1872.] 



' Shearwater 3 Scientific Researches. 



571 



be produced, even at a depth of from 400 to 500 fathoms, by the action of 

 Winds and Tides. 



55. On the 28th of August we proceeded to make a series of experi- 

 ments with the Current-drag in the deep water off Point Cires on the 

 African side ; first, however, making a series of observations on the Specific 

 Gravity of the water at different depths, for the purpose of ascertaining the 

 position of the plane of separation between the two strata. This was very 

 clearly indicated as lying between 200 and 225 fathoms ; for while the 

 Sp. Gr. of the surface-water was 1*02/9 (being evidently raised by the 

 westerly drift of Mediterranean water, kept up by the continuance of a 

 strong East wind), and that of the water at 200 fathoms was 1*0283, the 

 Sp. Gr. at 225 fathoms was 1*0298, and at 250 and 300 fathoms it was 

 1*0296. The continued strong East wind (force 6) obviously had a de- 

 cided effect in retarding the swr/izce-current, which, as the tide was ebbing 

 during the whole series of observations, would probably have set very 

 strongly to the eastward (it being near the date of full-moon) but for this 

 opposing influence. The trader-current, on the other hand, flowed east- 

 wards during the same ebb at a rate in the first instance exceeding that of 

 the surface-current, though subsequently much less, as is shown in the 

 following Table : — 



Surface-Current. Under-current. 



Time. Rate per hour. Depth. Rate per hour. 



1J to 2 hours after H.W. 



1*0 mile. 



225 faths. 



2*6 miles. 



91 



)» }i 



2*0 miles 



300 „ 



2*4 „ 



3 



)> )> 



3'8 „ 



50 „ 



3-8 „ 



3j a 



*> )i 



4*4 „ 



100 „ 



2*6 „ 



4 



>) )> 





150 „ 



1*9 „ 



H 



>} >> 



3'3 „ 



225 „ 



1-4 „ 



It thus appeared (1) that the influence of the ebb on the under- current 

 was exerted earlier and more strongly than on the upper current, the 

 former at first moving eastwards at the rate of 2*6 miles an hour, whilst 

 the rate of the latter was only 1 mile ; but (2) that at a more advanced 

 period of the ebb the relative rates were reversed, that of the surface- 

 current having increased to 4*4 miles an hour, and being still 3*3 miles 

 when the drag, hanging at the same depth (225 fathoms) as in the first 

 observation, showed a movement of the under-current of only 1*4 mile. — 

 This suggests the question whether the rapid flow of the Under-current on 



this occasion towards the east may not — anomalous as the fact may seem 



have been really due to the strength of the wind blowing/row* the east. For, 

 as was pointed out by Sir Wm. Thomson and Prof. Stokes, in the discussion 

 which took place on a paper which I submitted to the Mathematical and 

 Physical Section at the Edinburgh Meeting of the British Association *, 

 when a strong wind continues to blow into a loch or long narrow inlet a it 

 * Sec 'Nature,' Aug. 17, 1871, p. 316. 



