fastened, and which sustains the earth, must be conceived, not as an imaginary line, but as a solid cylinder, itself revolving; while the earth, being fastened round it, revolves round and along with it. The axis, in the sense of an imaginary line, cannot be found in the conception of Plato. Those contemporaries of Plato and Aristotle, who all agreed in asserting the revolution of the celestial sphere, did not all agree in their idea of the force whereby such revolution was brought about. Some thought that the poles of the celestial sphere exercised a determining force: others symbolised the mythical Atlas, as an axis traversing the sphere from pole to pole and turning it round. (Aristotel. De Motu Animal. 3. p. 699 a. 15-30.) Aristotle himself advocated the theory of a _primum movens immobile_ acting upon the sphere from without the sphere. Even in the succeeding centuries, when astronomy was more developed, Aratus, Eratosthenes, and their commentators, differed in their way of conceiving the cosmical axis. Most of them considered it as solid: but of these, some thought it was stationary, with the sphere revolving round it--others that it revolved itself: again, among these latter, some believed that the revolutions of the axis determined those of the surrounding sphere--others, that the revolutions of the sphere caused those of the axis within it. Again, there were some physical philosophers who looked at the axis as airy or spiritual--[Greek: to\ dia\ me/sou tê=s sphai/ras diê=kon pneu=ma]. Then there were geometers who conceived it only as an imaginary line. (See the Phaenomena of Aratus 20-25--with the Scholia thereon; Achilles Tatius ad Arati Phaenom. apud Petavium--Uranolog. p. 88; also Hipparchus ad Arat. ib. p. 144.) I do not go into these dissentient opinions farther than to show, how indispensable it is, when we construe the passage in the Platonic Timaeus, [Greek: peri\ to\n dia\ panto\s po/lon tetame/non], to enquire in what sense Plato understood the cosmical axis: and how unsafe it is to assume at once that he must have conceived it as an imaginary line. Proklus argues that because the earth is mentioned by Plato in the Phædon as stationary in the centre of the heaven, we cannot imagine Plato to affirm its rotation in the Timæus. I agree with M. Boeckh in thinking this argument inconclusive; all the more, because, in the Phædon, not a word is said either about the axis of the kosmos, or about the rotation of the kosmos; all that Sokrates professes to give is [Greek: tê\n i)de/an tê=s gê=s kai\ tou\s to/pous au)tê=s**] (p. 108 E). No cosmical system or theory is propounded in that dialogue.