4 F. L. Ekman, 



rents are the consequence. The heavier water goes towards the Hghter, 

 and the lighter whence the heavier comes; for two fluids differing in sp. 

 gravity, and standing at the same level, can no more balance each other 

 than unequal weights in the opposite scales of a true balance.» We see 

 that according to this' view the first impulse to the stream arises from 

 the heavier water, which, as standing at the same level as the lighter, 

 in consequence of its greater weight displaces the latter in the deeper 

 strata. In this manner MxVURy endeavours to explain the under-current 

 in straits, which unite oceanic basins of different degrees of saltness, 

 and this explanation is accepted by Carpenter and others. 



It seems to me, that this explanation of the under-current, which 

 has moreover been extensively applied to streams in the open ocean, 

 too implicitly reposes on the common notion of the fixed level of the 

 ocean's surface, and that the following theorems, however simple, deserve 

 especial attention, in as much as that they must be the foundation of a 

 correct comprehension of the phœnomenon. 



l:o) When the equilibrium between two masses of water has been 

 disturbed, two cases are possible, namely a), when the equilibrium is 

 disturbed throughout all the strata down to the bottom and b), when the 

 equilibrium is disturbed only to a certain depth below the surface. The 

 first case happens, when the specific gravity in both masses is the same, 

 but the level diff"erent, or the level the same, but the sp. gravity, at least 

 in the upper strata, different. The latter case presupposes, that the spe- 

 cific weight in both masses is in the lower strata the same, but different 

 in the strata situated above a certain level, and moreover that the 

 thickness of these latter strata is inversely as their specific gravities. 



2:o) Between two strata of water of the same level and depth, but 

 of different specific gravity, there will be a difierence of pressure, which at 

 the surface = 0, but increases with the depth below the surface. Should 

 however the lighter water be so much raised, that the difference of 

 pressure at its greatest depth vanishes, then at any point above that 

 depth the lighter water will produce a greater pressure than the heavier, 

 and the difference increase as we approach the surface; but the sum of 

 the differences of pressure of the two strata will be less in tlie latter 

 position than in the former. 



3:o) When the equilibrium between two masses of water is distur- 

 bed, the motion of the particles will be such as to restore the equili- 

 brium at the least possible expense of time and action. For a given 

 difference of pressure the required motion will occupy more time, when 



