1871.] 



Theory of the Ocean. 



545 



sure in that direction, from the greater density of the substratum at the 

 source of its origin ; but the temperature of the greater depths that are in 

 continuity seems to be of the same low normal condition below about 1000 

 or 1 200 fathoms, so that there is no such difference to set up an appre- 

 ciable horizontal movement in those deeps. 



Although undercurrents undoubtedly exist in the atmosphere, and thus 

 may lead to the possibility or belief in such general movements as a law 

 of the deeps of the sea also, yet the modes in which the solar influence 

 operates upon the two media are diametrically opposite. In the sea 

 the rarefying influence of the sun commences from, and therefore re- 

 mains at or near the surface, whilst in the atmosphere it commences from 

 below, and therefore disturbs and causes the lower strata to ascend. 



The sea is also a comparatively non-elastic fluid, whilst the air is the 

 most elastic, and thus yields to every local influence, whether of heat or cold. 



The isothermal temperature of the ocean deeps (viz. about 39|- Fahr.) 

 has been supposed to be that at which the water attains its greatest den- 

 sity, probably because it is found at the lowest tried depths of the At- 

 lantic and Antarctic seas, and because of its being the temperature of 

 greatest density of fresh water ; and therefore it has been said that a lower 

 temperature made sea-water lighter, causing it to float upon that at the 

 above-mentioned temperature. 



But this is contradicted by the temperatures found by Sir E. Parry, 

 and by the recent experiments of M. Edland, M. Despretz, and others, 

 which seem to show that the greatest density of sea- water is attained between 

 22° and 25° Fahr. 



It seems to me therefore (and I was impressed with the opinion before 

 knowing of this fact and the statements that confirm it) that this iso- 

 thermal temperature of 39|° — 4°, found throughout the Antarctic deeps, 

 is the settled mean temperature produced by the atmospheric influence 

 upon these areas, as about 59° Fahr. is of the eastern basin of the Mediter- 

 ranean, and about 55 J° Fahr. is of the deeps of the Greek Archipelago, and 

 54 G for the Sea of Marmora* — this difference in similar depths occurring in 

 consequence of the separation of the deeps of the two basins by a submerged 

 but comparatively shallow ridge between them, as the Mediterranean deeps 

 are separated from the Atlantic by the shallowest part of the Straits of 

 Gibraltar, and with an isothermal temperature of 59° for the deeps on one 

 side, and of 39^° on the other, subject to the deduction of 4° or 5° from 

 each. 



These facts suggest the view that there really may not be an exact 

 correspondence between the lowest temperatures of the Atlantic, Pacific, 

 and Indian oceans, although, when a temperature in excess of or under 

 39J° has been found, there has generally been supposed, since Sir James 

 Ross's establishment of this as the normal temperature of the ocean deeps 

 to be an error of observation, or a defect in the instrument used. 



* —4° for each. 



VOL. XIX. 2 u 



