338 GEOLOGY. 



lunar tide is more important than the solar, not because the attraction 

 of the moon is greater, for it is not, but because its differential attrac- 

 tion, the result of its lesser distance, is greater. 



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 favor- 

 able 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 

 essenlially 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 ex- 

 tremely 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 Libson in 1755, is greatly aggravated by accompanying sea- 

 waves. Earthquake- waves differ from ordinary waves in being trans- 

 latory, 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. 



