1871.] 



Theory of the Ocean. 



543 



Mediterranean as compared with the deeps of the Atlantic Ocean on the 

 west side of the 150- or 160-fathom barrier that separates the one from the 

 other at the western embouchure of the Straits of Gibraltar. 



The very high temperature of the depths of the Mediterranean below 

 about 200 fathoms, in all seasons, as compared with that of the Atlantic 

 and Pacific (where, according to Ross, Belcher, Denham, Pullen, and 

 others, it seems to remain at about 39j° Fahr. * in all latitudes between 

 the Arctic and Antarctic zones) results apparently from its insulation 

 from the Atlantic deeps by the 1 5 0-fa thorn bank or submarine ridge across 

 the entrance of the Gibraltar Straits, and thus appears to have settled 

 into a mean resulting from a small terrestrial influence from below and 

 the large solar influence above, since the normal temperature is constantly 

 at 59° t at all depths below 100 to 200 fathoms. 



The fluctuations of temperature in the Mediterranean Sea are conse- 

 quently confined to this upper zone of about 100 fathoms, in which the 

 temperature varies with the seasons, being in the summer and autumn 

 from 10° to 20° higher than the normal temperature, whilst in winter it 

 rises up at the surface to the normal temperature of 59° — 4°, viz. 55° ; 

 and is then even sometimes 10° lower at the surface and a few fathoms 

 below it, viz. in January and February, the coldest months. 



In the same parallel in the Atlantic the normal temperature of 39j° 

 — 4° is not reached in summer in less than 1000, or in 1200 fathoms in 

 the tropics. This is a peculiar condition of the two seas deserving notice. 

 Had the normal temperature of the Mediterranean been as low as that of 

 the Atlantic, the superficial influence would no doubt have extended down- 

 wards to the same depths as in the Atlantic. Upon the first consideration of 

 these facts, however, the inference seems to be, that the Atlantic deeps are 

 under the influence of cooling-down undercurrents from the poles. But 

 appreciable undercurrent movements as a universal movement (such as the 

 theory advocates) I have no belief in, except, probably, where two great 

 streams meet, such as the Arctic current and the Gulf-stream. 



It has been well shown, too, in support of this opinion, during the 

 soundings taken across the Atlantic, that perfectly still water reigns in a 

 large area of its deeps, by the fact of the sounding-line, on several occa- 

 sions, having coiled itself upon the sinker when some 200 or 300 fathoms 

 more than the actual depth had been accidentally or intentionally paid out 

 from the ship, and thus the coils came up in a bunch together round the 

 deep-sea lead, around which the line had become coiled as it stood upright in 

 the soft ooze or clay usual in great depths. This result was, therefore, a most 

 excellent test for showing that no appreciable movement or current existed 

 in a very considerable portion of those depths ; for it proved that the line 

 must have descended in the lower depths quite vertically when slack, with 



* —3° or 4° as the constant error now to be applied to all the earlier deep-sea 

 temperatures. 



t —the 3° or 4° as the constant error. 



