This discovery led him on to try and measure the cir cumference 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. CH. IV. CIRCUMFERENCE OF THE EARTH. FIG. 2 To the Sun D tine all ot B 29 Let the large circle represent the earth; B the gnomon at Syene, and a the gnomon at Alexandria. The length of the shadow C D of the gnomon A, will bear the same proportion to the circumference of the small circle (drawn from the top of the gnomon as a centre), that the distance from Alexandria to Syene (D to E) does to the whole circumference of the globe. This is true only if the rays from the sun to Alexandria and to Syene are parallel (or run at equal distances). They are not really quite parallel because they meet in the sun, but Eratosthenes knew that the sun was at such an enormous distance that their approach to each other was quite unimpor Diagram showing how Eratosthenes mea A, sured the circumference of the earth. tant. He now measured the distance between Alexandria and Syene and found it to be 5,000 stadia, or about 625 miles, and multiplying this by 50 he got 625 x 50 = 31,250 miles as the whole circumference of the earth, measured round from pole to pole. This result is not quite correct, but as nearly as could be expected from a first rough attempt. Eratosthenes also studied the direction of mountain-chains, the way in which clouds carry rain, the shape of the continents, and many other geographical problems. Hipparchus, 160.-Nearly one hundred years after Eratosthenes, the great astronomer Hipparchus was born, 160 B.C. Hipparchus was the most famous of all the astro nomers who lived before the Christian era. He collected and examined all the discoveries made by the earlier observers, and made many new observations; but astronomy had now become so complicated that the problems are too diffi cult to be explained here. Hipparchus made a catalogue of 1,080 stars, and showed how they are grouped with regard to the ecliptic. He also calculated accurately when eclipses of the sun or the moon would take place. But his great discovery was that called the 'Precession of the Equinoxes.' This is a very complicated movement which you can only understand by reading works on Astronomy; but I will try to give a rough idea of it, in order that you may always connect it with the name of Hipparchus. We have seen that the earth has two movements-one, turning on its own axis causing day and night; the other, travelling round the sun, causing the seasons of the year. But besides these it has a third curious movement, just like a spinning-top when it is going to fall. Look at a top a little while before it falls, and you will see that, because it is leaning sideways, the top of it makes a small circle in the air. Now our earth, because it is pulled at the equator by the sun, moon, and planets, makes just such a small circle in space; so that, instead of the north pole pointing quite straight to the polar star, it makes a little circuit in the sky, with the polar star in the centre. The pole moves very slowly, taking twenty-one thousand years to go all round this circle. To understand the effect of this movement we must examine more closely what the equinoxes are. Take your ball again and make it go round the lamp with its axis inclined (see p. 20). When you have it in such a position that the north pole is in the dark, or the northern winter solstice, you will find that a straight line drawn from CH. IV. PRECESSION OF THE EQUINOXES. 31 the sun to the centre of the earth will not meet the equator but a point to the south of it. But now pass the ball on to the next point when neither pole is in shade, and when it is equal day and equal night over the globe (our spring equinox), a line now drawn from the sun will fall directly upon the equator, so that the sun's path meets the equator at this point, which is called the equinoctial point. Pass on till the south pole of your ball is in the dark, the sun will now fall directly on a point north of the equator (making our summer solstice). Pass on again to the point of equal day and equal night, and the sun again falls direct on the equator, causing our autumn equinox. Now, if the earth did not make that small circle in space like the top, the sun would always touch the equator at exactly those same points of the earth's orbit or path round the sun; but the effect of that movement is to pull the equator slightly back, so that the points where the ecliptic and the equator cut each other are 50 seconds more to the west every year, and in this way the equinoxes travel all round the orbit from east to west in 21,000 years. Hipparchus discovered this pre-cession (or going forward) of the equinoxes; though he did not know, what Newton afterwards discovered, that it is caused by the sun and moon pulling at the mass of extra matter which is gathered round the equator. CHAPTER V. FROM A.D. 70 TO 200. Ptolemy founds the Ptolemaic System-He writes on GeographyStrabo, a great traveller, writes on Geography-Studies Earthquakes and Volcanoes-Galen the greatest Physician of Antiquity— Describes the Two Sets of Nerves-Proves that Arteries contain Blood-Lays down a theory of Medicine-Greece and her Colonies conquered by Rome-Decay of Science in Greece-Concluding remarks on Greek Science. Ptolemy, A.D. 70.—After Hipparchus there were many good astronomers at Alexandria, but none whom we need notice until the year 70 after Christ, when Ptolemy Claudius, a native of Egypt, was born. He was not one of the Ptolemies who governed Alexandria, and the place of his birth is unknown, but he is famous for having made a regular system of astronomy founded upon all that the Greeks had learnt about the heavens. His discoveries, like those of Hipparchus, are too complicated for us to discuss here; they related chiefly to the movements of the moon and the planets; but the one great thing to be remembered of him is, that he founded what is called the Ptolemaic System of astronomy, which tries to explain all the movements of the sun, stars, and planets, by supposing the earth to stand still in the centre of them all. This system is contained in Ptolemy's great work called 'the Syntaxis.' It may seem strange that, as it is not true that the earth is the centre, Ptolemy should have been able to explain so much by his system, but you |