NARRATIVE OF THE CRUISE. 
971 
winds drive the surface water of the Atlantic bodily to the westward, thus drawing the 
water away from the eastern side of the ocean, it is evident that while it pushes the 
water westward it must drag water from some other direction to supply its place. No 
doubt much of it is supplied by surface drift from the southward and eastward, but if the 
temperature curves of the water oft’ the coast of Guinea be studied, it will be seen that 
much of the water which is brought into the eastern part of the Atlantic to supply that 
drained away by the Equatorial Current and other westerly streams is supplied from the 
deeper, colder, and less dense strata of southern seas. On the lltli August 1873, in 
the Guinea Current the surface temperature was 78° - 7, and at 50 fathoms it was only 54° - 2, 
the decrease being at the average rate of 0 o, 5 F. per fathom. The latitude of this Station 
was 12° 5 ' N. If this be accepted as an indication of the deep water of the southern 
hemisphere penetrating to the northward of the Equator, it follows necessarily that as it 
goes farther north it will experience a thrust to the eastward, and will therefore be 
thrown up against the western shores of Africa, and the observed cold current coming 
from the south and entering the Strait of Gibraltar would be satisfactorily explained. It 
must be remembered that to produce and sustain the observed temperature gradient of 
7° F. in less than 20 miles, and a difference of 5° F. between the temperature of the 
surface water and that of the air above it, a considerable and active supply of cold water 
is absolutely necessary, and it is observed to have a motion from the southward. 
It is difficult to make an accurate estimate either of the mean depth or the mean 
salinity of the ocean, but taking the one at 2000 fathoms (3660 metres) and the other at 
35 '5 grammes per litre, there would be an average of 1300 kilogrammes of salt dissolved 
per square decimetre of sea surface. Taking the specific gravity of sea salt at 2 '5, this 
would give the amount of salt dissolved as equivalent to a layer of it 52 metres, or 170 
feet thick over the whole area of the ocean. 
It may be pointed out that organisms which secrete silica are much more abundant 
in the Pacific, where the specific gravity of the water is relatively low ; and, on the other 
hand, organisms which secrete carbonate of lime are relatively more abundant where the 
specific gravity of the water is high, as, for instance, in the tropical regions of the Atlantic. 
The ratio of total salts to water has been discussed when treating of the 
density of the water at constant temperature. During the cruise samples of sea 
water from very various depths and localities were collected and preserved in 
glass-stoppered bottles, and from time to time sent home. After the return of 
the Expedition a large number of these were handed over to Professor Dittmar 
for analysis. On his analyses of a portion of these, and his work in connection 
with the subject, Professor Dittmar has furnished an exceedingly valuable Report , 1 
1 Report on the Composition of Ocean-Water, Pliys. Chem. Chall. Exp., part i., 1884. 
