1870.] 



Deep-sea Researches. 



163 



and Jebel Musa or Apes Hill, which lies opposite to it, at a distance of only 

 8 geographical (9^ statute) miles, on the African coast, the Strait being here 

 nearly at its narrowest : and it was also that at which the greatest depth 

 (510 fathoms) was indicated by the Soundings marked on the Chart. 

 With this depth our own Sounding, which gave a bottom at 5 1 7 fathoms, 

 agreed very closely ; and having thus at once found the position most ad- 

 vantageous for our work, that position was precisely determined by angles 

 taken by sextant from the Ship between conspicuous objects on the shore. 

 The 5o^r>m-temperature obtained in the first sounding was between 5° and 

 6° higher than that which had been met with at corresponding depths on 

 the bed of the Atlantic about 100 miles to the westward; whilst the 

 swr/ac<?-temperature was lower by from 5°*3 to 6°, as will be seen by the 

 following comparative statement : — 



Surface- Bottom- 





Station. 



Depth. 



temperature. 



temperati 



Strait of Gibraltar . 



. 39 



517 



66-0 



o 



55-5 





. 31 



477 



71*3 



50-5 





. 32 



651 



71.5 



50-0 





33 



554 



72-0 



497 





. 34 



414 



717 



50-0 



33. This striking difference led us to take a set of serial soundings at 

 intervals of 50 fathoms ; and these gave a result which, though it appeared 

 anomalous at the time, was afterwards fully explained, and proved to be 

 of unexpected import. The Temperature fell, at 50 fathoms from the sur- 

 face, to 56° ; at 100 fathoms it was 55°*7 ; at 150 it was 55 0, 5 ; and from 

 that depth to the bottom, at 517 fathoms, there was no further descent. 

 Now it will be shown hereafter (§ 8S) that the thermal condition which 

 here so much surprised us by its contrast with that of the Atlantic waters, 

 is that universally met with in the Mediterranean; the Temperature of 

 which, whatever may be its surface-elevation, falls to within 1° Fahr. above 

 or below 56° at a depth of 50 fathoms, to a degree or two lower at 100 

 fathoms, and then remains uniform down to the greatest depth (1743 

 fathoms) at which we examined it. And it thus appears that whilst the 

 surface- water in this part of the Strait is certainly derived from the Atlantic, 

 the deeper water, partaking of the thermal condition which so remarkably 

 characterizes that of the Mediterranean basin, may be fairly regarded as 

 belonging to the latter. 



34. This inference is in harmony with another fact ascertained on the 

 same occasion, viz. the great excess in Salinity shown by water brought up 

 from the depth of 250 fathoms over the water of the surface. Whilst the 

 Specific Gravity of the latter was found to be TO 271, that of the former 

 was T0293 ; and whilst the proportion of Chlorine in the latter was 20*324 

 per 1000, it was 21*775 in the former. Now in these particulars the 

 Surface-water agreed well with what had been found to be the condition 



p 2 



