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.
[Illustration: Fig. 13.--View of Mecca in the Seventeenth Century.]
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.
[Illustration: Fig. 14.--Cordova after the Expulsion of the Moors.]
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.
[Illustration: Fig. 15.--Light travels in Straight Lines.]
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.
[Illustration: Fig. 16--Effect of Refraction of Light by 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.