Mr. J. Croll on the Physical Cause of Ocean-currents, 271 



Now, in determining the actual difference of level between 

 the Mediterranean column and the Atlantic, we must leave out 

 of account the water under the level of the ridge. This point 

 must be so obvious to every one familiar with hydrostatics that I 

 need not go into further detail in the matter. 



Taking Dr. Carpenter's own data as to the density of the 

 Mediterranean and Atlantic waters, what, then, is the difference 

 of density? The submarine ridge comes to within 167 fathoms 

 of the surface ; say, in round numbers, to within 1000 feet. What 

 are the densities of the two basins down to the depth of 1 000 feet ? 

 According to Dr. Carpenter there is little, if any, difference. 

 His own words on this point are these : — "A comparison of these 

 results leaves no doubt that there is an excess of salinity in the 

 water of the Mediterranean above that of the Atlantic; but 

 that this excess is slight in the surface-water, whilst somewhat 

 greater in the deeper water " (§ 7). " Again, it was found by 

 examining samples of water taken from the surface, from 100 

 fathoms, from 250 fathoms, and from 400 fathoms respectively, 

 that whilst the first two had the characteristic temperature and 

 density of Atlantic water, the last two had the characteristics 

 and density of Mediterranean water " (§ 13). Here, at least to 

 the depth of 100 fathoms or 600 feet, there is little difference 

 of density between the waters of the two basins. Consequently 

 down to the depth of 600 feet there is nothing to produce any 

 sensible disturbance of equilibrium. If there be any sensible 

 disturbance of equilibrium, it must be in consequence of differ- 

 ence of density which may exist between the depths of 600 

 feet and the surface of the ridge. We have nothing to do with 

 any difference which may exist between the water of the Medi- 

 terranean and the Atlantic below the ridge ; the water in the 

 Mediterranean basin may be as heavy as mercury below 1000 

 feet; but this can have no effect in disturbing equilibrium. The 

 water, to the depth of 600 feet, being of the same density in 

 both seas, the length of the two columns acting on each other 

 is therefore reduced to 400 feet — that is, to that stratum of water 

 lying at a depth of from 600 to the surface of the ridge 1000 

 feet below the surface. But, to give Dr. Carpenter's theory full 

 justice, we shall take the Mediterranean stratum at the density 

 of the deep water of the Mediterranean, which he found to be 

 about 1*029, and the density of the Atlantic stratum at 1*026. 

 The difference of density between the two columns is therefore 

 •003. Consequently, if the height of the Mediterranean column 

 be 400 feet, it will be balanced by the Atlantic column of 401*2 

 feet ; the difference of level between the Mediterranean and the 

 Atlantic cannot therefore be more than 1*2 foot. The total 

 amount of work that can be performed by gravity in the case 





