210 



Messrs. Carpenter and Jeffreys on [Dec. 8, 



fact that both the Temperature and the Density of the bottom -water 

 brought up at two stations (Nos. 65 and 67), from 198 and 188 fathoms 

 respectively, unmistakably indicated its derivation from the Mediterranean 

 basin. Although its density corresponded rather with that of the 400 

 fathoms' stratum than with that of the 250 fathoms' stratum at the other 

 end of the Strait, yet it may be very well conceived to be the water of the 

 250 fathoms' stratum reduced in density during its outward flow through 

 the Strait by intermixture with the less dense water of the in-current. 



117. It now no longer then admits of doubt that the water of the deeper 

 part of the Mediterranean basin, which has undergone concentration by 

 evaporation, is continually flowing outwards into the Atlantic, notwith- 

 standing that in doing so it has to be brought nearer the surface, so as to 

 pass over the ridge ; and that the increase of density in the Mediter- 

 ranean water, which would otherwise go on without check so long as the 

 loss by evaporation is in excess of the fresh water returned into the basin, 

 is thus kept within a very narrow limit. 



118. The essential phenomena of the Gibraltar Current having been 

 thus determined, we have to consider how they are to be accounted for ; 

 that is to say, to inquire (1) what is the power which gives motion to 

 the enormous body of water continually flowing from the Atlantic into 

 the Mediterranean ; (2) what it is which not only gives motion to the 

 undercurrent flowing from the Mediterranean to the Atlantic, but draws 

 up the heavier water from the depths of the former to the comparative 

 shallow of its limiting ridge ; and (3) in what way the power is generated 

 in each case. 



119. These questions have been answered — as we believe correctly — by 

 Captain Maury*, on the hypothetical assumption of the existence of an 

 undercurrent, which has now been verified. He shows that in each case 

 Gravity is the impelling power ; and that in both cases this power ori- 

 ginates from a common source — the excess of evaporation beyond the 

 return of fresh water by rain and rivers, which produces at the same time 

 a reduction of the level, and an increase in the density, of the water 

 within the Mediterranean basin ; the former drawing in surface-water by 

 gravitation from the higher level outside, whilst the latter forces out deeper 

 water by the excess of pressure of the superincumbent column. As the 

 vertical circulation thus occasioned has not yet, so far as we are aware, been 

 formularized under First Principles, and as these principles have a much 

 more extended application than Capt. Maury himself seems to have sup- 

 posed, we shall now present them in a systematic form. 



120. The following appear to be self-evident propositions : — 



I. That wherever there is a difference of level between two bodies of Water 

 in free communication with each other, there will be a tendency towards 

 the equalization of their levels by a surface-flow from the height towards 

 the lower. 



Physical Geography of the Sea, I860, pp. 194-196. 



