REPORT ON OCEANIC CIRCULATION. 
17 
Thus, the most powerful causes concerned in deflecting the isothermals of tempera¬ 
ture of sea-water and the lines of specific gravity from their normal east and west 
directions are the prevailing winds and the surface currents of the ocean they originate 
and maintain ; and further, as regards the specific gravities, the rainfall of the different 
regions powerfully influences the direction of the lines, and, as will appear further on, 
the vertical and horizontal movements in the ocean to which they give rise. 
Temperature of the Ocean at Different Depths. 
The Atlantic and Pacific Oceans. 
Depth of 100 Fathoms (Map 3 1 ). — On this map the isothermals are drawn for 
every five degrees of temperature, except in the cases of the highest and lowest 
temperatures of the oceans respectively, which are given to the nearest degree. From 
all the observations the mean temperature at this depth is 60 o- 7, and on the map, as 
on all the others, isothermals above the mean temperature at the particular depth are 
coloured red, and below it are coloured blue. 
Looking first at the distribution of the temperature from about lat. 30° N. to lat. 
30° S., it is at once seen that the higher temperatures lie to the westwards of the 
anticyclonic regions of the different oceans, and that the temperature of the western 
portions of these oceans is very much higher than in the eastern portions. The great 
difference observed is occasioned by the ocean currents setting in to the west, which 
are originated and maintained by the north-east and south-east trade winds. These 
winds drive the warm surface waters before them onwards towards the eastern coasts 
of the continents. In those cases where the. higher temperatures do not quite reach 
the continents, it is owing to the interposition of islands in their paths ; as, for example, 
Madagascar, the West Indies, and the East Indies, as respectively affecting Africa, 
North America, and Asia. 
The North and South Atlantic present a strong contrast to each other. It will be 
seen that on the whole the surface specific gravities of the two oceans (Map 1) do not 
very materially differ from each other, but their temperatures at the 100 fathom depth 
are widely different. In the South Atlantic the area showing a temperature above 60 o- 7, 
the mean temperature of the oceans, is small as compared with the extent of the same 
temperature and upwards in the North Atlantic. Further, in the South Atlantic the 
highest isothermal is only 63°, whereas in the North Atlantic there is a large area with 
a temperature exceeding 65°, which encloses two small areas each with a temperature 
rising to 70°. 
1 A large number of maps were constructed for depths intermediate to those discussed in this Report, particularly 
for depths down to 200 fathoms, in connection with the question as to the depth to which the influence of season is 
experienced. 
(PHY3. OH EM. CHALL. EXP.-PART VIIT.- 1895 .) 
3 
