12 
THE VOYAGE OF H.M.S. CHALLENGER. 
whole globe during all seasons of the year. Over the whole of this vast space westerly 
and north-westerly winds sweep as strong winds, subject to little, if any, variation 
with season. From the enormous quantities of warm waters they impel before them 
into the Southern Ocean, they may be regarded as playing the most conspicuous part, 
of all the prevailing winds, in the circulation of the waters of the ocean. 
Specific Gravity of the Ocean. (Map 1.) 
Of all oceans the South Atlantic is the one containing fewest islands, and it is 
traversed by virtually the same winds at all seasons of the year. In that ocean, accord¬ 
ingly, the distribution of the specific gravity of the surface is presented in its simplest 
form. From the equator to the estuary of the Rio de la Plata the prevailing winds are 
south-easterly trades. It is also to be noted that these winds blow from higher to lower 
altitudes, and consequently from regions of lower to regions of higher temperatures (see 
Map 2). From these conditions result a drier atmosphere, increased evaporation from 
the surface of the sea, and a small rainfall, much smaller than would otherwise be, owing 
to the almost total absence of islands which would, in proportion to their height, 
condense the aqueous vapour into rain, and thus lower the specific gravity in this 
ocean. Hence, the specific gravity increases on nearing the South American coast, 
attaining the maximum, 1'0275 to P0278, from Bahia to Rio de Janeiro. Off the west 
coast of Africa, in the same latitudes, specific gravity is much lower, even though that 
coast, from lat. 10° to 30" S., is one of the driest regions of the globe, much drier, 
indeed, than the corresponding coast of Brazil. This is due to the fact that the 
prevailing westerly winds on that coast are but the in-draught from the sea towards the 
heated land of Africa, and, necessarily, owing to the distribution of pressure, they extend 
only a short distance westwards into the Atlantic. Further, as the whole wind-system, 
together with the surface currents of this portion of the South Atlantic, is drifted west¬ 
ward by the south-east trades, there is an up welling along the more easterly portion of 
the ocean to make good the surface drain, and the specific gravity of this deep water 
brought to the surface is much lower than that of the surface waters to the westward 
owing to the large evaporation of this region, to which these surface waters are subjected 
during their long course over this ocean. 
On the other hand, the North Atlantic presents features strikingly different. For a 
considerable portion of the year northerly and north-westerly winds set in on the north¬ 
west coast of Africa, and, since they have traversed a considerable extent of the Atlantic, 
and have been steadily advancing into warmer latitudes, that region is characterised 
by a specific gravity higher than anything observed off the coast of South Africa. Again, 
the north-east trades do not blow home on the shores of the Eastern States of America. 
On the contrary, the north-east winds from the anticyclone of the North Atlantic curve 
