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.