24 
THE VOYAGE OF H.M.S. CHALLENGER. 
There is still seen an increase, though it is now a small one, in the temperature of 
the Atlantic and Pacific in the extreme east of the equatorial region near the continents. 
Depth of 500 Fathoms (Map 7).—Very marked changes have now been brought 
about in the distribution of temperature over the Atlantic. The mean temperature of 
the ocean has now fallen to 40°T or 1 0, 7 lower than at 400 fathoms, and the whole of 
the water of the South Atlantic is now under the average temperature of the whole 
ocean, except a narrow tongue which projects southward about long. 10° W. nearly as 
far as lat. 10° S., where a temperature of 40 o, 3 is found. 
On the other hand, nearly the whole of the North Atlantic to the north of the line 
of lowest atmospheric pressure, marking the region of calms, is above the average, and the 
region of highest temperature is no longer located in the western division of the ocean 
in the line of the drift of the trades. But the highest temperature, 54°, is now to be 
found immediately to the west of Gibraltar, thus pointing to a new distinct source of 
the high temperature of the North Atlantic from 500 fathoms downwards. 
The specific gravities of the North Atlantic are still much higher than any elsewhere 
observed, but at this depth there are no observations near Gibraltar. Those observed 
at greater depths will afterwards be referred to in connection with this appearance of 
high temperature in the east of the ocean. 
No area indicating a region of higher temperature is now seen in the North 
Pacific, but in the South Pacific there is such a well-marked area between New 
Zealand and Australia, showing an isothermal of 44°, being thus 4° higher than the 
highest in the South Atlantic, but 10° lower than the highest in the North Atlantic. 
Observations of specific gravities at this depth are almost entirely wanting in the 
Pacific. 
Depth of 600 Fathoms (Map 8).—The outstanding feature of the temperature 
at this depth is presented by the North Atlantic, where the higher temperatures no 
longer appear in the western part of the ocean. The highest isothermal seen anywhere, 
51°, is to the west of Gibraltar, from which temperature gradually falls to 39° imme¬ 
diately off the coast of America. Between the Canaries and the Azores the specific 
gravity of 1 ‘0267 has been observed, and a little to westward P0263. These are very 
high specific gravities for this depth, being much higher than any elsewhere observed 
in the ocean, and they point unmistakably to the warm dense undercurrent which issues 
from the Mediterranean as the source of the remarkably high temperature now overspread¬ 
ing the Atlantic. Further, the disposition of the isothermals alone would place the 
matter beyond dispute. The all but total absence of temperatures above the average from 
the South Atlantic is a noteworthy feature of this ocean, for only on its northern side 
does the temperature rise 0 °‘7 above the mean. This striking feature of the tempera¬ 
ture of the South Atlantic, seen also at the depths immediately above and below the 
600 fathoms depth, may be referred to two causes, the one being the transference to the 
