1870.] 



Deep-sea Researches. 



197 



down ; but so soon as the temperature of the Air falls much below that 

 of the Sea, the surface-layer being cooled will become heavier and sink, and 

 will thus carry down cold instead of heat, so as to lower the temperature 

 of the stratum below. In no instance, however, so far as we can learn, has 

 the surface-temperature of the Mediterranean ever been seen so low as 56°, 

 even in midwinter. 



91. That it is by this sinking of the surface-films successively concen- 

 trated by evaporation that the Solar heat, which acts so powerfully on 

 the Mediterranean basin during the summer, is transmitted downwards, 

 appears certain from the fact, of which the particulars will be presently 

 given, that the Salinity of the water of the Mediterranean is greater below 

 the surface than at the surface, instead of diminishing as it does in the 

 Atlantic (§ 84) ; and we thus see how important an influence is exerted by 

 that Salinity in diffusing the heat imparted to the surface through the waters 

 beneath. In the great /res^-water lakes of Switzerland, the deeper water 

 retains all through the year a temperature but little above 39°, the small 

 excess being probably derived from the warmth of its bed ; for the whole 

 mass of water down to the bottom must be cooled to this degree in winter 

 before any ice can form on its surface ; and as the heating of the surface 

 in summer makes all the water affected by it specifically lighter, none 

 of it will descend and carry heat downwards, as it does in the Mediter- 

 ranean. 



92. Density. — The determination of the actual Salinity of the water of 

 the Mediterranean basin, alike at the Surface and at various Bottom-depths, 

 was one of the special objects of our inquiries ; for although various Ana- 

 lyses had been previously recorded, they had been made upon samples of 

 water which had been kept in bottles for a more or less considerable period ; 

 and the depths from which those samples had been collected were not by 

 any means the greatest known to exist in this basin. — The number of Chlo- 

 rine-determinations of Surface-MVQ.ter was 25 ; and their Geographical 

 range was from the Strait of Gibraltar to the edge of the Eastern basin 

 (Station 60). A marked difference in density was observable between the 

 Surface-waters of the Western and of the Eastern portions of this area ; for 

 whilst those of the latter invariably showed a considerable excess in Salinity 

 above the maximum of Atlantic water, that excess was so much reduced in 

 some of the samples taken nearer to the Strait, as almost certainly to show 

 that the surface-stratum there consists in great degree of Atlantic Water. 

 Thus at Station 47, in which a like indication was given by the Tempera- 

 ture of the Surface-water (§ 87), we found the proportion of Chlorine to 

 be 20*46, or only 0*27 above the maximum we had encountered in Atlantic 

 water ; and when we crossed to the neighbourhood of the opposite Algerine 

 Coast (where, however, the density of the Surface-water was probably re- 

 duced by the entrance of River-water), we found the proportion of Chlo- 

 rine as low in one case as 19*69, and in another as 19*99. When ap- 

 proaching the Strait on our return voyage, we took a series of five samples 



