A soap-bubble in the wind could hardly be more flexible in form and sensitive to influence than is the earth.  On the morning of May 9th, 1876, the earth’s crust at Peru gave a few great throbs upward, by the action of expansive gases within.  The sea fled, and returned in great waves as the land rose and fell.  Then these waves fled away over the great mobile surface, and in less than five hours they had covered a space equal to half of Europe.  The waves ran out to the Sandwich Islands, six [Page 145] thousand miles, at the rate of five hundred miles an hour, and arrived there thirty feet high.  They not only sped on in straight radial lines, but, having run up the coast to California, were deflected away into the former series of waves, making the most complex undulations.  Similar beats of the great heart of the earth have sent its pulses as widely and rapidly on previous occasions.

The figure of the earth, even on the ocean, is irregular, in consequence of the greater preponderance of land—­and hence greater density—­in the northern hemisphere.  These irregularities are often very perplexing in making exact geodetic measurements.  The tendency of matter to fly from the centre by reason of revolution causes the equatorial diameter to be twenty-six, miles longer than the polar one.  By this force the Mississippi River is enabled to run up a hill nearly three miles high at a very rapid rate.  Its mouth is that distance farther from the centre of the earth than its source, when but for this rotation both points would be equally distant.

If the water became more dense, or if the world were to revolve faster, the oceans would rush to the equator, burying the tallest mountains and leaving polar regions bare.  If the water should become lighter in an infinitesimal degree, or the world rotate more slowly, the poles would be submerged and the equator become an arid waste.  No balance, turning to 1/1000 of a grain, is more delicate than the poise of forces on the world.  Laplace has given us proof that the period of the earth’s axial rotation has not changed 1/100 of a second of time in two thousand years.

[Page 146] Tides.

But there is an outside influence that is constantly acting upon the earth, and to which it constantly responds.  Two hundred and forty thousand miles from the earth is the moon, having 1/81 the mass of the world.  Its attractive influence on the earth causes the movable and nearer portions to hurry away from the more stable and distant, and heap themselves up on that part of the earth nearest the moon.  Gravitation is inversely as the square of the distance; hence the water on the surface of the earth is attracted more than the body of the earth, some parts of which are eight thousand miles farther off; hence the water rises on the side next the moon.  But the earth, as a whole, is nearer the moon than the water on the opposite side, and being drawn more strongly, is taken away from the water, leaving it heaped up also on the side opposite to the moon.