1872.] 



f Shearwater ' Scientific Researches. 



559 



of A and raising that of B, will produce a further surface -indraught from 

 B into A. Supposing B to be cut off by a septum from the rest of the 

 basin, as A was in the first instance, this interchange would proceed until 

 half of the heavier water of column A has flowed into the lower part of 

 column B, and half of the lighter water of column B has flowed into the 

 upper part of column A, so as to equalize the pressure and the level in the 

 two. If now the septum be removed between columns B and C, the excess 

 of pressure which B shares with A will operate in like manner (though in 

 a diminished measure) against C ; and the same interchange will take place 

 until the equilibrium is restored. And thus by a propagation of the like 

 interchange through the whole succession of columns in a progressively 

 diminishing ratio, the stratum of colder and heavier water (supposing its 

 temperature not to be altered by conduction from below or from above) 

 would ultimately spread itself at a uniform level over the entire floor of the 

 basin. This extension, in fact, would be simply a question of time, like 

 the equalization in the level of its surface ; the rate of the movement being 

 determined by the "viscosity " of the water. 



33. But we have now to trace out the result of the continued action of 

 severe Cold on the surface of the limited area at one extremity of the basin. 

 As fast as the bottom-outflow tends to restore the equilibrium between the 

 Polar column and the rest of the basin, a new disturbance will take place 

 by the cooling of the water brought in by the surface-inflow ; and this 

 disturbance will be propagated onwards from column to column, along the 

 whole length of the basin, in the manner just indicated. If the glacial 

 stratum were not to acquire any heat in its diffusion over the floor of the 

 basin, its thickness would be continually augmented by fresh exchanges 

 between column and column, — the warmer su?face-wa.ter flowing towards 

 the cold extremity of the basin, as fast as cold bottom-water flows from 

 it ; until at last the entire mass of the water in the basin (supposing its 

 surface-temperature not to be kept up by insolation) would have its tem- 

 perature progressively reduced by downward convection in the area over 

 which the cold directly acts, and thence by lateral convection, until the 

 whole would be brought down to 27°, when no further movement would 

 take place. 



34. But since the Glacial stratum, as it creeps along the sea-bed, is con- 

 tinually receiving heat from the crust of the earth beneath and from the 

 warmer layer of water above, its thickness would diminish with the increase 

 of distance from the Polar area ; or, in other words, its surface would form 

 an inclined plane, lying at a greater and yet greater depth, the further it 

 has flowed from its source, — which is just what, r so far as our present 

 knowledge extends, proves to be the case. And further, as the upper 

 stratum is being continually draughted off in the opposite direction, 

 the portion of the glacial stratum which has had its temperature raised by 

 the agencies just noted, and which overlies the rest, will be lifted into its 

 place by the intrusion of a fresh arrival of colder water beneath, so as to 



