32 THE OCEAN WORLD. 



assuming its depth to be 400 feet only, and its width seven miles, 

 and that it contained the average proportion of solid matter, estimated 

 at one-thirtieth, it appears that salt enough to make eighty-eight cubic 

 miles of solid matter were carried into the Mediterranean in those 

 ninety days. " Now," continues Dr. Maury, " unless there were 

 some escape for all this solid matter which has been running into this 

 sea, not for ninety days, but for ages, it is very clear that the Medi- 

 terranean would long ere this have been a vat of strong brine, or a 

 bed of cubic crystals of salt." 



For the same reason. Dr. Maury considers it certain that there is 

 an under current to the south of Cape Horn, which carries into the 

 Pacific Ocean the overflowings of the Atlantic. In fact, the Atlantic 

 is fed unceasingly by the Great American rivers, while the Pacific 

 receives no important affluent, therefore ought to be, and is, sub- 

 jected to enormous losses, in consequence of the evaporation con- 

 tinually taking place at the surface. 



Tides. 



Tides manifest themselves upon our planet by the alternate 

 rising and falling of the waters of the ocean, and by currents also 

 in narrow seas. These effects are produced by two pairs of waves 

 which travel round the earth every day — a greater pair of waves 

 caused by the attraction of the moon, and a smaller pair caused by 

 the sun. The principal pair of waves which form the lunar tide 

 occupy a lunar day, which consists of twenty-four hours and fifty-four 

 minutes, in travelling round the earth ; while the smaller waves, 

 caused by the sun, take only twenty-four hours to traverse the same 

 distance. It thus sometimes happens that the crests and depressions 

 of the solar and lunar waves coincide. When this occurs we have 

 what are called spring tides, in which, owing to the union of the two 

 waves, the waters advance to their greatest height at the flood, and 

 retreat farthest in the ebb. The spring tides are followed after a 

 certain number of days by neap tides, in wliich the rising and falling 

 of the water is least. This comes to pass because the solar tide 

 travels quicker round the earth than the lunar tide, and accordingly 

 gains upon it day by day, until in about six or seven days after 

 spring tides the crest of the solar wave has advanced into the depres- 

 sion of the lunar wave, and partly fills it. 



By measuring the height of the spring tides, we get a quantity 

 equal to the sum of the heights of the solar and lunar wayes, and by 

 measuring the neap tides we get their difference. We may thus learn 

 by observation that the lunar wave is about two and a half times 



