"Lives through all life, extends through all extent, Spreads undivided, operates unspent."[65] With reference to the habitability of other planets, Tennyson has beautifully said-- "Venus near her! smiling downwards at this earthlier earth of ours, Closer on the sun, perhaps a world of never fading flowers. Hesper, whom the poets call'd the Bringer home of all good things; All good things may move in Hesper; perfect people, perfect kings. Hesper--Venus--were we native to that splendour, or in Mars, We should see the globe we groan in fairest of their evening stars. Could we dream of war and carnage, craft and madness, lust and spite, Roaring London, raving Paris, in that spot of peaceful light? Might we not in glancing heavenward on a star so silver fair, Yearn and clasp the hands, and murmur, 'Would to God that we were there!'" The ancient Greek writer, Diogenes Laertius, states that Anaximander (610-547 B.C.) believed that the earth was a sphere. The Greek words are: μισην τε την γην κεισθαι, κεντρυ ταξιν επεχουσαν ουσαν σφαιροειδη.[66] With reference to the Aurora Borealis, the exact nature of which is not accurately known, "a good story used to be told some years ago of a candidate who, undergoing the torture of a _vivâ voce_ examination, was unable to reply satisfactorily to any of the questions asked. 'Come, sir,' said the examiner, with the air of a man asking the simplest question, 'explain to me the cause of the aurora borealis.' 'Sir,' said the unhappy aspirant for physical honours, 'I could have explained it perfectly yesterday, but nervousness has, I think, made me lose my memory.' 'This is very unfortunate,' said the examiner; 'you are the only man who could have explained this mystery, and you have forgotten it.'"[67] This was written in the year 1899, and probably the phenomenon of the aurora remains nearly as great a mystery to-day. In 1839, MM. Bravais and Lottin made observations on the aurora in Norway in about N. latitude 70°. Bravais found the height to be between 62 and 93 miles above the earth's surface. The cause of the so-called Glacial Epoch in the earth's history has been much discussed. The Russian physicist, Rogovsky, has advanced the following theory--