So Alexander went right ahead with his father’s plan to conquer Persia. The time had come to pay back Persia for that last invasion one hundred and fifty years before. He got together an army and crossed the Hellespont into Asia and won battle after battle against the first Persian armies sent out to stop him. He kept moving on, for Persia was a vast empire. Soon he came to a town where in a temple there was kept a rope tied into a very far-famed and puzzling knot. It was called the Gordian Knot, and it was very famous because the oracle had said that whoever should undo this knot would conquer Persia. But no one had ever been able to untie it. When Alexander heard the story, he went to the temple and took a look at the knot. He saw at once that it would be impossible to untie it, so, instead of even trying, as others had done, he drew his sword and with one stroke cut the knot in two. So now when a person settles something difficult, not by fussing with it as one untangles a snarl, but at a single stroke, cutting through all difficulties, we say he “cuts the Gordian Knot.” From that time on, Alexander conquered one city after another and never lost any battle of importance until he had conquered the whole of Persia. Then he went into Egypt, which belonged to Persia, and conquered that country, too. To celebrate this victory, he founded a town near the mouth of the Nile and named it after himself, Alexandria. Then he started there a great library which later grew to be so big that there were said to be five hundred thousand books in it--that is, half a million--and was the largest library of ancient times. The books were not like those in the library of Assur-bani-pal nor the kind we have now, of course, because printing had not been invented. They were every one of them written by hand, and not on pages, but on long sheets which were rolled up on sticks to form a scroll. [Illustration: A scroll, pens and ink.] In the harbor of Alexandria was a little island called Pharos, and on this island some years later was built a remarkable lighthouse named from the island, the Pharos, and its light could be seen for many miles. It was really a building more like a modern sky-scraper with a tower. It was over thirty stories high, which seemed most remarkable at that time when most buildings were only one or two stories high, and its light could be seen for many miles. So the Pharos of Alexandria was called one of the Seven Wonders of the World. You have already heard of three others, so this makes the fourth. Alexandria grew in the course of time to be the largest and most important seaport of the ancient world. Now, however, the Pharos and the library and all the old buildings have long since disappeared.