572 



Dr. W. B. Carpenter on the 



[June 13_, 



raises a " head " of water, the pressure of which must produce an outward 

 Under-current ; as is evidenced by the continuance of the surface in-current, 

 which would otherwise cease as soon as the rise of the water is sufficient 

 to neutralize by excess of gravity the force of the wind. Now here, 

 although the westerly drift produced by the wind is not antagonized by 

 the direct obstacle of a land-barrier, yet the strong in-current which meets 

 it will obviously have the like tendency to raise a " head" of water, especially 

 in that narrowest part of the Strait which lies between Tarifa and Point 

 Alcazar ; and the increase of pressure produced by this, acting in the direc- 

 tion of the Tidal movement of the under-current, would obviously tend to 

 accelerate it. 



56. With the view of obtaining another set of observations on the 

 deeper (southern) part of the "ridge," we proceeded thither during the 

 night, and took up our position, on the morning of the 29th, about 7 

 miles W.S.W. of Cape Spartel. There was a slight Westerly wind, with 

 a smooth sea. The Specific Gravity of the surface-water, taken 2 hours 

 after low water, when the flow of the tide was giving a westerly movement 

 to both strata, was 1'0275 ; thus showing, by its excess above the ordinary 

 standard of Atlantic water, that the surface-drift from the Mediterranean 

 produced by the continuance of strong Easterly wind had reached this end 

 of the Strait. The Sp. Gr. at 130 fathoms was 1-0282, at 155 fathoms 

 it was T02S4, and was the same on the bottom at 180 fathoms. Thus it 

 appeared that as the lower stratum here consists of a mixture of Mediter- 

 ranean and Atlantic water, the predominance of the outward under-current 

 is not indicated so strongly by Sp. Gr. as it is on the shallower (northern) 

 part of the ridge, where the lowest stratum consists of nearly pure Medi- 

 terranean water (§52). — The results of this day's experiments with the 

 Current-drag confirmed the conclusion indicated by the previous series, as 

 to the complete Tidal reversal of both currents at this end of the Strait ; 

 whilst they gave even stronger evidence of the excess of zraward movement 

 in the upper stratum, and of ow^ward movement in the lower. From 2 to 

 5 hours after Low water, the surface-current was running ivestvr&r&s at the 

 rate of 0'75 mile ; it then slackened, became stationary at High water, and 

 then flowed eastwards, its rate increasing at 3| hours after High water to 

 1*2 mile per hour. On the other hand, the under-current at 3 \ hours 

 after Low water was shown, by a drag hung at 155 fathoms depth, to be 

 flowing ztfesftvards at the estimated rate of 1*25 mile per hour, and from 

 3 J to 4^ hours was still running at the rate of 1*10 mile ; whilst at from 

 3 to 3^ hours after High water it flowed eastwards at the estimated rate 

 of only 0'67 mile per hour ; the excess of zwflowof the upper-current, and 

 that of outflow of the under-current, being thus each 0*45 mile per hour. 



3 hours after L. W. 3 hours after H. W. 



Surface-current W.^ — ■ 0*75 mile per hour. — -^-E. 1*20 mile per hour. 

 Under-current W.-*— 1*12 mile per hour. «~-^E. 0*67 mile per hour. 



57. On the 30th of August (Full Moon) the ship was taken into the 



