READINGS IN PHYSICAL SCIENCE.[H]
IV.—THE SEA.
It has been ascertained that water covers about three times more of the earth’s surface than the land does. We could not tell that merely by what we can see from any part of this country, or indeed of any country. It is because men have sailed round the world, and have crossed it in many directions, that the proportion of land and water has come to be known.
Take a school-globe and turn it slowly round on its axis. You see at a glance how much larger the surface of water is than the surface of land. But you may notice several other interesting things about the distribution of land and water.
In the first place you will find that the water is all connected together into one great mass, which we call the sea. The land, on the other hand, is much broken up by the way the sea runs into it; and some parts are cut off from the main mass of land, so as to form islands in the sea. Britain is one of the pieces of land so cut off.
In the second place, you cannot fail to notice how much more land lies on the north than on the south of the equator. If you turn the globe so that your eye shall look straight down on the site of London, you will find that most of the land on the globe comes into sight; whereas, if you turn the globe exactly round, and look straight down on the area of New Zealand, you will see most of the sea. London thus stands about the centre of the land-hemisphere, midway among the countries of the earth. And no doubt this central position has not been without its influence in fostering the progress of British commerce.
In the third place, you will notice that by the way in which the masses of land are placed, parts of the sea are to some extent separated from each other. These masses of land are called continents, and the wide sheets of water between are termed oceans. Picture to yourselves that the surface of the solid part of the earth is uneven, some portions rising into broad swellings and ridges, others sinking into wide hollows and basins. Now, into these hollows the sea has been gathered, and only those upstanding parts which rise above the level of the sea form the land.
When you come to examine the water of the sea, you find that it differs from the water with which you are familiar on the land, inasmuch as it is salt. It contains something which you do not notice in ordinary spring or river water. If you take a drop of clear spring water, and allow it to evaporate from a piece of glass, you will find no trace left behind. Take, however, a drop of sea water and allow it to evaporate. You find a little white point or film left behind, and on placing that film under a microscope you see it to consist of delicate crystals of common or sea salt. It would not matter from what ocean you took the drop of water, it would still show the crystals of salt on being evaporated.
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.