146 National Geographic Magazine. 



ically the tides of an ideal ocean of known depth and configura- 

 tion remains still unsolved. According to Ferrel our present 

 knowledge of tidal phenomena is comparable to that possessed 

 2,000 years ago of the science of astronomy. 



Temper A.TURE of the Sea. 



The temperature of sea water had already been observed by 

 Ellis, in 1749, in the Atlantic, and subsequent expeditions have 

 furnished a great number of temperature observations in various 

 seas and for various depths. The diversity of instruments and 

 of methods employed by the earlier observers, and the faulty 

 methods of recording, have made the uniform reduction of many 

 of these observations difficult or impossible. The most complete 

 and valuable collection of these older observations up to 1868, 

 with an account of the instruments and methods used by each 

 observer, was published by Prestwich, in 1876, in the Philo- 

 sophical Transactions, Vol, 165. 



With the advent of the great scientific expeditions, which were 

 supplied with modern and refined instruments, our knowledge 

 of the thermal conditions of the sea has progressed immensely, 

 and we are now able to construct charts of all the oceans, show- 

 ing the distribution of the isotherms with considerable accuracy. 



The annual average surface temperature has been found higher 

 in the Indian Ocean than in either the Atlantic or Pacific ; the 

 North Atlantic is slightly warmer than the North Pacific, but the 

 South Pacific is warmer than the South Atlantic ; this holds gen- 

 erallvgood also for the temperatures between surface and bottom. 



The tempei-ature generally decreases more or less rapidly from 

 the surface down to about 500 fathoms, at which depth it is quite 

 uniformly between 39° and 40° F. From that depth it decreased 

 slowly towards the bottom: in the Polar seas to between 27° and 

 28° F.; in the middle and higher latitudes of the northern hemi- 

 sphere and at depths of 2,000 to 3,000 fathoms, to between 34° 

 and 36° F. ; at the equator and in southern latitudes it remains 

 in the neighborhood of 32° F. 



The low temperatures at the bottom are thought to be due to 

 a steady but slow circulation of water from the Polar seas 

 towards the equator, and, where the circulation is most free and 

 unobstructed, as in the South Atlantic, South Pacific and Indian 

 Ocean, the bottom temperature is slightly lower than in the 

 North Atlantic and North Pacific, both of which are connected 

 with the Polar Sea by comparatively narrow and shallow straits. 



