THE VOYAGE OF H.M.S. CHALLENGER. 
*28 
At the 100 fathoms depth the temperature conditions have materially altered. As 
regards the entrance to the Gulf, it is within the North Atlantic isothermal of 65°, but 
over the Gulf itself temperature, to the west of long. 90° W., falls below 60°, and over 
a small region between that line and the continent it falls to 55°. Here, then, 
observation gives no evidence that the dense surface waters descend to any great depth 
carrying with them their high temperature downward. The evidence, on the contrary, 
goes to show that the strong surface current issuing through the Straits of Bernini, with 
the deeper current it generates, drains away, to a very large extent, the whole of the 
warm dense waters of the surface, thus bringing about an upwelling of colder water in 
the western part of the Gulf, so that the temperature there is 15° lower than it is 
immediately to the south-west of Cuba. Unfortunately, no specific gravities have 
been available at this and lower depths in the Gulf of Mexico, which would probably 
show a diminished specific gravity corresponding to the diminished temperature. 
Three specific gravities have, however, been observed in the western part of 
the Atlantic, viz., 1’0278, l - 0278 and U0274, which may be regarded as pointing to a 
mixing of the higher surface specific gravities of the Gulf of Mexico with the water 
of the Atlantic at this depth. 
The relation of the temperature of the west of the Gulf of Mexico to that of the 
Atlantic continues to hold at greater depths till the temperature falls at 700 fathoms to 
39 :- 5, at which depth the Gulf of Mexico becomes “a closed sea,” and the temperature 
remains at this point dowm to the greatest depth, 2067 fathoms, of the Gulf of 
Mexico. The following gives a comparison of the lowest temperature in the Gulf of 
Mexico and the temperature at the entrance to the Gulf at the different depths :— 
Depth in 
Fathoms. 
Temperature at 
Entrance. 
Lowest Tempera¬ 
ture in Gulf. 
Difference. 
200 
55°-0 
46°-0 
9°-0 
300 
50°-0 
44° - 0 
6°-0 
400 
43°-5 
41°-0 
2° -5 
500 
41°-5 
40°-0 
r-5 
600 
41°-0 
40°-0 
r-o 
700 
40 o, 0 
39°-5 
0°-5 
These facts point to a strong upwelling of the deep waters in the western division 
of the Gulf of Mexico, necessitated by the strong current which issues from it, and which 
extends to a considerable depth in the ocean. This view is borne out by the fact that 
the Atlantic easterly current is there embayed before starting to join the Atlantic north¬ 
eastern current on its way towards the Arctic Ocean, and southerly and south¬ 
westerly winds become prevalent, thus inducing upwelling in the western part of the Gulf. 
It has been calculated that, owfing to the heaping up of the waters driven by the 
