CHAPTER VIII 



HYDROGRAPHY 



We pass now to a consideration of the chief physical 

 characteristics of the oceans — the Earth is supposed to be 

 the only planet in our solar system which has oceans. These 

 physical characteristics may aU be grouped under the general 

 term Hydrography, and the following may serve as a con- 

 venient list of the more important subdivisions : — Size, 

 Depth, Temperature, Salinity, Density, Pressure, Colour, 

 Penetration of Light, Viscosity, and Alkalinity. There 

 are a few other physical phenomena of the ocean which for 

 various reasons are omitted from this brief summary of 

 the subject. 



Size of the Ocean. 



First, as to the extent of the oceans relatively to the land, 

 it is known that water covers more than two-thirds of the 

 surface of the globe, and it has been calculated that the 

 volume of the dry land above sea-level is 23 millions of cubic 

 miles, while the volume of the ocean is many times more, 

 about 300 to 320 miUions of cubic miles according to different 

 estimates. The mean height of the land is 2,300 feet and 

 the average depth of the sea 11,500 feet ; but the greatest 

 height of the land (Mount Everest, 29,002 feet) and the 

 greatest known depth of the sea (5,348 fathoms = 32,089 feet) 

 are nearly the same, the mountain being over 5| and the 

 sounding a httle over 6 miles. The disproportion between 

 land and sea is constantly increasing in consequence of the 

 wearing down of the land. It is supposed that the material 

 carried from the land to the oceans is about 3-7 cubic miles 



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