The weight and power of such on-rushing masses of water are tremendous, as appears from the effect on coasts where they strike; but this opens up a subject which is too large for treatment here, and I must refer readers to geological treatises, and to such special works as Professor N. S. Shaler’s excellent “Sea and Land,” where the work of the ocean in tearing down and building up its coasts is fully and entertainingly explained. I shall have something more to say on this point, also, when I come to the chapter “Dangers of the Deep,” and speak of the terrible destruction caused by earthquakes, and in certain other agitations of the sea not due to the wind, and often styled “tidal waves.” There is only one kind of “tidal wave,” properly speaking, however; and this is a theoretical rather than an actual one, perceptible usually only in that rising and falling of the water along coasts twice each twenty-four hours that we call the flow and ebb of the tides; and here we see the effect rather than the thing itself. [Illustration: LOW TIDE, ST. JOHN’S HARBOR, N. B.] The tide has been an inevitable circumstance of the existence on the earth of the ocean, or any other great body of water, ever since its origin, yet it was not until Sir Isaac Newton made us comprehend the law of gravitation that its mystery was explained. We now know with certainty—if you want the mathematical formulæ and so forth, consult some good modern encyclopædia under the word _tide_—that this periodical rising and falling of the sea is due to the attraction of the sun and moon,—to the last three times as much as to the first, because it is so much nearer. This attraction is exerted toward the globe as a whole; and its visible effect upon the movable water is to lift it bodily on that side nearest the moon, and at the same time to pull away the earth from the water on the opposite side, which amounts to the same thing; and thus high tides are simultaneously produced at these antipodes, which accounts for the two a day. At the same time, however, the intermediate spaces have low tides caused by an attraction there toward the center of the earth. “There are thus always simultaneously and directly under the moon two high waters opposite each other, and two low waters at equal distances between them. Owing to the rotation of the earth, this permanent system of swells and troughs travels from east to west over every part of the ocean and of its coast, and explains the regular succession of rising and falling waters, at equal intervals of time, which we call the tides.” [Illustration: THE EARTHQUAKE WAVE PASSING OVER THE LIGHTHOUSE ON POINT ANJER.]