x The Hydrosphere 175 



the same surface temperature, sinks to 70 at about 200 

 fathoms, and falls as low as 37 at 1200, where it is pre- 

 vented from entering the Red Sea basin by the ridge. The 

 surface water in the Red Sea is densest when its tempera- 

 ture is lowest in winter, and the dense layers at 70 tem- 

 perature sink to the bottom, so that the whole basin below 

 the level of the barrier assumes and maintains the lowest 

 average winter temperature of the air above. The hotter 

 water in summer being less dense on account of its expan- 

 sion, though it contains more salt, remains floating on the 



Indian Ocean 



Red Sea 



FIG. 34. Temperature Section of the Red Sea and Indian Ocean ; showing the 

 action of a barrier in separating bodies of water at different temperatures. 

 The shading is darker as the temperature is lower. Not drawn to scale. 



surface, and its heat passes down only by the slow process 

 of conduction. The Mediterranean furnishes another in- 

 stance of the same distribution of temperature. The sill 

 separating its basin from the Atlantic is 190 fathoms below 

 the surface, and the water on it is at 55, a temperature 

 which prevails to the bottom, of the Mediterranean, while 

 in the Atlantic the temperature falls as low as 35 at the 

 same depth. 



237. Circulation of Seas by Concentration. The 

 great evaporation in the Red Sea raises the density of its 

 surface water (at 60 F.) to 1-0300, and the salinity is 4 

 per cent. The level of the sea is lowered by evaporation 

 to the extent of from 10 to 25 feet a year, and a surface 

 current of the fresher but equally hot water of the Indian 

 Ocean is consequently always pouring in. If there were no 

 return current of dense salt water, it is calculated that the 



