64 HARBOUR CONSTRUCTION. 



its turn, be warmed as it approaches the surface, and the warm 

 off-flowing stream will in this way be maintained. 



It is evident that, so long as the flow of warm water from 

 the surface continues, the supply of cold water must be kept up. 

 Whence, then, comes this supply ? 



From investigations which were carried on during the 

 voyage of H.M.S. Challenger* it would appear that the tem- 

 perature of the ocean is in general, though not always, highest 

 at the surface, and that it decreases rapidly for the first 

 hundred fathoms, then less rapidly down to 500 or 600 fathoms, 

 after which it decreases very slowly, but generally progressively, 

 to the bottom. 



These differences in temperature are undoubtedly connected 

 with the great ocean currents, which form a general system of 

 circulation, the movement of the warm surface water from the 

 tropics towards the poles necessitating a compensating return 

 current, or drift, from the poles towards the equator. 



It is worthy of note that this underlying mass of cold water, 

 just referred to, which at its lower depths has a temperature 

 scarcely, if at all, above the freezing-point of fresh water, should, 

 immediately under the equator and in other hot regions where 

 the surface water has a temperature of somewhere about 70 

 Fahr., approach nearer to the surface than elsewhere. The fact 

 that it does so seems to support the view, already expressed, 

 that the comparatively light column of warm water is raised by 

 the denser cold water forcing itself underneath it, whereby 

 the warmer water is made to flow off in the form of surface 

 currents. 



The mean normal temperature of the superficial portion of 

 the earth's crust, and therefore of the bed of the sea, may be 

 taken at not less than 45 or 46 Fahr. It is clear, therefore, 

 that this mass of cold water, which must be undergoing constant 

 renewal, owing to the flowing-off and evaporation of the surface 

 water, cannot derive its low temperature from equatorial regions, 

 but must come from a place where the conditions are such as to 

 impart to it a freezing temperature. 



Sir Wyville Thomson states 2 that he was able to trace a 



band of water of similar low temperature, continuously from the 



equator to the Antarctic Sea. There seems, therefore, to be no 



room for doubt that this cold water is supplied from the region 



1 "Voyage of the Challenger? Thomson, vol. ii. 2 Ibid , vol. ii. p. 302. 



