gravity of different substances, that is, the weight of any particular substance compared to an equal bulk of water.

It will be quite sufficient if you remember the experiment as I have explained it; but as you may perhaps be puzzled to see how it can have anything to do with weight, you can, you wish, try to master the following explanation of Fig. 1,

Diagram showing the difference of specific gravity between equal weights of gold, silver, and mixed metal.

A B C, Spring balances. d, Gold ball weighing 19 oz. e, Silver ball weighing 19 oz. f, Crown of mixed metal weighing 19 oz.

which shows how specific gravity is measured. You must begin by remembering that the crown, the gold ball, and the silver ball, when weighed in the air, will all pull the marker of the spring balances A, B, C, down to 19; that is, they will all weigh 19 ounces. But when they are immersed in water they

CH. III.

ARCHIMEDES-SPECIFIC GRAVITY.

25

will no longer weigh the same, because the water round them buoys them up just as much as it would buoy up the quantity of water which they displace.

Now, the gold ball takes the place of as much water as would weigh one ounce if you could take it out and weigh it in the air. So the gold ball is buoyed up one ounce by the water round it, and accordingly you see it only pulls the marker down 18 ounces instead of 19. But the silver ball, although it weighs the same, is larger, and takes the place of nearly two ounces of water, therefore it is buoyed up nearly two ounces, and only pulls the marker down to 17. Now, as the crown weighs the same as the two balls, its shape is of no consequence; if it was made all of gold it would take as much room, and be buoyed up as much as the gold ball. If it was all silver it would be buoyed up as much as the silver ball, and therefore, as it pulls the marker down half-way between 17 and 18 ounces, it must be half gold and half silver.

In this way Archimedes showed how we can learn the weight of any substance compared to an equal bulk of water, and this is called the 'specific gravity' of the substance.

He also invented a screw for pumping up water, which is still called the 'screw of Archimedes.'

Archimedes was unfortunately killed in the city of Syracuse when it was besieged by the Romans during the second Punic war. The General Mecenas had given special orders that his life should be spared; but he was so deeply engaged in solving a problem that he heard nothing of the din of war around him, and a common soldier not being able to get any answer from him, killed him without knowing who he was.

3

CHAPTER IV.

280 TO 120 B.C.

Erasistratus and Herophilus study the Human Body-Eratosthenes the Geographer lays down the First Parallel of Latitude, and the First Meridian of Longitude-He measures the Circumference of the Earth-Hipparchus writes on Astronomy-Catalogues 1,080 Stars -Calculates when Eclipses will take place-Discovers the Precession of the Equinoxes.

Erasistratus and Herophilus.-At the time when Archimedes was studying in Alexandria, two physicians were teaching there, who are famous in the history of anatomy, or the structure of the body. The one was Erasistratus and the other Herophilus. The birthplaces and dates of these two physicians are doubtful, but we know that they were the first men who dissected the human body, and gave a clear account of its parts. Erasistratus, in particular, described the brain and its curious windings or convolutions, and the division between the cerebrum or front part and the cerebellum or hinder and lower part. He seems also to have known that it is in our brain that we feel everything, and that it is the nerves which carry messages from different parts of our body to our brain. Herophilus traced out the tendons or strong cords which fasten the muscles to the bones, the ligaments or fibrous cords which unite one bone to another; and the nerves. He is the first physician who pointed out that in feeling a pulse you must notice three

CH. IV. ERATOSTHENES-PARALLEL OF LATITUDE. 27

things: 1st, how strongly it throbs; 2nd, how quickly; 3rd, whether the beats are regular or irregular. Many of the names which Erasistratus and Herophilus gave to parts of the body are still used by anatomists, and the school of medicine founded by them in Alexandria was renowned for more than six hundred years.

Eratosthenes, 276.-We must now turn to the science of geography, which at this time began first really to be studied by a Greek named Eratosthenes, born at Cyrene 276 B.C. Like all men of science of that day, he too came to Alexandria, where the king, Ptolemy Evergetes, made him keeper of the Royal Library. He made a map of all the world that was then known, and described the countries of Europe, Asia, and Libya. But his two great works were, laying down the first parallel of latitude, and trying to measure the circumference of the earth. He laid down the parallel of latitude in the following manner. He knew that at all places on the equator the day was exactly the same length all the year round, and that the length of the days and nights varied more and more as you went northwards; therefore he reasoned that, if he could draw a line east and west through a number of places whose longest day was exactly the same length, those places would all be at the same distance from the equator. He began at the Straits of Gibraltar, where the longest day was exactly 14 hours, and then observing all those places whose longest day was also 14 hours, he drew a line through the south coast of Sicily, across the south of the Peloponnesus, the island of Rhodes, the bay of Issus, and across the Euphrates and Tigris, out to the mountains of India. If you follow this line on a map you will find it is the 36th parallel of north latitude, and that Eratosthenes' observation was perfectly correct.

This discovery led him on to try and measure the circumference of the earth. Having found a line straight round the earth from east to west, he knew that if he drew a line at right angles to it, that is exactly north and south, he should have a line which would describe a circle round the earth from pole to pole, as the equator marks a circle round the earth midway between the two poles. This second line he drew from Alexandria, and it passed right through Syene, now called Assouan, one of the southern cities of Egypt, and thus he knew that Alexandria and Syene were on the same meridian of longitude.

Now he found that at Syene the sun was exactly overhead at midday, at the time of the summer solstice. He knew this by means of a gnomon, or upright pillar (B, Fig. 2), which was used by the Greeks to measure the height of the sun in the sky. At Syene this pillar cast no shadow at noon of the summer solstice, proving that the sun shone straight down upon the top of it; and this was further proved by the sun shining down to the bottom of a deep well, which it would not do unless it were directly overhead. But at Alexandria the gnomon did cast a shadow, because, as Alexandria was further north and the earth is round, the sun there was not directly overhead. Now, as light travels in straight lines (see p. 21), a line drawn from the extreme point of the shadow cast by the pillar or gnomon up to the top of the pillar itself would, if carried on, run straight into the sun, and thus the angle between this line and the pillar showed at what angle the sun's rays were falling at Alexandria. By measuring this angle, Eratosthenes found that Alexandria was th of the whole circumference of the earth north of Syene, where the rays were perpendicular. You can form an idea of this from the accompanying diagram, Fig. 2.