THE VOYAGE OF H.M.S. CHALLENGER. 
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also is high. In the same ocean, between lat. 10° to 40° S., and a little to the west 
of long. 10° W., the Challenger took eight observations, with a mean of 1*0264, 
which is still a high mean, but less than the mean of 1*0268 found to westward as we 
near South America. But within or close to the region where temperature falls to 
53° the five specific gravities observed by the Challenger were 1*0261, 1*0262, 1*0260, 
1*0260, and 1*0261, giving a mean of 1*0261, thus corroborating the view that the 
low temperature of 53° found here is brought, by upwelling, from greater depths. 
Depth of 200 Fathoms (Map 4).—The mean temperature of the ocean at this 
depth is 50°*1, showing thus a fall of 10°*6 from the mean at 100 fathoms. The part 
of the North Atlantic where temperature is above the mean, 50°*1, embraces now a very 
much wider area than that of the South Atlantic ; further, within the same area the 
temperature rises much higher in the North than in the South Atlantic, the highest 
isothermal in the former being 64° whereas in the latter it is only 55°. In the North 
Atlantic the mean specific gravity is 1*0270 over the warm area, but over the similar 
area in the South Atlantic it is only 1*0261. Thus in the North Atlantic the specific 
gravity conditions favour a more rapid descent of the warmer surface water, which, as 
shown by the temperature observations, is what actually occurs. 
In the South Pacific the warm regions embrace a much wider area than in the 
North Pacific, the temperature at 200 fathoms being still higher in that half of the 
ocean which receives the overflow of warm surface waters from the other half, precisely 
as obtains between the two divisions of the Atlantic. In the South Pacific the mean 
specific gravity is 1*0259, and of the North Pacific 1*0256, these, it is to be noted, being 
both very’ much less than the mean specific gravity of the North Atlantic, which is 
1*0270. 
In both the Atlantic and Pacific a local higher temperature is still found in the 
eastern equatorial regions of both oceans, but the increase from the lower temperature 
to the westward is now much less than it was at 100 fathoms, being only from 2° to 3° 
instead of 12° and 6°. With these reduced differences the specific gravity differences 
are also reduced. 
Depth of 300 Fathoms (Map 5).—The mean temperature of the ocean at 300 
fathoms is 44°*7, or 5°*4 less than at 200 fathoms. The higher temperature of the 
North as compared with the South Atlantic is even more pronounced than at less depths, 
as seen from a comparison of the regions of the two oceans included within the 
isothermal of 45°. Further, while the highest isothermal in the South Atlantic is 48°, 
covering a very small area, the same isothermal in the North Atlantic covers about half 
the area of the whole ocean, and includes two areas within which the temperature rises 
to fully 60° ; in the western of the two it rises to 63°, or 15° higher than the highest 
temperature anywhere found in the South Atlantic. It is also to be noted that these 
two areas of warm water are both well to the west of the ocean, in other words, they 
