1870.] 



Deep-sea Researches. 



191 



hand, in the Equatorial- Polar flow of the warm stratum its lower layer would 

 be gradually reduced in temperature by moving over with the cold stratum 

 beneath*. Moreover as its surface would be exposed to a lower and 

 yet lower Atmospheric temperature the further North it moves, each super- 

 ficial layer as it is cooled will descend into the colder stratum, of which the 

 thickness will be progressively augmented at the expense of that which 

 overlies it. 



83. The Third group of ito^om-temperatures, which includes those taken 

 within the embouchure of the Strait of Gibraltar (Nos. 25-28, p. 220), 

 presents some peculiarities which are worthy of notice, when taken in con- 

 nection with the fact already stated as to the constant Temperature of about 

 55° found in the water of the Mediterranean at depths greater than 100 

 fathoms. For if we compare the Bottom-temperatures of Stations 25, 28, 

 29, 30, and 38, with those of Stations 31, 32, 33, and 34, we find in the 

 former a distinct elevation above the latter. Thus at Stations 25 and 

 28 the temperature was 53°'5, at 374 and 304 fathoms respectively ; 

 at Station 29 it was 55° at 227 fathoms ; at Station 30 it was 52°*7 at 

 386 fathoms ; and at Station 58 it was 54° at 503 fathoms : whilst at 

 Station 31 it was 50 o, 5 at 477 fathoms ; at Stations 32 and 34 it was 50° 

 at 651 and 414 fathoms respectively ; and at Station 33 it was 49°*7 at 

 554 fathoms. This difference seems fairly attributable to the influence of 

 the undercurrent which is now known to carry out the warmer Mediterra- 

 nean water to mingle with the colder water of the Atlantic, and which, after 

 flowing over the ridge between Capes Trafalgar and Spartel (§ 69), may 

 still for a time maintain its distinctness on the descending slope of the 

 Atlantic basin. It would probably not be difficult to trace its further 

 course by a sufficient number of observations on the Temperature and Spe- 

 cific Gravity of the bottom-water to the west of the Strait ; and it would 

 be very interesting thus to ascertain how far this undercurrent makes its 

 way before blending with the general mass of Atlantic water. — If a detailed 

 examination of the phenomena of the double current should be undertaken 

 by the Authorities at Gibraltar, this point should not be neglected. 



84. Density. — In order to determine the Salinity of the water of the At- 

 lantic Ocean as a basis for comparison with that of the Mediterranean Sea, 

 the proportion of Chlorine in 34 samples of the former, taken between Fal- 

 mouth and Lisbon, was determined by Volumetric analysis. Of these sam- 

 ples, 12 were of surface-water, 12 of bottom-water from various depths 

 down to 1095 fathoms, and 12 of intermediate water. The results are 

 expressed in Grammes per 1000 Cubic Centimetres of water. 



* It was long ago shown by Dr. Arnott, in his ' Elements of Physics,' that if two 

 layers of water originally of different temperatures, separated by a good conductor, move 

 in contrary directions, they will gradually exchange temperatures ; and this principle is 

 now applied in the construction of Coolers for Breweries and Distilleries, in which a 

 hot liquid which it is desired to cool is made to impart nearly all its heat to a cold 

 liquid whose temperature it is desired to raise. 



