Page:South African Geology - Schwarz - 1912.djvu/26

 there through Russia to Nova Zembla, is in the course of being measured, and when it is completed we shall have better data on this side of the globe to argue from.

The Relation between Land and Sea. The greater portion of the earth's surface is covered with water, roughly about three-quarters are ocean, and the continents project above the ocean basins as broad flat plateaux. The moon, as it revolves round the earth, causes the water to heap up below it, owing to the attraction of its mass to the liquid mass. On the other side of the globe to the moon the gravity due to the earth's attraction is lessened by the attraction due to the moon, so that an equal tidal bulge of water occurs on both sides of the globe. The sun, although much larger than the moon, is farther away, and hence the attraction due to the sun is less than that due to the moon; numerically, if we take the attraction of the moon as 100, that of the sun is 38. If the sun and moon are both on the same side of the globe, as at new moon, then the two attractions act together, and spring tides occur; the same happens when the two luminaries are on opposite sides at full moon. At the quarters of the moon the joint tide is the difference of the attractions due to the sun and moon, when neap tide occurs.

So that the difference between spring and neap tide is 138 to 62, or about 7 to 3. If the sun and moon can exert such an attraction on the waters of the globe, the continents, which are close at hand, must equally attract the water, and hence the ocean surface does not conform to the shape of the solid