CHAPTER II. The Wise Men of Alexandria

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.

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.

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.

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.

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.

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.

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.”