128 The Colour of the Sea 



Roy. Soc., 1888, XLIII. p. 356), supplied the evidence which 

 definitively established the fact that coral islands are not 

 a product of subsidence. 



When the survey of this shoal had been completed, in 

 so far as the time at the disposal of a steamer engaged on 

 a commercial mission permitted, a line of soundings was run 

 from the "Patch" to the African coast at Mogador. Inde- 

 pendently of the high land which is visible from the sea at a 

 distance of many miles, the approach of the coast is indicated 

 by a fall in the temperature of the water of the sea surface, 

 and a remarkable change in its colour. Outside, the tempera- 

 ture of the surface water was 21 C., and its colour was ultra- 

 marine. After sighting the land its temperature fell, at first 

 slowly, then rapidly, and, when at a distance of two miles 

 from Mogador, it was only 16 C. The colour at the same 

 time had become a pure olive-green, which maintained its 

 transparency until close to the shore, where it became masked 

 by the solid matter kept continually in suspension by the 

 mechanical energy of the breaking waves. 



The pure green colour of the water and its temperature, 

 so much lower than that which could persist at the surface 

 of the sea in the latitude of Mogador, made me for a moment 

 think that it might be in reality Antarctic water which had 

 found its way, at or near the bottom, into the northern hemi- 

 sphere, having been diverted first to the west while in the South 

 Atlantic, then to the east after crossing the line. But this 

 idea could persist only for a moment, because the temperature 

 and the density of the bottom water were found to be those 

 characteristic of the bottom water of the eastern basin of 

 the North Atlantic, as shown by the "Challenger" observations, 

 and these are much higher than those of any other ocean. 



The low temperature of the water showed that it could 

 not come on the surface from the north or south or west of it, 

 and the only source from which it could come was from below 

 the surface. Deep water comes close to the coast, and the 

 water at 2000 fathoms was found to have a temperature of 

 2-5 C., so that the supply of cold from this source was adequate, 

 and it was available with a very small expenditure of energy. 



