214 



Messrs, Carpenter and Jeffreys on 



[Dec. 8, 



and the Baltic Sound, may be brought about otherwise than by the 

 excess of evaporation which maintains it in the one case, or by the 

 continual dilution with fresh water which maintains it in the other. 

 It may be easily shown that a constant and decided Difference of Tem- 

 perature must have exactly the same effect. Let the Mediterranean 

 basin be supposed to be filled with water of the same density as that of 

 the Atlantic, and up to the same level ; and to be then cooled down below 

 the freezing-point of fresh water by the withdrawal of Solar heat, whilst 

 the surface of the Atlantic continues to be heated, as at present, by the 

 almost tropical sunshine of the Gibraltar summer. The cooling of the 

 Mediterranean column, reducing its bulk without any diminution of weight, 

 would at the same time lower its level and increase its density. An in- 

 draught of Atlantic water must take place through the Strait to restore that 

 level ; but this indraught would augment the weight of the column, giving 

 it an excess above that of the column at the other end of the Strait ; and 

 to restore the equilibrium a portion of its deeper water must be forced out 

 as an undercurrent towards the Atlantic, thus again reducing the surface- 

 level of the Mediterranean. Now so long as the warm Atlantic water 

 which comes in to maintain that level is in its turn subjected to the same 

 cooling, with consequent lowering of level and increase of density, so long 

 would the vertical pressures of the two columns, which would be speedily 

 restored to equilibrium if both basins were subjected to the same heat or 

 the same cold, remain in a constant state of inequality; and so long, 

 therefore, on Principles V. & VI. (§ 120), must this vertical circulation 

 continue. 



1 25. Now the case thus put hypothetically has a real existence. For the 

 Mediterranean cooled down by the withdrawal of Solar heat, let us sub- 

 stitute the Polar Basin ; and for the Atlantic, the Equatorial Ocean. The 

 antagonistic conditions of Temperature being constantly sustained, a con- 

 stant interchange between Polar and Equatorial waters, through the seas 

 of the Temperate Zone, must be the result. The reduction in the tem- 

 perature of the Polar column must diminish its height whilst augmenting 

 its density; and thus a flow of the upper stratum of Equatorial water 

 must take place towards the Poles, to maintain the level thus lowered. 

 But when the column has been thus restored to an equality of height, it 

 will possess such an excess of weight that its downward pressure must 

 force out a portion of its deeper water ; and thus an underflow of ice-cold 

 water will be occasioned from the Polar towards the Equatorial areas. 



126. The agency of Polar Cold will be exerted, not merely in reducing 

 the bulk of the water exposed to it, and thereby at the same time lowering 

 its level and increasing its density ', but also in imparting a downward 

 movement to each new surface- stratum as its temperature is reduced, 

 whereby a continual indraughtVill be occasioned from the warmer surface- 

 stratum around. For the water thus newly brought under the same cool- 

 ing influence will descend in its turn ; and thus, as the lowest stratum will 



