On the Depth and Saltness of the Ocean. 31 



- ture is met with both within the polar circles and in proceed- 

 ing towards the equator. In the higher latitudes above 60°, 

 the ocean, in descending, increases in temperature until it 

 arrives at its mean point; while proceeding towards the 

 equator, it decreases from the surface downwards ; this de- 

 ci'easebeyond the tropical circle is about twenty-three fathoms, 

 for every degree of latitude. Within the ti'opics it is l'" for 

 every thirteen fathoms of depth until 400 fathoms, after 

 which it requii'es a descent from 200 to 300 fathoms to effect 

 a like change. 



From the observations of Admiral D'Urville, it would' ap- 

 pear that the waters of the Mediterranean do not follow tbe 

 rate of descent of the Atlantic and Pacific Oceans. He esti- 

 mated the mean temperatui'e of that sea, below 200 fathoms, 

 at 55°, and this from the fact of his having obtained that tem- 

 perature at the depth of 1000 fathoms. If this be so, it leads 

 to an interesting inquiry as to whether it may not be in con- 

 sequence of the vast internal fii'es that are known to prevail 

 in the countries that surround it. 



The penetrations of solar light, or the depths at which it 

 becomes totally absorbed, is another subject which claimed 

 particular attention during the cruise of the Exploring Ex- 

 pedition. The mode of obtaining results was to let down a 

 pot, bottom upwards, painted white, some eighteen inches 

 in diameter, by the deep sea-line until it ~was lost sight of, 

 noting the depth at which it disappeared, and then again its 

 reappearance, the mean being taken for the result ; these sel- 

 dom differed more than a fathom ; the eye was placed five 

 feet above the surface in the direction with the line by which 

 the pot was held. It would appear at first that the depth at 

 which an object could be seen would depend upon the inten- 

 sity as well as the angle at which the rays of light fell upon 

 the surface of the ocean. They undoubtedly have some effect, 

 but seldom made a greater difference than one and a half fa- 

 thoms. Under different latitudes, and in different tempera- 

 tures of the water, the anomalies far exceeded this, and were 

 indeed too great not to excite inquiry and call attention to 

 other causes. Tliere is little doubt that the great cause of the 

 variation noticed in the temperature of the waters affected in 



