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 which 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 waves, 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 
