60 THE OCEAN 



interposition of the little Sea of Marmora 

 between the two principal seas. The result is 

 an outflowing surface current of brackish 

 water through the Bosphorus into the Medi- 

 terranean, and a return undercurrent of dense 

 saline Mediterranean water into the Black 

 Sea (see Fig. 2). There is a similar interchange 

 of water by currents at the surface and 



A tl an tic y 

 Ocean ^ 



FlQ. 2. — Diagrammatic section showing the inflowing and outflowing 

 currents between the Mediterranean and the North Atlantic on the 

 one hand, and the Mediterranean and Black Sea on the other. 



below it between the Red Sea and the Indian 

 Ocean through the Strait of Bab-el-Mandeb. 



Gases in Sea-Water. — Atmospheric air is, 

 roughly speaking, composed of four-fifths of 

 nitrogen to one-fifth of oxygen, but oxygen 

 is much more soluble in water than nitrogen, 

 so that these two gases are not present in the 

 same proportion when air is dissolved in 

 water, the ratio of oxygen to nitrogen being 

 much increased. Dry air free from extrane- 

 ous substances contains about 21 per cent, of 

 oxygen ; air extracted from rain-water con- 

 tains about 31 per cent, of oxygen, and air 



