1870.] 



Deep-sea Researches. 



205 



attributable. These effects will, of course, be most marked when the tidal 

 movement is augmented by a strong wind in its own direction*. — The 

 constant current does not occupy by any means the entire breadth of the 

 Strait. In its narrowest part the rapid central in-current is said by Admiral 

 Smyth not to average more than 4 miles in width ; and on either side the 

 stream when moving inwards is usually much less rapid, its rate, indepen- 

 dent of tide, being about 1 mile per hour in the neighbourhood of Tarifa, 

 and 2 miles an hour on the coast of Africa. These lateral currents are 

 much more affected than the central current by the Lunar tide, which 

 produces a complete periodical reversal of them ; the flood at springs run- 

 ning westwards off Tarifa at the rate of from 2 to 3 miles per hour, whilst 

 even at neaps it runs westwards at 1 mile per hour. The ebb, on the other 

 hand, concurs with the general current, and augments both its volume and 

 its rate. Thus, when the water is falling, the whole stream is running 

 eastwards ; but when it is rising, the tide on either shore sets westwards. 

 By taking advantage of this periodical westerly flow in the lateral streams, 

 it is possible for sailing-vessels to make their way outwards f in opposition 

 to a continued westerly wind. 



107. The rate of the general in-current diminishes immediately that it 

 discharges itself into the Mediterranean basin, over the surface of which 

 it seems to spread itself, in virtue of its lower Specific Gravity (§ 92). 

 But the influence of its motion is sensibly experienced along the Spanish 

 Coast as far as Cape de_Gat, and along the African Coast even as far as 

 the Bay of Tunis, — its force and direction, however, being greatly affected 

 by the prevalent winds. 



108. Various hypotheses have been put forward at different times to ac- 

 count for this continual influx of Atlantic water into the Mediterranean. 

 The motion of an undercurrent flowing in the opposite direction was very 

 early suggested by Dr. Smith J ; but he did not attempt to show in what 

 way motion is given either to the surface inflow or to the deep outflow. 

 Quite recently the extraordinary hypothesis has been seriously put forward, 

 that the influx through the Strait may be due to a gradual depression of 

 the bottom now going on§. The explanation usually received is that first 

 offered by Dr. HalleyH, who attributed it to the excess of evaporation 

 from the surface of the Mediterranean Sea over the whole amount returned 

 to its basin either directly by rainfall or by the rivers which discharge 

 themselves into it ; so that the level would be progressively lowered, if not 

 kept up by an inflow from the Atlantic. The obvious objection to this 

 explanation is, that as the water which passes off by evaporation leaves 

 its Salt behind it, and as the water which enters through the Strait is 



* Admiralty Sailing Directions, p. 309. 



f See Admiral Smyth's ' Mediterranean,' p. 176. 



J Philosophical Transactions, vol. xiv. p. 364. 



§ Mr. George Maw in Geological Magazine, December 1870, p. 550. 

 || Philosophical Transactions, vol. xvi. p. 366. 



