OTTRKENTS OF THE SEA. 183 



377. Upper and under currents through straits explained .—Vmt tho 

 salt water, which has lost so much of its freshness by evapo- 

 ration, becomes salter, and therefore heavier. The lighter water 

 at the Straits cannot balance the heavier water at the Isthmus, 

 and the colder and salter, and therefore heavier water, must 

 either rim out as an under current, or it must deposit its surplus 

 salt in the shape of crystals, and thus gradually make the bottom 

 of the Ked Sea a salt-bed, or it must abstract all the salt from the 

 ocean to make the Eed Sea brine — and we know that neither the 

 one process nor the other is going on. Hence we infer tha.t 

 there is from the Red Sea an under and outer current, as there is 

 from the Mediterranean through the Straits of Gibraltar, and 

 that the surface waters near Suez are salter than those near the 

 mouth of the Eed Sea. And, to show why there should be an 

 outer and under current from each of these two seas, let us 

 suppose the case of a vat of oil, and a vat of wine connected by 

 means of a narrow trough — the trough being taken to represent 

 the straits connecting seas the waters of which differ as to 

 specific gravity. Suppose the trough to have a flood-gate, which 

 is closed until we are ready for the experiment. Now let the 

 two vats be filled, one with wine the other with oil, up to the 

 same level. The oil is introduced to represent the lighter water 

 as it enters either of these seas from the ocean, and the wine the 

 same water after it has lost some of its freshness by evaporation, 

 and therefore has become salter and heavier. Now suppose the 

 flood-gate to be raised, what would take place? Why, the oil 

 would run in as an upper current, overflowing the wine, and the 

 wine would run out as an under current. 



378. TJie Mediterranean current. — The rivers which discharge 

 their waters into the Mediterranean are not sufficient to supply 

 the waste of evaporation, and it is by a process similar to this 

 that the salt which is carried in from the ocean is returned to 

 the ocean again : were it not so, the bed of that sea would be a 

 mass of solid salt. The unstable equilibrium of the seas is a 

 physical necessity. Were it to be lost, the consequences would 

 be as disastrous as would be any derangement in the forces of 

 gravitation. Without doubt, the equilibrium of the sea is pre- 

 served by a system of compensation as exquisitel}' adjusted as 

 are those by which the "music of the spheres" is maintained. 

 It is difficult to form an adequate conception of tho immense 



