16 
THE VOYAGE OF H.M.S. CHALLENGER. 
south-easterly. They therefore blow home on that coast, bringing from tropical regions 
the warm waters of the surface over the whole of the northern portion of this sea. 
On the western part of the North Atlantic the isothermal of 75° is carried to a 
higher latitude than anywhere else in the ocean, owing to the prevailing southerly and 
south-westerly winds, which carry thither the high temperature waters immediately to the 
south. On the other hand, the isothermal of 45° is carried to a lower latitude near 
Nova Scotia than anywhere else in the ocean, owing to the prevailing north-westerly 
winds there in the cold months of the year, and the cold currents that later on descend 
on these coasts from the Arctic regions. The isothermal of 45° is further remarkable in 
that it extends over a far larger number of degrees of latitude than any other isothermal, 
viz., from lat. 45° to 68° N., or twenty-three degrees of latitude. 
A marked feature of the distribution of the temperature of the North Atlantic is 
the crowding of the isothermals off the coast of America, and their opening out into 
higher latitudes as they approach Europe and to lower latitudes as they near Africa. 
Thus on the American coasts the isothermals from 45° to 75° extend over twelve 
degrees of latitude, but on the eastern side of the Atlantic to forty-eight degrees. An 
important point is that the highest temperature as regards latitude is not near the coast, 
but in the ocean a considerable distance to westwards. The lowering of the temperature 
off the north-west of Africa deserves attention, as being the direct result of the curving 
round of the prevailing winds in this part of the North Atlantic anticyclone to north¬ 
west and north, thus transferring the surface waters from higher to lower latitudes. A 
similar effect is even more emphatically expressed in the isothermals of the air in this 
part of the ocean (see Map 13 in the Report on Atmospheric Circulation). A similar 
lowering of the temperature, from similar causes, is seen off the west coasts of all the 
continents, the prevailing winds in these regions passing from higher to lower latitudes. 
On the other hand, as the prevailing winds on the western sides of the oceans pass from 
lower into higher latitudes, temperatures there are higher, except in the higher latitudes, 
where icebergs and icefloes, polar currents, and upwelling occur. 
The warm currents to the east of South Africa, and the cold currents to the 
west, are w T ell shown on Map 2, on which the results are now stated with great 
accuracy of detail from the Meteorological Charts for the Ocean District adjacent to 
the Cape of Good Hope, issued by the Meteorological Council. 
The lowest isothermal on the map is 30° in the Arctic Regions. The isothermals 
here, to the north of the British Islands, are taken from Professor Mohn’s great work on 
The Meteorology of the Norwegian North Atlantic Expedition, 1876-78 (Plate XVI.). 
These mean annual temperatures of the Norwegian Sea and Arctic Ocean, so carefully 
prepared by Mohn, are of first importance in their bearings on ocean circulation,' 
and the same remark may be extended to the deep-sea temperatures of the same 
expedition. 
