The distance of the moon from the earth is about 240,000 miles. If this be taken as the distance from the center of the moon to the center of the earth, 236,000 and 244,000 miles respectively are the distances from the center of the moon to the nearest and most distant points on the earth. The distance of the sun from the earth is about 93,000,000 miles. If this be taken as the distance between the centers of these bodies, then the distances from the center of the sun to the nearest and most distant points on the earth’s surface are 92,996,000 and 93,004,000 miles respectively. The ratio of 4000 to 236,000 or to 244,000 is much greater than the ratio of 4000 to 92,996,000 or to 93,004,000. Hence the tide-producing force of the moon is greater than that of the sun. The tides show themselves along shores in the form of waves which, in shallow water, become translatory. They differ from the wind-waves in their periodicity, and locally in their greater height. The effects of the tidal waves on the shores of the sea, and on the bottom in shallow water, are the same as the effect of wind-waves of equal strength, and need not be separately considered in connection with the gradation of the sea-bottom. In passing through narrow straits or narrow passes of any sort, the tidal movement becomes a current which, under favorable conditions, abrades or “scours” the bottom effectively. The tidal currents in the narrow passes about New York harbor may serve as an illustration. =Aperiodic movements.=—In addition to the wind-waves which are essentially constant and universal, and to the tidal waves, which are periodic, there are accidental waves which are locally and temporarily of importance. Such are earthquake-waves, which are sometimes extremely destructive. Thus an earthquake-wave on the coast of Peru in 1746 swept a frigate several miles inland and deluged Lima, seven miles from the shore. The havoc of most earthquakes affecting coasts, such as that of Lisbon in 1755, is greatly aggravated by accompanying sea-waves. Earthquake-waves differ from ordinary waves in being translatory, and so in being more effective on the bottom in deep water. Their greatest force, however, is felt in shallow water and on shores. Volcanic eruptions likewise give rise to exceptional aperiodic waves. The same is true of landslides where they affect the coast or any part of the sea-bottom. The fall of glacier ends and the capsizing of icebergs likewise generate strong waves. To the category of exceptional waves also belong those generated by the winds of exceptional storms, such as that which devastated Galveston in 1900.[149] =Summary.=—From the point of view of their direct geological results in shallow water, all movements of the sea-water may be grouped into two main classes—(1) waves, with the undertow and the littoral currents they generate, and (2) ocean-currents.[150] WAVES.