66 HARBOUR CONSTRUCTION. 



transported hither and thither, in the form of vapour, by the 

 winds. 



It is, moreover, all returned to the ocean again, either directly 

 in the form of rain or by rivers, after they have collected it from 

 their catchment areas, thus doubling the effect. 



It is obvious that so large a volume of water as this cannot 

 be abstracted from the ocean and returned to it again, in the 

 course of a year, without the circulation of its waters being, in 

 a material degree, affected by it. 



Respecting this, Sir Wy ville Thomson states 



"My general views with regard to ocean-circulation remain un- 

 altered. I think, however, that we have now good reason to believe 

 that the indraught of water at a low temperature into the Atlantic 

 and Pacific gulfs as we may almost call them from the Southern 

 Sea, is to a great extent due to an excess of precipitation over 

 evaporation in the ' water-hemisphere,' and a corresponding excess 

 of evaporation over precipitation in the ' land-hemisphere ; ' that, in 

 fact, a part of the circuit of general ocean-circulation passes through, 

 the atmosphere." l 



Evaporation affects the motion of water in another way. 



When it takes place from salt water, the water which remains 

 is salter, and therefore heavier, than it was before. Thus, though 

 raising the temperature of salt-water reduces its specific gravity, 

 evaporation tends to increase it. When, therefore, salt water 

 is being heated, two processes are in operation, tending to 

 counteract each other. 



The surface water of the ocean, as it becomes salter by 

 evaporation, is prone to sink. Its subsidence is, however, 

 impeded by its relatively high temperature and by the manner 

 in which the waves stir up the water to considerable depths and 

 cause its particles to mingle. 



During the voyage of the Challenger the question of the 

 specific gravity of ocean water was very fully investigated. 

 Numerous samples of surface water and of water from various 

 depths and localities were obtained, and their specific gravities 

 carefully ascertained and compared, due allowance being made 

 for differences in temperature. From these tests it was found 

 that, usually, the specific gravity of the surface water was the 

 highest. It would, therefore, appear that in the open ocean the 



1 " Voyage of the Challenger." 



