198 



Messrs. Carpenter and Jeffreys on 



[Dec. 8, 



for the purpose of testing the reality of this difference ; and we found the 

 proportions of Chlorine to be respectively 2077, 20*67, 20*56, 20*51, and 

 20*47. The mean of these five determinations, together with the one pre- 

 viously taken at Station 47, but excluding the two taken on the coast of 

 Algiers, is 20*57; and the mean Specific Gravity was 1*0274. On the 

 other hand, at the Sicilian end of the basin, where the water was that of 

 the Mediterranean proper, the mean of ten Chlorine-determinations was 

 21*05, with a corresponding Specific Gravity of 1*0280. The maximum of 

 Chlorine was there 21*32, with Specific Gravity 1*0284 ; and the minimum 

 20*77, with Specific Gravity 1*0277. The combination of these 16 obser- 

 vations gives a mean of 20*87 for the Chlorine, and 1*0278 for the Specific 

 Gravity, of the Surface-water of the Mediterranean generally *. 



93. The number of Chlorine- determinations of Bottom-water taken in 

 the proper Mediterranean basin, at depths between 207 and 1700 fathoms, 

 was 20. They show a general excess of Salinity over the Surface-water, 

 the mean of the whole being 21*38 (as against 20*87), with a maximum of 

 21*88 (Sp. Gr. 1*0292) and a minimum of 21*08 (Sp. Gr. 1*0281). On 

 grouping them into three Series according to their depth, we arrive at a 

 curious result : — 



Fathoms. Chlorine. Sp. Gr. 



200 to 400, Mean of 7 observations 21*53 1*0287 



400 to 800, „ 7 „ 21*38 1*0285 



1300 to 1700, „ 6 s , 21*21 1 0283 



94. Thus it appears that the excess of Salinity is greatest in the shallower 

 water, and that it gradually diminishes with the depth. This is also shown 

 most strikingly by comparing the sample taken from the least depth 

 (207 fathoms) with that taken from the greatest depth (1703 fathoms) ; 

 for it was the former that showed the maximum of 21*88, and the latter 

 that showed the minimum of 21*08. — Now this fact, though at first unex- 

 pected (since we had supposed that the heaviest water would gravitate 

 to the greatest depths), seems not difficult to account for, if we consider 

 the mode in which the concentration of the surface-film will be likely to 

 affect the water below. For it can be shown experimentally, by pouring a 

 strong saline solution tinged with colour upon the top of a weaker colour- 

 less solution, that the former will in the first instance sink " bodily," but 

 will gradually impart its excess of salt to the liquid through which it falls ; 

 the descent of the coloured stratum becoming slower and slower, and its 



* This mean accords closely with that of 20*845 obtained by Prof. Forchhammer from 

 his examination of samples collected at different times and by different persons from 

 various parts of the surface of the Mediterranean (Phil. Trans. 1865, p. 252). His 

 maximum of Chlorine, 21 '718, was higher than ours ; but this seems to have been an 

 exceptional case ; and the sample may have undergone some concentration in keeping. 

 On the other hand, his minimum was lower, being only 20*16 ; but this sample, having 

 been taken in the Strait of Gibraltar, contained a large proportion of Atlantic water. 



