20 
THE VOYAGE OF H.M.S. CHALLENGER. 
These two examples may be regarded as typical of the distribution of the tempera¬ 
ture over the greater portion of the intertropical part of the Pacific from the surface, 
where the differences are small, downwards to 100 fathoms depth, where the difference 
exceeds 20°. It is further seen that the difference nearly ceases at about 200 fathoms. 
Similar areas of low temperature are seen in the Atlantic to the west of Lower Guinea, 
and again in the Indian Ocean to the south of Ceylon. From their evident relations 
to the systems of atmospheric circulation, temperatures, and specific gravities of their 
respective oceans, a similar origin is suggested for them all. 
These areas of low temperature occur to the north of the equator in the Pacific and 
Indian Oceans, but to the south of it in the Atlantic. Now these are the sides of the 
equator where are located at this depth areas of high temperature of less extent and 
intensity and of lower specific gravity than are found on the other side. In other words, 
they occur on the side of the equator where the warm waters of the surface, owing to 
the diminished specific gravity, are not diffused or conveyed so quickly downwards to 
greater depths. 
Again, looking at the Pacific, the expanse of low temperature is bounded on each 
side by the trade winds, particularly by those tracks of the ocean where they are strongest. 
These trades impel to the westward warm surface currents, which by friction extend to a 
considerable depth in the ocean. To the eastwards these currents are fed by the current 
succeeding them generated by the trades, but on their equator side and the interven¬ 
ing space, the current, depth for depth, is more sluggish, because the surface winds 
there are either very light or wholly wanting. The result is that the currents of the 
trades proper take the largest source of their supply from the sea directly underlying the 
calm intervening region between the north and south trades. Hence there necessarily 
sets in an upwelling from lower depths to make good the drain, and the waters thus 
drawn from greater depths brings the low temperature of these depths up with them. 
If this were all, these areas of low temperature would be found close up to the 
respective continents on the east side of the ocean. Now in no instance are they found 
in such a situation, but instead, some distance to westward in the ocean, apparently 
proportional to the breadth of the ocean, and having between them and the continent 
to the east a subsidiary area of high temperature. These small areas of high tempera¬ 
ture have their explanation in the back water surface currents flowing eastward between 
the trades inforced by the westerly winds of a monsoonal character which are encountered 
for some distance out at sea before arriving at the western shores of the continents. 
It is probable that the greater volume of, and distance traversed by, these currents in 
the broad Pacific as compared wfith other oceans directly result in a stronger and more wide¬ 
spread upwelling, accompanied with a correspondingly extensive diminution of tempera¬ 
ture. Again, the South Atlantic pours vast volumes of its warm surface waters into the 
North Atlantic to join the westerly current already formed and flowing there, and this 
