Then, life still being absent from the earth, the _Oceanic Era_ began. The waters condensed into an ocean over the earth, or else collected in some great oceanic depression. Lands presently emerged from it. It was a hot ocean, steaming no doubt, for its temperature was perhaps about 500° F. Some one may ask--Why, then, did it not steam away into clouds? The answer is that the atmosphere was still very heavy in that past era, probably still exerting a pressure as much as fifty times as great as to-day. The pressure of the atmosphere at the earth's surface to-day is usually about fifteen pounds to the square inch. In such circumstances water boils rapidly away at the temperature of 212° F. But if the water be taken up to the top of Mont Blanc, where the air pressure is less than that at the sea-level (or if, which amounts to the same thing, we reduce the pressure on the water's surface by placing it under the receiver of an air-pump and partially exhaust the air), it will boil at a lower temperature than this. If, on the other hand, we increase the pressure on the surface of the water by any means, such, for example, as by placing it in a chamber of compressed air, the water can be heated to a higher temperature without boiling away. In the bygone era of which we are speaking the pressure of the atmosphere on the water's surface was 700 lb. or 800 lb. to the square inch; and therefore it could be heated up to a high temperature without evaporating rapidly. Another thing began to happen in those days. All bodies in space attract one another; the Sun attracts its planets; the planets attract the Sun and their satellites; and the satellites in their turn attract the planets. Ages before the earth had a moon these forces were at work. But the attractions of solid bodies for one another do not bring about any very perceptible alterations in their shapes; though if the bodies are spinning they effect slow changes in their speed of rotation. It is different when the bodies are liquid, or if they have liquid surfaces. Then the attractions of a sun or a moon on a planet begin to draw up the waters of the planet and produce tides. The attraction of the earth would produce tides on the Moon if an ocean existed there; and, it is suspected, do produce something resembling tides on the present surface of the Sun.[8] As soon, therefore, as oceans appeared on the earth the waters began to ebb and flow in tides. (Another consequence of this constant ebb and flow was that the friction of these movements began to diminish the speed of the earth's rotation--just as a string that was placed round the circumference of a spinning-top would, if constantly pulled backwards and forwards, gradually help to slow down the top.) Then oceanic waves and currents would begin to eat a way into the land that was on their borders, or which was emerging from their depths. Rivers would begin to arise, and they would carry on the work of erosion.