18 
THE VOYAGE OF H.M.S. CHALLENGER. 
It has been long recognised that in the Atlantic the region of calms between the 
north and south trades is in all seasons north of the equator, reaching its northern 
limit, lat. 13° N., in July, and its southern limit, lat. 2° N., in January. Hence, in all 
seasons the south-east trades penetrate into the northern hemisphere, driving before 
them the warm surface waters of the South Atlantic. Thus vast quantities of the 
heated surface water of the South Atlantic are transferred bodily to the North Atlantic, 
resulting in these oceans presenting, as regards their temperature, the strongest 
contrasts to each other at all depths down to the bottom of the sea. The mean annual 
position of this region of calms may be shown by drawing on Map 51 of the Report 
on Atmospheric Circulation a line across the Atlantic, marking out the path of least 
barometric pressure of that region. Now it is this simple circumstance, the position of 
this line, which gives to the North and the South Atlantic Oceans their distinct 
contrasted features. 
Turning now to the Pacific Ocean on the same map, we find that the line of least 
barometric pressure lies not north, but south, of the equator, from long. 160° E. to 
long. 130° W. An examination of the maps for the separate months shows that for 
eight months of the year this state of things substantially holds good, culminating in 
December, January, and February when barometric pressure is very low in Australia. 
In these months the north-east trades and ocean currents of the Western Pacific extend 
into the South Pacific to about lat. 15° S., as shown by the current charts now in course 
of preparation by the Meteorological Council. 
The isothermals for the depth of 100 fathoms (Map 3) show that the manner of 
the distribution of the temperature in the North and South Pacific is precisely the 
reverse of what obtains in the Atlantic. In the North Pacific the highest temperature, 
70°, is restricted to two very small areas, whereas in the South Pacific the area marked 
out by the isothermal of 70° covers a very extensive region, and encloses another region, 
also very extensive, where the temperature exceeds 72°. In truth at this depth the 
South Pacific presents a region with a temperature above the general mean of the ocean, 
larger than the high temperature regions of all the other oceans combined. The role 
played by Australia, the low atmospheric pressure of its warmer months mainly 
bringing about the result, deserves careful consideration. 
A feature of the temperature in all the oceans is a pronounced increase in the 
eastern equatorial region of each ocean on nearing the continents. Thus in the Pacific 
about the Galapagos Islands, temperature is 62°, or 12° higher than in the same latitudes 
some distance to westward; in the Atlantic it rises to 59° in the Gulf of Guinea, or 6° 
warmer than it is to westward; and in the Indian Ocean it rises to 60°, or 4° higher 
than it is to westward. This peculiarity arises from two causes, one being the monsoonal 
deflection of the trade winds, for some distance out to sea, out of their course to west¬ 
ward, so as to blow towards and in upon the heated land of the continents ; and this 
