616 report— 1878. 



tling against the palseocrystic ice, keeps imperfectly open the water-way by which 

 Nordenskjold hopes to work his course to Behring's Strait. The southern de- 

 flections are practically lost, being to a great extent, though not entirely, dissipated 

 in the great westerly current of the southern anti-trades. 



One of the most singular results of these later investigations is the establish- 

 ment of the fact that all the vast mass of water, often upwards of 2000 fathoms in 

 thickness, below the neutral band, is moving slowly to the northward ; that in fact 

 the depths of the Atlantic, the Pacific, and the Indian Oceans are occupied by 

 tongues of the Antartic Sea, preserving in the main its characteristic temperatures. 

 The maintenance of a low temperature while the temperature of the floor of the ocean 

 must be higher, and that of the upper layers of the sea greatly higher, is in itself a 

 conclusive proof of steady movement of the water from a cold source ; and the 

 fact that the temperature of the lower layers of water, both in the Atlantic and 

 the Pacific, is slightly but perceptibly raised to the northward, while the continuity 

 of every layer with a corresponding layer in the southern sea can be clearly traced, 

 indicates the southern position of that source. 



The immediate explanation of this very unexpected phenomenon seems simple. 

 From some cause or other, as yet not fully understood, evaporation is greatly in 

 excess of precipitation over the northern portion of the land-hemisphere, while 

 over the water-hemisphere, and particularly over its southern portion, the reverse 

 is the case ; thus one part of the general circulation of the ocean is carried on 

 through the atmosphere, the water being raised in vapour in the northern hemi- 

 sphere, hurried by upper wind currents to the zone of low barometric pressure in 

 the south, where it is precipitated in the form of snow or rain, and, welling thence 

 northwards in the deepest channels on account of the high specific gravity depen- 

 dent on its low temperature, supplies the place of the water which has been re- 

 moved. 



The cold water wells northwards, but it meets with some obstructions on its 

 way, and these obstructions, while they prove the northward movement, if further 

 proof were needed, bring out another law by which the distribution of ocean tempe- 

 rature is regulated. The deeper water sinks slowly to a minimum at the bottom, 

 so that if we suppose the temperature at a depth of 2000 fathoms to be 36° F., 

 the temperature at a depth of 3000 may be, say, 32°. Now, if in this case the 

 slow current meet on its northward path a continuous barrier in the form of a sub- 

 marine mountain ridge rising to within 2000 fathoms of the sea-surface, it is clear 

 that all the water below a temperature of 36° will be arrested, and, however deep 

 the basin beyond the ridge may be, the water will maintain a minimum of 30° from 

 a depth of 2000 fathoms to the bottom. In many parts of the ocean we have most 

 remarkable examples of the effect upon deep-sea temperature of such barriers inter- 

 secting cold in-drauglits, the most marked instance, perhaps, a singular chain of 

 closed seas at different temperatures among the islands of the Malay Archipelago ; 

 but we have also a striking instance nearer home. Evaporation is greatly in excess 

 of precipitation over the area of the Mediterranean, and consequently, in order to 

 keep up the supply of water to the Mediterranean, tbere is a constant inward cur- 

 rent through the Straits of Gibraltar from the Atlantic ; I need not at present refer 

 to an occasional tidal counter-current. The minimum temperature of the Mediter- 

 ranean is about 54° F. from a depth of 100 fathoms to the bottom. The tempe- 

 rature of 54° is reached in the Atlantic at the mouth of the Straits of Gibraltar at 

 a depth of about 100 fathoms, so that in all probability future soundiu>rs will show 

 that the free water-way through the Straits does not greatly exceed 100 fathoms 

 in depth. 



The Depth of the Sea, and the Nature of Modern Deposits. — It seems now to be 

 thoroughly established by lines of trustworthy soundings which have been run 

 in all directions, that the average depth of the ocean is a little over 2000 fathoms, 

 and that in all probability it nowhere exceeds 5000 fathoms. Depths beyond 4000 

 fathoms are rare and very local, and seem to be usually pits in the neighbourhood 

 of volcanic islands. In all the ocean basins there are depressions extending over 

 considerable areas where the -depth reaches 3000 fathoms or a little more, and 

 these depressions maintain a certain parallelism with the axes of the neighbouring 

 continents. 



