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



basins, the sides of which are sufficiently strong to resist all 

 pressure. It follows that, however denser the water of the 

 Mediterranean may be than the water of the Atlantic, it is 

 only the water above the level of the ridge that can possibly 

 exercise any influence in the way of disturbing equilibrium, so 

 as to cause the level of the Mediterranean to stand lower than 

 that of the Atlantic. Suppose both basins empty, and dense 

 water to be poured into the Mediterranean, and water less dense 

 into the Atlantic, until they are both filled up to the level of the 

 ridge ; still the heavier water in the one basin exercises no in- 

 fluence in raising the level of the lighter water in the other 

 basin, the entire pressure being borne by the sides of the basins. 

 But if we continue to pour in water till the surface is raised, 

 say, 1 foot, above the level of the ridge, then there is nothing 

 to resist the lateral pressure of this 1 foot of water in the 

 Mediteranean but the counter pressure of the 1 foot in the 

 Atlantic. But as the Mediterranean w T ater is denser than the 

 Atlantic, this 1 foot of water will consequently exert more pres- 

 sure than the 1 foot of water of the Atlantic. We must there- 

 fore continue to pour in more water into the Atlantic until its 

 lateral pressure equals that of the Mediterranean. The two seas 

 will then be in equilibrium, but the surface of the Atlantic will 

 of course be at a higher level than the surface of the Mediterra- 

 nean. The difference of level will be proportionate to the differ- 

 ence in density of the waters of the two seas. But here we 

 come to the point of importance. In determining the difference 

 of level between the two seas, or, what is the same thing, the 

 difference of level between a column of the Atlantic and a 

 column of the Mediterranean, we must take into consideration 

 only the water which lies above the level of the ridge. If there 

 be 1 foot of water above the ridge, then there is a difference of 

 level proportionate to the difference of pressure between the 1 

 foot of water of the two seas. If there be 2 feet, 3 feet, or any 

 number of feet of water above the level of the ridge, the differ- 

 ence of level is proportionate to the 2 feet, 3 feet, or whatever 

 number of feet there may be of water above the ridge. If, for 

 example, 13 should represent the density of the Mediterranean 

 water and 12 the density of the Atlantic water, then if there 

 were 1 foot of water in the Mediterranean above the level of the 

 ridge, there would require to be 1 foot 1 inch of water in the 

 Atlantic above the ridge in order that the two might be in 

 equilibrium. The difference of level would therefore be 1 inch. 

 If there were 2 feet of water, the difference of level would be 2 

 inches; if 3 feet, the difference would be 3 inches, and so on. And 

 this would follow, no matter what the actual depth of the two 

 basins might be ; the water below the level of the ridge exer- 

 cising no influence whatever on the level of the surface. 



