212 



Messrs. Carpenter and Jeffreys on 



[Dec. 8, 



moving force will then act as a constantly accelerating one, until any further 

 increase in rate is prevented by the opposing influence of friction, &c. 



121. Now such an agency as that which is required by Principles VI. 

 and VII. to maintain a double current in a narrow Strait actually exists 

 in the cases of the Mediterranean and the Red Sea. It must be borne in 

 mind in considering these, that whilst their basins are limited, the Ocean- 

 basins at the other end of their respective Straits are practically unlimited ; 

 so that the levels and densities of the latter may be regarded as constant. 

 Now as the excess of evaporation in the Mediterranean basin at the same time 

 lowers the level and increases the density of the water which remains, the 

 reduction of the level gives rise to a continual surface-inflow. But, on the 

 other hand, the restoration of the level by an inflow of salt water, the den- 

 sity of the contents of the Mediterranean basin being already in excess, 

 occasions a constant want of equilibrium between the columns of water at 

 the two extremities of the Strait ; and as the lighter water of the Atlantic 

 cannot balance the heavier water of the Mediterranean, a portion of the 

 latter is forced outwards as an undercurrent, — thus again producing a de- 

 pression of the level, to be again restored by a surface-inflow from the At- 

 lantic. — Thus the original moving force of both currents is the heat of the 

 Sun. 



122. The case may perhaps be made still plainer, by considering the effect 

 of changes in its conditions. If the whole amount lost by the evaporation 

 from the surface of the Mediterranean were replaced by the fresh water of 

 rain and rivers, there would be neither lowering of its surface nor increase 

 of its density ; and there would be neither influx nor efflux through the 

 Strait of Gibraltar. If, again, with the present excess of evaporation, the 

 Atlantic were to supply fresh water instead of salt, the influx through the 

 Strait of Gibraltar would be only that required to maintain the level, and 

 thus to supply the loss by excess of evaporation ; and as the columns at the 

 two extremities of the Strait would remain in constant equilibrium, there 

 would be no efflux. But as the water which flows in from the At- 

 lantic is salt instead of fresh, and is itself rendered still more dense by con- 

 centration in the Mediterranean, the constantly renewed excess in the 

 weight of the Mediterranean column can only relieve itself by as continual 

 an efflux : this efflux, by lowering the surface-level, in its turn occasions 

 an indraught to maintain it ; and thus the in-current has to replace not 

 only the fresh water lost by excess of evaporation, but also the denser water 

 forced out by its excess of weight. These two agencies, like the pertur- 

 bations of the Planets, are so balanced against one another, as to maintain 

 a constant mean. If the evaporation were to increase, more Atlantic water 

 would flow in ; but the increase of density in the Mediterranean water 

 would cause more of it to flow out, which again would occasion a larger 

 indraught of the less dense water of the Atlantic. And thus the excess 

 of density would be kept down to a very moderate amount, — as is actually 

 found to be the case in the Red Sea, notwithstanding that the enormous 



