The floating of the earth on water or some other liquid long held ground. It was adopted by Thales, and six centuries later Seneca adopts the same opinion, saying that the humid element that supports the earth's disc like a vessel may be either the ocean or some liquid more simple than water. Diodorus tells us that the Chaldeans considered the earth hollow and boat-shaped--perhaps turned upside down--and this doctrine was introduced into Greece by Heraclitus of Ephesus. [Illustration: FIG. 29.--THE EARTH OF ANAXIMANDER.] Anaximander represents the earth as a cylinder, the upper face of which alone is inhabited. This cylinder, he states, is one-third as high as its diameter, and it floats freely in the centre of the celestial vault, because there is no reason why it should move to one side rather than the other. Leucippus, Democritus, Heraclitus, and Anaxagoras all adopted this purely imaginary form. Europe made the northern half, and Lybia (Africa) and Asia the southern, while Delphi was in the centre. Anaximenes, without giving a precise opinion as to the form of the earth, made it out to be supported on compressed air, though he gave no idea as to how the air was to be compressed. Plato thought to improve upon these ideas by making the earth cubical. The cube, which is bound by six equal faces, appeared to him the most perfect of solids, and therefore most suitable for the earth, which was to stand in the centre of the universe. [Illustration: FIG. 30.--PLATO'S CUBICAL EARTH.] Eudoxus, who in his long voyages throughout Greece and Egypt had seen new constellations appear as he went south, while others to the north disappeared, deduced the sphericity of the earth, in which opinion he was followed by Archimedes, and, as we have seen, by Aristotle.