THE PHYSICAL CONDITION OF THE OCEAN. 353 



of enormous area and volume — is apparently again cut ofltby a subma- 

 rine ridge. The nortlr,vestern part of the Indian Ocean is for similar 

 reasons assumed to be divided from the main body, the shallower water 

 probably running from the Seychelles to the Maldive Islands. 



Mr. Buchan has pointed out why some parts of oceans, deep and vast 

 though they be, are, Avhen cut off from communication with others, 

 ■warmer at the bottom. Water can only sink through lower layers 

 when it is the heavier, and though a warmer surface current becomes 

 from evaporation denser, its heat makes it specifically lighter than the 

 strata below. It is only when such a current parts gradually with its 

 heat, as in traveling from tropical to temperate regions, that it sinks 

 and slowly but surely carries its temperature with it, modifying the 

 extreme natural cold of the bottom layers. In the North Atlantic and 

 Pacific we have such a condition. The great currents of the Gulf 

 Stream and Japan current as they flow to the north sink, and in the 

 course of ages have succeeded in raising the bottom temperatnre 3 or 

 I degrees. In the southern seas this influence is not at work, and 

 directly connected with the more open water around the South Pole, 

 there is nothing to carry to the abysmal depths any heat to raise them 

 from their normal low temperatures, due to the absence of any heating- 

 influence. The ice masses around the South Pole have probably little 

 or no effect on bottom temperature, as the fresher, though colder, water 

 will not sink; and, as a matter of fact, warmer water is found at a few 

 huiulred fathoms than at the surface. The lowest temperature ever 

 obtained was by Sir John Eoss in the Arctic Ocean in Davis Strait at 

 a depth of 080 fathoms, when he recorded a reading of 25° F. This 

 probably requires confirmation, as thermometers of those days were 

 somewhat imperfect. In the great oceans the greatest cold is found on 

 the western side of the South Atlantic, where the thermometer stands 

 at 32.3° F., but temperatures of 29° F. have been obtained of recent 

 years east of the Faroe Islands, north of the ridge which cuts oft" the 

 deeper waters of the Arctic from the Atlantic. 



Though scarcely within the limits of my subject, which is the sea 

 itself, I must say a few words on the sea floor. The researches carried 

 on in the Challenger revealed that, while for a certain distance from the 

 continents the bottom is comi)osed of terrestrial detritus, everywhere 

 in deep water it is mainly composed of the skeletons or remains of 

 skeletons of the minute animals that have lived in the water. In com- 

 paratively small depths we find remains of many shells. As the depth 

 increases to 500 fathoms or so we get mainly the calcareous shells of 

 the globigerina', which may be said to form by far the greater iiart of 

 the oceanic floor. In deei)er water still, where pressure, combined with 

 the action of the carbonic acid, has dissolved all calcareous matter, we 

 find an impalpable mud with skeletons of the siliceous radiolaria of 

 countless forms of the greatest beauty and complexity. Deeper still, 

 i. e., in water of, speaking generally, over 3,000 fathoms, we find a 

 SM 91 23 



