970 
THE VOYAGE OF H.M.S. CHALLENGER. 
boundary of the ocean; on the 29th, 120 miles to the eastward, the bottom of the 
basin had been reached in 2800 fathoms. On the 28th the “dip” in the tempera- 
ture took place at 1700 to 1900 fathoms, while on the 29th it was found at 2100 to 
2300 fathoms. 
The temperature of the water from 2400 fathoms to the bottom was uniform, the 
mean result of six observations being 32° - 43. This water underruns the body of the 
Atlantic water, which at 1500 to 2000 fathoms has here a temperature of 37°, producing a 
temperature gradient of about 1 0, 3 per hundred fathoms at the steepest. For the pre- 
servation of this gradient a considerable supply of cold water is requisite, and it must 
be drawn from higher latitudes. But any motion of the water towards the Equator will 
be accompanied by a strong deflection to the westward (proportional to the change of 
the cosine of the latitude). A measure of this deflecting force is furnished by the rise 
of this cold water at the more inshore Station on the 28 th, where the maximum gradient 
is at about 1750 fathoms, while on the 29th, at a distance of 120 miles, it is at about 
2100 fathoms. Hence the average incline produced on the surface of the cold water by 
this tangential force is 350 fathoms in 120 miles, or approximately a gradient of 1 in 350. 
Between latitudes 36° and 35° the tangential force increases in the proportion of 81:82. 
In the northern hemisphere there is no polar current at the bottom which can be 
compared with this one. The North Atlantic water which penetrates into Arctic regions 
is cooled, and fills up the whole of the deep basin of the Norwegian Sea with cold but 
dense water, but its passage southward is barred by the ridge stretching from Greenland 
by Iceland, Faeroe, and the Shetlands to Norway, the greatest depth of water on which 
does not exceed 300 fathoms. The northward drift at the surface is largely influenced 
by tidal currents which stir up the water to the very bottom, keeping the summit of the 
ridge clear of mud. 1 Were it not for this ridge there would doubtless be a similar current 
occupying the deep water along the edge of the continental plateaus of Europe and Africa. 
A very remarkable current exists in the North Atlantic close to the shores of Morocco, 
and extends from Cape Bojador to Cape Spartel and even through the Strait of Gibraltar 
along the African coast into the Mediterranean. This current, which flows in a northerly 
direction, is confined to the immediate neighbourhood of the coast, not extending more 
than 20 miles out to sea. Its waters are characterised by their low density and 
temperature as compared with the water immediately outside, and by their deep olive- 
green colour. In October 1883 the S.S. “Dacia” found the temperature of the wuiter 
out at sea to be very constantly 69° F. ; when the African coast in the neighbourhood of 
the town of Mogador was approached the temperature and the density began to fall 
perceptibly and close in within a couple of miles of the shore, and in less than 50 fathoms, 
the temperature fell to 61° in water of a pure olive-green colour, and the density (at 60° F.), 
which had been 1‘0268 outside, fell to D0264. When it is remembered that the trade 
1 See Encycl. Brit. Article, Norwegian Sea, vol. xvii. p. 592, 9tli eel., 1884. 
