THE WISE MEN OF ALEXANDRIA
WHEN Alexander the Great overcame the Egyptians, he sailed
along the African coast past the mouths of the river Nile until he came
to a place which seemed to him suitable for building a great city. It
was a strip of land on the sea-shore with a great lake behind it and
an island in front. “Here,” said the mighty conqueror, “shall be my
capital city, to be called Alexandria after my name.” So he sent for
his great architect Dinocrates, and told him to plan the streets and
palaces of the new royal city. Thousands of slaves were soon set to
work. The streets were made straight and wide, and canals were dug to
join the lake to the open sea and to the Nile, so that great merchant
vessels would find shelter from the storms.
The island was joined to the city by a causeway a mile long, and a
lighthouse 400 feet high was built on the island. The lighthouse was
in the shape of a round tower. On the top of the tower a bright fire
was kept constantly burning, so that the smoke by day and the flame by
night should guide ships safely into the harbour. This tower was the
first lighthouse ever built, and is said to have cost a quarter of a
million pounds (£250,000).
Within fifty years of its foundation Alexandria became one of the
foremost centres of commerce. The second of the Egyptian kings founded
the famous Library of Alexandria, and thus secured for the city a place
in the front rank of learning, which it kept for six hundred years.
Many of the great works of Greek writers were saved from destruction
by being preserved in Alexandria. The third king compelled every
stranger who passed through Alexandria with a book in his possession to
leave a copy of it at the Library. He also built a Museum, which was an
institution closely resembling what we should now call a University.
It had dwelling-rooms for professors, it had lecture halls, and a
great dining-room. The University grew and flourished, and attracted
clever young men from all the shores of the Mediterranean Sea. One of
these students was Euclid, who wrote the books on Geometry. There were
also poets, critics, and historians. Many books from all nations were
translated into Greek, including the Old Testament, at which seventy
scribes are said to have worked.
It was an Alexandrian astronomer named Aristarchus, who about 270 B.C.
first determined the distance of the sun from the earth in comparison
with that of the moon. He knew that the moon was a round ball, and
that its various shapes or phases were produced by the sun shining
upon it at different angles. He carefully watched the growing sickle
of the moon from day to day, waiting for the time when the moon should
appear to be exactly a half-circle. When this is the case an imaginary
line drawn from the eye of the observer to the centre of the moon is
at right angles to a similar line drawn from the centre of the moon
to the sun, as shown in the picture (Fig. 6). At the instant when the
sun’s light was seen to be shining squarely on the moon, that is just
before sunset, Aristarchus pointed one leg of an instrument like a
pair of compasses to the moon, and the other leg to the sun. The angle
between the two directions was found to be not quite a right angle. It
measured, in fact, 87 degrees (Fig. 6), whereas a right angle contains
90 degrees.
[Illustration: Fig. 6.--Comparing the Distances of the Sun and Moon.]
Now, if a triangle be drawn like that in Fig. 6, in which the angle
at M is a right angle, and that at E is 87 degrees, this triangle
illustrates the results of the observations. The moon’s distance is
represented in the triangle by the line EM, and the sun’s distance by
SM. It does not matter how large or small the triangle is drawn; for,
if the lines are inclined at the proper angles, the line SM will always
be the same number of times longer than EM.
Aristarchus concluded from his observations that the sun was eighteen
or nineteen times farther away than the moon. In reality, it is about
four hundred times that distance, but considering that the ancients
had very imperfect instruments, and that it is almost impossible to
decide the exact instant when the moon is just half-full, it is not
remarkable that the result obtained was far too small. As the result
of other observations, Aristarchus found that 720 suns, placed edge to
edge, would just circle the sky, and this is very close to the truth.
Men of science honour him for his ingenious methods of measuring the
sun’s distance and size, and for his painstaking observations, though
the results were not perfect.
Another famous astronomer of Alexandria, who was also the custodian of
the Library, was the first to measure the size of the earth. He was in
the habit of sailing up the Nile, and found that the farther he sailed
the more new stars appeared in the south, while the northern stars
disappeared gradually. This fact convinced him that the earth, like the
moon, is a round globe, and he thought that if he could go very much
farther in the same direction towards the south, he would eventually
circle the earth, and arrive at Alexandria again from the north. But
to measure the circle of the earth it was not necessary to travel
all round it. It was sufficient to find the fraction of the circle
traversed by travelling a certain distance, and this could be done by
measuring how much higher the southern stars appeared in the sky after
travelling a certain distance. Since the stars are very much farther
away than the sun, the difference in their height could not be due to
the distance travelled, but only to the roundness of the earth.
The astronomer sailed from Alexandria as far as the Falls of Aswan, and
carefully measured the distance he had travelled, which he found to be
520 miles. He found that at midday the sun stood about 7 degrees higher
in the sky at Aswan than at Alexandria. This angle was just about
1-50th of a whole circle. The astronomer concluded that if he continued
to travel south for fifty times that distance he would travel round the
globe, and in doing so he would cover the distance of 26,000 miles. The
real circumference of the earth is 23,700 miles, so that the learned
Alexandrian was not far wrong. But he would have been even more correct
had he known that Aswan is not really due south of Alexandria, but a
little east of south. This made his line too long.
The name of this great astronomer was Eratosthenes. His end was a sad
one. He lost the sight of his eyes and his powers of observation.
He starved himself to death, saying that life without the means of
pursuing his studies was not worth living.
Another remarkable man who lived in those days at Alexandria was Heron.
He was employed at the Museum, lecturing on Mechanics, Optics, and
the principles of Surveying.
[Illustration: Fig. 7.--Heron’s Steam-Engine.]
He invented a number of contrivances and machines which were the wonder
of his age. The people of Babylon had invented clocks driven by water.
[Illustration: Fig. 8.--The Jumping Ball.]
Heron improved these by letting the water drop through a small hole
bored in a precious stone of great hardness, so that the water should
not be able to enlarge the hole and to make the clock run faster. The
water dropped into a vessel containing a little boat. The boat had a
mast which was graduated, so that as the water rose, one graduation
after another appeared above the edge of the vessel.
These graduations indicated the hours. He also made the boat turn a
wheel, which at each hour caused a number of balls to fall into a
silver goblet. The number of balls indicated the number of hours, so
that the clock struck the hour very much like our clocks of to-day.
Heron is famous for having invented a kind of steam-engine, consisting
of a hollow ball with two nozzles pointed in opposite direction (Fig.
7). The steam passed into the ball through the arms by which it was
suspended, and the steam hitting against the air outside caused the
ball to spin round. In another form of the apparatus (Fig. 8) a ball
was kept jumping up and down by the steam issuing from a vertical pipe.
Heron found that when a narrow tube is put into water, and the upper
end is stopped with the finger while still under water, then on lifting
the tube out of the water with the stopped end uppermost, the water
does not flow out at the open end below until the finger is removed.
He, therefore, constructed a vessel (Fig. 9) which became known as the
“Vestal’s Goblet”; it was filled by plunging it into water, stopping
the upper opening with the finger, and lifting it out. The water
escaped in a spray through the small holes below on removing the finger.
[Illustration: Fig. 9.--The Vestal’s Goblet.]
[Illustration: Fig. 10.--The Double Goblet.]
In another form of the instrument there were two compartments which
were filled in the same manner with wine and water respectively. It was
a custom among the Greeks to mix wine with water, and offer the same
mixture to the gods in the temples.
Heron also made a self-feeding wick for oil lamps, which were then in
common use. The oil was burnt in an open vessel (Fig. 11), and as it
burnt away the wick was consumed also, but Heron made the oil itself
turn up the wick. He made a plate of wood float on the oil; as the oil
burnt away the plate of wood gradually sank, and in doing so it moved a
cog wheel. The cog wheel in turning moved a straight rack, or toothed
bar of wood, at the end of which the wick was fastened. In this way
the wick was moved on as the oil was consumed, and the lamp was kept
steadily burning.
The most important instrument which Heron improved was the Surveyor’s
Level. Egypt depends for its fertility upon the floods of the Nile, as
there is very little rain in Egypt.
[Illustration: Fig. 11.--Heron’s Self-Feeding Wick for Lamps.]
From the oldest times the Egyptians had to see that they could
distribute the land after the flood in the same way as it was
distributed before, each man getting his own land back. This was found
to be very difficult, as many landmarks were destroyed by the flood.
The art of Surveying was practised at a very early date in Egypt, and
there is no doubt that the science of Geometry was born in Egypt,
as it was necessary in order to be able to determine the extent and
boundaries of land.
[Illustration: Fig. 12.--Modern form of Heron’s Level.]
An important problem in connection with Surveying was to find when
two points are on the same level. This is done nowadays with a
spirit-level if the points are close together, or with an instrument
called a theodolite if the points are far apart. But a theodolite
requires a telescope, and the Alexandrians lived long before the
telescope was invented. Heron, however, got over the difficulty by
constructing a long box filled with water and provided with openings,
in the form of a cross, at each end. Above the crosses were glass tubes
communicating with each other, in which the water was kept at the same
level, and when that was the case the two crosses were exactly at the
same level, and any object seen through both crosses together was at
the same level as the crosses themselves. Heron probably used rods very
similar to the black and white rods used by surveyors at the present
day for measuring distances and levels.
Heron also invented a mechanical stone crusher, an organ driven by
water power, a counting machine to show the distance travelled by a
ship or a chariot, and an arrangement of mirrors which he called a
spy-glass.
Many of the works of Heron have been preserved to us. They are
written in Greek, which was the language used in Alexandria since its
foundation. Heron was always careful to make his reasoning quite clear
to his pupils, and he insisted that they must not believe anything
without proof.
“It is necessary,” he said, “that those who wish to become acquainted
with mechanical art should know what causes are at work in every
motion. It is important that nothing should be put before students
without proof, and that nothing should remain doubtful for them. In our
presentation every problem is to find a solution. We therefore recall
various principles taught by the ancients which are connected with our
subject.”