642 TEMPERATURE OF THE OCEAN. 



only the Temperate but also the inter-Tropical portions of the oceanic area, 

 a bottom-temperature prevails of between 32" and 35^" Fahr. ; whilst 

 within the Polar areas this temperature falls to 28°. Further, it may be 

 asserted, (2) that this vast oceanic basin, whose average depth may be 

 estimated at about two miles and a half, is occupied to within 400 fathoms 

 of its surface (save in the exceptional case of the Gulf Stream region), by 

 water whose temperature is below 40° F., — this cold water actually coming 

 up nearer to the surface in the Equatorial Atlantic (as several of the older 

 observers had noticed) than it does beneath the Tropics. 



Were the bottom of the Atlantic Ocean a perfect basin, the bottom- 

 temperatures might continue the same across its whole breadth, but, as 

 shown in the diagram, the Atlantic sea-bed is divided by ridges into 

 several basins. Now, it has been found that, however deep a basin or 

 depression of the sea-bed may be, yet that the temperature of its water at 

 the bottom will not be less than that which can pass across its ridge, from 

 outside sources. This was observed by Captain Chimmo in the Sulu Sea, 

 a striking example, where at and below a depth of about 300 fathoms 

 (the depth of the enclosing barrier) to the bottom, at 1,778 fathoms, the 

 temperature remained uniform at about 50° F. ; whereas, in the China Sea, 

 the flow of cold water from the Polar to the Equatorial regions reduced 

 the temperature, at a depth of 416 fathoms, to 41°, and at the bottom 

 (1,546 fathoms) to 37° F., 13° less temperature than at the deeper station 

 of the Sulu Sea. 



The same system holds good in such large areas of depressed sea-bed as 

 those in the North Atlantic Ocean, and it has been found that the bottom 

 temperature is never below 35-3° F. in the eastern basin, however deep the 

 water may be ; nor below 346° F. in the north-western. The only place 

 on the Atlantic sea-bed where the water has been found below 32° (freezing 

 point of fresh water), in the Temperate or Tropical regions, is in the 

 depression between the coast of South America and the central ridge to 

 the Soaih of the Equator. 



In (Kpnclusion, it may be remarked, that these deep-sea temperature 

 soundings are of great practical utility to telegraph engineers, and they will 

 be alluded to presently as affecting the question of Circulation. 



Salinity and Specific Gravity. — It is considered that the salt contained 

 in »ea-water is mainly derived from the land, the rain and springs dissolv- 

 ing substances from the surface and lower strata, which are carried down 

 to the sea by the streams and rivers. In regard to this subject, we cannot 

 do better than give first a few extracts from a very interesting essay, 

 laid before the Royal Society, by Professor George Forchhammer, of the 

 University, and Director of the Polytechnic Institute, at Copenhagen.* 



" The elements which occur in the greatest quantity in sea- water have 

 been long known, and the latest enquiries, my own included, have brought 

 the number of them up to twenty-seven, which are as follow : — 1, oxygen ; 

 2, hydrogen; 3, chlorine; 4, bromine; 5, iodine; 6, fluorine; 7, sulphur; 



* " On the Composition of Sea-water in different parts of the Ocean," in the " Philo- 

 sophical Transactions of the Royal Society,'' 1865, pages 203 — 262. 



