But it is not merely the surface water which moves. You have learnt a little about icebergs; and one fact about them which you must remember is that, large as they may seem, there is about seven times more of their mass below water than above it. Now, it sometimes happens that an iceberg is seen sailing on, even right in the face of a strong wind. This shows that it is moving, not with the wind, but with a strong under-current in the sea. In short, the sea is found to be traversed by many currents, some flowing from cold to warm regions, and others from warm to cold. Here, then, are four facts about the sea:—1st, it has a restless surface, disturbed by ripples and waves; 2ndly, it is constantly heaving with the ebb and flow of the tides; 3dly, its surface waters drift with the wind; and 4thly, it possesses currents like the atmosphere. For the present it will be enough if we learn something regarding the first of these facts—the waves of the sea. Here again you may profitably illustrate by familiar objects what goes on upon so vast a scale in nature. Take a basin, or a long trough of water, and blow upon the water at one edge. You throw its surface into ripples, which, as you will observe, start from the place where your breath first hits the water, and roll onward until they break in little wavelets upon the opposite margin of the basin. What you do in a small way is the same action by which the waves of the sea are formed. All these disturbances of the smoothness of the sea are due to disturbances of the air. Wind acts upon the water of the sea as your breath does on that of the basin. Striking the surface it throws the water into ripples or undulations, and in continuing to blow along the surface it gives these additional force, until driven on by a furious gale they grow into huge billows. When waves roll in on the land, they break one after another upon the shore, as your ripples break upon the side of the basin. And they continue to roll in after the wind has fallen, in the same way that the ripples in the basin will go on curling for a little after you have ceased to blow. The surface of the sea, like that of water generally, is very sensitive. If it is thrown into undulations, it does not become motionless the moment the cause of disturbance has passed away, but continues moving in the same way, but in a gradually lessening degree, until it comes to rest. The restlessness of the surface of the sea becomes in this way a reflection of the restlessness of the air. It is the constant moving to and fro of currents of air, either gentle or violent, which roughens the sea with waves. When the air for a time is calm above, the sea sleeps peacefully below; when the sky darkens, and a tempest bursts forth, the sea is lashed into waves, which roll in and break with enormous force upon the land.