1870.] 



Deep-sea Researches. 



195 



reduction showed itself in the uppermost stratum of 10 fathoms; but 

 though the further reduction took place at a very slow rate, the tempera- 

 tures at this Station were decidedly below those at the other two, down to 

 100 fathoms, at which there was not a difference of 1° among the three. 



88. Bottom-Temperature. — The uniformity which showed itself in the 

 Temperature of the bottom (see Table, p. 221), at all depths below 100 

 fathoms, was very remarkable. The lowest bottom-temperature we anywhere 

 met with was 54° ; and this presented itself at a depth of 790 fathoms. The 

 highest we anywhere met with was 56°*5 ; and this presented itself in three 

 instances, at depths of 266, 390, and 445 fathoms. But that the trifling 

 elevation was not in any way dependent upon the smaller depth of these 

 Soundings, was obvious from two considerations : — first, that our deepest 

 sounding gave a temperature of 56° on a bottom of 1743 fathoms, whilst 

 we found 55° at depths of 1456 and 1508 fathoms; and second, that the 

 slight variations observable among the fo^om-temperatures occurred also 

 between the temperatures taken at 100 fathoms. In fact, whatever the 

 temperature was at 100 fathoms, that was the temperature of the whole 

 mass of water beneath, down to the greatest depth explored. In that part 

 of the Western basin of the Mediterranean which lies between Gibraltar 

 and Sardinia, the bottom -temperature ranged between 54° and 55 0, 5, the 

 average being 54°* 9. East of this, in the neighbourhood of Sicily, the 

 bottom-temperature ranged between 55° and 56°*5, the average being 

 55 0, 8. It was because we thought it possible that the slight excess of 

 Bottom-temperature on this area might be due to Volcanic heat beneath, 

 that we directed our homeward course by way of Etna and Stromboli, for 

 the purpose of ascertaining if the near neighbourhood of a constantly 

 active Volcano has any influence in raising the temperature of the bottom. 

 No such influence, however, was perceptible ; the Temperatures obtained 

 at Stations 61 and 62,— namely 55°7 at 392 fathoms, and 55°"3 at 730 

 fathoms, — being rather below than above the average. 



89. The remarkable contrast thus presented to the slow but continuous 

 reduction of temperature encountered in the successive strata of Oceanic 

 water in the great Atlantic basin, and to the sudden fall which presents itself 

 as the Thermometer descends to its lower depths (§ 79), excites enquiry 

 into the cause of the difference. It is now clear that no amount of surface- 

 heat has power directly to affect the temperature of sea-water to a greater 

 depth than 100 fathoms, the elevation of temperature it produces below 

 30 fathoms being very slight ; and it seems also clear that the uniform 

 temperature of from 54° to 56°'5 encountered below the 100 fathoms' 

 stratum, represents the permanent temperature of the great mass of water 

 which occupies the Mediterranean basin. Now this mass is entirely cut off 

 from the influe of the G-eneral Oceanic Circulation, the surface-inflow 

 through the Strait of Gibraltar having no other effect than slightly to lower 

 the general temperature at the western extremity of the basin. And the 

 uniform permanent temperature of the mass of Mediterranean water may 



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