas prowl about the desert, which is also infested with ants, scorpions, and locusts, as well as a horrible kind of spider with double pincers. The Arabs are called the sons of Ishmael. They are traders and workers in leather. They are calm and dignified in appearance, and have never been conquered.

Thirteen centuries ago these Arabs were divided into a great number of tribes who made war upon each other. Then Mohammed arose, and proclaimed that he was a prophet sent by God to teach the Arab people, and through them to convert all the world to a new religion. He suffered much persecution at first, but in the end he overcame all his enemies and ruled the whole of Arabia. He destroyed all idols, and taught his people to give up alcoholic drinks, gambling, and usury. His successors, who were called Khalifs, went forth to conquer the world. Those who would not be converted to the Mohammedan religion they forced to pay heavy taxes, those who resisted by force of arms they slaughtered without mercy, but those who became Mohammedans they rewarded with a share of their booty and power of government. They captured Damascus and Jerusalem, they conquered Egypt and North Africa, and besieged Constantinople. In the end they conquered Spain in the west and Persia in the east, so that their Empire stretched from the Atlantic Ocean to India.

The Arabs destroyed many kingdoms, but they also knew how to build. They founded great cities, the most glorious of which was Bagdad on the river Tigris, where Harun-al-Rashid reigned and Queen Scheherazade told her tales of goblins, battles, and treasures, and Aladdin worked his wonderful lamp. They also founded Cairo, within sight of the Egyptian pyramids, and made it the capital of Egypt after they had pillaged Alexandria. But Mecca in Arabia, where Mohammed first preached his new doctrine, remains the centre of the Mohammedan religion to this day.

When the Roman Empire was destroyed by the barbarians from the north, it was the Arabs, strange to say, who took up the cultivation of science where it had been left by the Greeks and Romans. The Khalifs who built Bagdad, Cairo, and Cordova in Spain, loved to surround themselves with learned men from all parts of the world. An Arabian writer says, “To Cordova came from all parts of the world students eager to cultivate poetry, to study the sciences, or to be instructed in divinity or law, so that it became the meeting-place of the eminent in all matters, the abode of the learned, and the place of resort for the studious. Its interior was always filled with the eminent and the noble of all countries, its literary men and scholars were continually vying with each other to gain renown, and its precincts never ceased to be the arena of the distinguished, the recourse of readers, the halting place of the noble, and the repository of the true and virtuous. Cordova was to Spain what the head is to the body, or what the breast is to the loin.”

The sciences generally cultivated among the Arabs were Chemistry and Optics. In chemistry they discovered a number of new metals and acids. They knew how to distil a liquid and crystallise a solid, and how to mingle various metals to form new alloys with variable properties. They were excellent makers of sword-blades, and their scimitars or curved swords were dreaded by all the soldiers of Christendom.

The science of Optics was originally cultivated by a native of Mesopotamia named Alhazen, who died in the year 1038 A.D. Alhazen was called to Egypt by one of the Khalifs who had heard that he had thought out plans for regulating the flow of the Nile in such a manner that each year there should be plenty of water for inundations. Alhazen went to Egypt and inspected the land, but found that his plan was not suitable. The Khalif was angry, and would have punished Alhazen, but the latter pretended to be mad, and managed to hide himself until the Khalif died. He then reappeared and became famous as a scholar and instructor of youth, copying old manuscripts and writing books on Astronomy, Mathematics, and Optics.

Alhazen’s greatest work was done on the subject of the laws which govern the production of light. He proved, first of all, that light travels along straight lines. “If the light of the sun,” he says, “or the light of the moon, or the light of fire, enters a dark room through a narrow slit, and dust is in the room, or dust is made to fly in it, the light entering through the slit is made clearly visible in the air mixed with dust; it is also visible on the floor or on the opposite wall of the room. And it is found that the light travels through the slit to the floor or to the opposite wall along straight lines. And when a straight rod is held along that visible light, it is found that the light travels along the straight rod.

But if there is no dust in the room, and the light appears on the floor or on the opposite wall, and a straight rod is held between the slit and the patch of light, or between both a thread is stretched, and a body is brought between the patch and the slit, the light becomes visible on the opaque body and disappears from the place at which it was visible. If then the opaque body is moved to and fro in the space indicated by the rod, the light always remains visible on the opaque body. It is, therefore, clear that the light proceeds along straight lines from the slit to the place where it is visible.”

That light travels in straight lines is only true so long as it travels through air or through the same kind of substance. When a looking-glass is held in the path of the rays in a dusty room, it is seen that a beam of light starts from the looking-glass in a direction quite different from that of the original beam. This fact did not escape the notice of Alhazen, and he carefully observed the direction taken by the reflected beam in various circumstances. He had read the books written by the learned Greeks, and he knew from them that the reflected beam makes the same angle with the mirror as the original beam does. To this observation he added another, and a very important one. He found that when a flat surface, such as a piece of paper or cardboard, is held against the mirror so that it touches the original and the reflected beam, that surface or sheet is always upright on the mirror, and stands at right angles to its surface. This law, discovered by Alhazen, explains how we see trees reflected in a river or a pond pointing straight down, and not slanting to the right or to the left. If we had a big fiat surface which we could make to stand vertically on the water, that surface, if made to pass, through the tree, would also pass through its reflection in the water.

Most of us have observed that when a kitten sees its image in a looking-glass for the first time, it takes it for another kitten, but after a few attempts to make friends with the supposed companion it finds out that the image is only an illusion. This observation was explained by Alhazen, by pointing out that the eye always perceives objects in the direction in which the beam of light enters the eye. He investigated another curious illustration of this. When a coin is placed at the bottom of a dish, and we move away from the dish until the rim just hides the coin, the latter is, of course, invisible. But on filling the dish with water without changing our position, we find that the coin becomes visible again, being apparently raised, although it really remains at the bottom of the dish as before.

Alhazen proved that this is another case of illusion due to the bending of the beam of light. He showed this bending of the beam of light in a very ingenious manner. He took a big glass vessel, and filled it with water containing a few drops of milk. He took it into a dark room where he had already studied the reflection of light. He found that when the water in the glass vessel was placed in the beam of light, the beam appeared broken at the surface of the water, bending suddenly down more towards the floor, and making it shorter. The beam was made visible in the air by the suspended particles of dust, and in the water by the suspended particles of milk. Alhazen showed that the beam under water always makes a greater angle with the surface than does the original beam, but he did not succeed in finding out the exact relation between the two angles, nor was that relation discovered until several centuries afterwards.

Like a true Arab, Alhazen was very fond of studying illusions of all kinds. But there was one appearance which was before his time not considered an illusion, but a reality. The sun and moon appear much larger when they are rising or setting than they do when they are high up in the sky. Alhazen, however, showed that the apparent size is in reality the same in both cases. He held a coin at arm’s length so that it just covered the rising moon. He again held the same coin at the same distance from his eye between himself and the moon when she was up in the sky. He found that the coin again just covered the moon, but that if the moon were seen reflected in a distant mirror so that it was apparently on the horizon it again appeared larger. This, he said, was simply owing to our habit of judging things on the earth by comparison with earthly objects.

The Arabs, whether in Bagdad, Damascus, Cairo, or Cordova, were well acquainted with the principle of Archimedes, and they knew that a body in water weighs less than in air, the difference being the weight of the water displaced by the body. But they were first to use the idea of what is now known as specific gravity, and they explained that the specific gravity of a body is obtained by dividing the weight of the body by the weight of the same volume of water. This definition is used even now, and tables of specific gravity are found in all books dealing with the properties of various materials.

Before the destruction of the great Arabian Empire, which put an end to the cultivation of science by the Arabs, and transferred their work to the new nations of Europe, they had reached the summit of fame in the whole world. Their last achievement was one which gave rise to many remarkable developments. It is described in a book called The Book of the Balance of Wisdom. The Arabian author says in this book that even air must have some weight, and that the true weight of a body cannot be the weight as measured in air, because air, like water, must somewhat reduce its weight. This was the observation upon which later the great inventions of the barometer and the air-pump were based.