There are some other things beside common salt in sea water. But the salt is the most abundant, and we need not trouble about the rest at present. Now, where did all this mineral matter in the sea come from? The salt of the sea is all derived from the waste of the rocks. It has already been pointed out how, both underground and on the surface of the land, water is always dissolving out of the rocks various mineral substances, of which salt is one. Hence the water of springs and rivers contains salt, and this is borne away into the sea. So that all over the world there must be a vast quantity of salt carried into the ocean every year. The sea gives off again by evaporation as much water as it receives from rain and from the rivers of the land. But the salt carried into it remains behind. If you take some salt water and evaporate it the pure water disappears, and the salt is left. So it is with the sea. Streams are every day carrying fresh supplies of salt into the sea. Every day, too, millions of tons of water are passing from the ocean into vapor in the atmosphere. The waters of the sea must consequently be getting salter by degrees. The process, however, is an extremely slow one. Although sea water has probably been gradually growing in saltness ever since rivers first flowed into the great sea, it is even now by no means as salt as it might be. In the Atlantic Ocean, for example, the total quantity of the different salts amounts only to about three and a half parts in every hundred parts of water. But in the Dead Sea, which is extremely salt, the proportion is as much as twenty-four parts in the hundred of water. Standing by the shore and watching for a little the surface of the sea, you notice how restless it is. Even on the calmest summer day, a slight ripple or a gentle heaving motion will be seen. Again, if you watch a little longer, you will find that whether the sea is calm or rough, it does not remain always at the same limit upon the beach. At one part of the day the edge of the water reaches to the upper part of the sloping beach; some six hours afterward it has retired to the lower part. You may watch it falling and rising day by day, and year by year, with so much regularity that its motion can be predicted long beforehand. This ebb and flow of the sea forms what are called tides. If you cork up an empty bottle and throw it into the sea, it will of course float. But it will not remain long where it fell. It will begin to move away, and may travel for a long distance until thrown upon some shore again. Bottles cast upon mid-ocean have been known to be carried in this way for many hundreds of miles. This surface-drift of the sea water corresponds generally with the direction in which the prevalent winds blow.