Thales was an astronomer as well as a philosopher and engineer. He learned more exactly the true length of the year than it had been known before; and he also made some calculations of eclipses, at least so far as to predict the year in which they would happen. One eclipse which he predicted happened to occur on the day of a great battle between two contending armies. It was cloudy, so that the combatants could not see the sun. This circumstance, however, which concealed the eclipse itself, only made the darkness which was caused by it the more intense. The armies were much terrified at this sudden cessation of the light of day, and supposed it to be a warning from heaven that they should desist from the combat. Thales the Milesian was the author of several of the geometrical theorems and demonstrations now included in the Elements of Euclid. The celebrated fifth proposition of the first book, so famous among all the modern nations of Europe as the great stumbling block in the way of beginners in the study of geometry, was his. The discovery of the truth expressed in this proposition, and of the complicated demonstration which establishes it, was certainly a much greater mathematical performance than the measuring of the altitude of the pyramids by their shadow. But to return to Croesus. Thales undertook the work of transporting the army across the river. He examined the banks, and found, at length, a spot where the land was low and level for some distance from the stream. He caused the army to be brought up to the river at this point, and to be encamped there, as near to the bank as possible, and in as compact a form. He then employed a vast number of laborers to cut a new channel for the waters, behind the army, leading out from the river above, and rejoining it again at a little distance below. When this channel was finished, he turned the river into its new course, and then the army passed without difficulty over the former bed of the stream. The Halys being thus passed, Croesus moved on in the direction of Media. But he soon found that he had not far to go to find his enemy. Cyrus had heard of his plans through deserters and spies, and he had for some time been advancing to meet him. One after the other of the nations through whose dominions he had passed, he had subjected to his sway, or, at least, brought under his influence by treaties and alliances, and had received from them all re-enforcements to swell the numbers of his army. One nation only remained--the Babylonians. They were on the side of Croesus. They were jealous of the growing power of the Medes and Persians, and had made a league with Croesus, promising to aid him in the war. The other nations of the East were in alliance with Cyrus, and he was slowly moving on, at the head of an immense combined force, toward the Halys, at the very time when Croesus was crossing the stream.