THE SEA AND ITS WORK It is well known that the waters of the sea cover nearly three-fourths of the surface of the earth. We think of the United States as being a large piece of land of over three million square miles--but the sea is about forty-five times as large, that is, it covers approximately 140,000,000 square miles. It is a remarkable fact that the average depth of the great oceans of the earth is nearly two and one-half miles. If the sea were universally present everywhere with the same depth, it would be almost two miles deep. Yet this vast body of water is an extremely thin layer when compared with the earth's diameter of 8,000 miles. The Pacific is the deepest of the oceans with an average depth of about two and three-fourths miles. The deepest ocean water ever sounded is 32,114 feet (over six miles), not far from the Philippine Islands. This is known as the Planet Deep, and was discovered in 1912. Second deepest is 30,930 feet near the island of Guam. In the Pacific Ocean there are five places where the water is over five miles deep and eleven places were it is over four miles deep. The deepest sounding ever made in the Atlantic Ocean was 27,972 feet, not far from Porto Rico. Many substances are known to be in solution in sea water, but in spite of this the composition is remarkably uniform. The most abundant substance by far in solution is common salt. In every 100 pounds of sea water, there are 3.5 pounds of mineral matter of various kinds dissolved. Nearly 78 per cent of the dissolved matter is common salt. The principal other constituents in solution are chloride and sulphate of magnesia, and the sulphates of lime and potash. All other dissolved mineral substances together make up less than one per cent of the total. It has been estimated that if all the dissolved mineral matter should be brought together, it would form a layer 175 feet thick over the whole sea bottom. The salts of the sea have been mostly supplied by the rivers, which in turn have derived them from the disintegration and chemical decay of the rocks. If we make a general comparison with the surface of the land, the floor of the ocean is a vast monotonous plain. None of the sea bottom compares with the ruggedness of mountains, and even the more level portions of land surface show many sharp minor irregularities such as stream trenches. But the sea bottom is characterized by its smoothness of surface. There are under the sea, however, mountain-like ridges, plateaus, submarine volcanoes and valleys known as "deeps." But these rarely show ruggedness of relief like similar features on land.