It is pottery again that has been the basis of this chronological reconstruction. The beautiful Cretan many-coloured ware of the Middle Minoan period, exported to Egypt during the Middle Kingdom and found with objects of the Twelfth Dynasty, forms the chief equating factor between those two periods, and the other equations are based on similar facts. Pottery can be made in some cases to fix approximate dates without the help of equations. Buildings, for instance, cannot have stood later than the date of the particular kind of pottery found in their ruins. It may be remarked in passing that the Egyptian trade thus indicated by the remains of Cretan pottery was responsible for a great improvement in that pottery. Towards the end of the early Minoan period the two great inventions of the firing furnace and the potter’s wheel were brought to Crete from Egypt. Before that time the vases had been roughly shaped by hand and hardened in the sun. They now were “thrown” with such a mastery of technique as to attain egg-shell thinness. Traces of commercial intercourse overseas can be found as far back as the Neolithic Age. Among the deposits of stone implements in Crete are great quantities of obsidian knives, and the only source of obsidian in the Ægean was the island of Melos. Obsidian is a kind of volcanic glass which flakes off into layers, giving a natural edge. Excavators, who are as childish as most people, have shaved, and have had near shaves, with obsidian knives. It is probable that the Minoan Empire had a navy as well as a merchant marine. Minos was commonly represented as “Ruler of the Waves,” and the Greek historians, Herodotus and Thucydides, refer to him as a mythical character celebrated as the first possessor of a fleet. The extent of the Minoan Empire can be gauged by the survival of many trading stations and naval outposts on all the shores of the Ægean, from Sicily in the East to Gaza in the West, which bore the name “Minoa.” There was a bad chapter, according to tradition, in the Empire’s history. When the King’s son Androgeos went to Athens to compete in the games, he won everything, and was killed in jealousy; and the powerful Minos therefore decreed that seven Athenian boys and seven girls should be sent every nine years (or as other versions of the story say, every year) to be eaten by the Minotaur, a monster half man, half bull, which lived in the maze called the Labyrinth. That happened twice; but on the third occasion the hero Theseus volunteered to go as one of the victims; and with the aid of Ariadne, the King’s daughter, who fell in love with him, he killed the monster. She gave him a sword and some string, which he fastened to the entrance of the maze as he went inside. He was thus able to find his way out again. Theseus had promised his father, the old King Ægeus, that if he returned alive, his ship would show white sails in place of the usual black, so that the news of his safety could be read in the distance.