14 
THE VOYAGE OF H.M.S. CHALLENGER. 
Sea, the specific gravity of the Arabian Sea is high, but everywhere else to the north of 
lat. 10 S. it is rather low, owing no doubt to the large and, over wide areas, heavy rain¬ 
fall occurring there. For a considerable portion of the year the prevailing winds of West 
and North Australia are from the sea, where barometric pressure is much higher than in 
the interior of the continent, and consequently the specific gravity is higher for some 
distance round these coasts. Similarly, over the western half of this ocean the prevail¬ 
ing winds blow home on the coast of Southern Africa, in the interior of which in the same 
months pressure is also low, and, just as happens off South America, the specific gravity 
is largest on the coast to the south of the latitude of Madagascar, which lies quite open to 
the ocean. The deflection of the lines round the South African coast is very interest¬ 
ing in connection with the well-known warm and cold ocean currents which are so strik¬ 
ing a feature of that part of the globe. Fortunately, these lines are drawn from good 
annual mean specific gravities, calculated from numerous observations made during each 
of the twelve months of the year, which have been published by the Meteorological 
Council. It will be observed that the highest specific gravities of the Indian Ocean 
are found in the region of the anticyclones of that ocean, just as occurs with respect to 
the other oceans, particularly the Pacific Ocean. 
The lower specific gravities observed in central Atlantic, between the higher specific 
gravities of the North and South Atlantic, are also annual averages, based on numerous 
observations, and mark with great distinctness the region between the two trade 
winds. It will be observed that in this region from long. 30° W. the specific gravity 
constantly diminishes eastward to the head of the Gulf of Guinea, where it falls below 
1 ’0260, being lower than occurs anywhere on the coasts from the English Channel to 
the Cape of Good Hope. 
The region of lowest specific gravity anywhere hitherto observed is the southern 
part of the Sea of Okhotsk where the mean falls to 00240, as calculated from numerous 
observations made there by Admiral Makaroff on board the “ Vitiaz,” and the next 
lowest between Greenland and Spitzbergen, where Professor Mohn, also from numerous 
observations, gives the low mean specific gravity of 1 - 0245. The mean of the ten 
observations made by the Challenger to the south of lat. 60° S. is 00247, and of the 
six of these made near to or to the south of the line of 00245 on Map 1, the mean 
is 00245, being the same as Mohn’s line of least mean specific gravity. The low specific 
gravities round the southern part of South America are occasioned, on the west coast 
chiefly, by the heavy rainfall of that region, and on the east coast, in addition to the 
rainfall, by the upwelling there resulting from the strong westerly winds blowing from 
the land seawards. 
