THE VOYAGE OF H.M.S. CHALLENGER. 
966 
gravity is lower than in the south, the contrary being the case in the Atlantic, and no 
doubt the configuration of these two oceans is the chief cause of their' diverging con- 
ditions, the North Pacific being, as already stated, a wide open bay, whereas the North 
Atlantic is more like a lake. 
Generally speaking, the density diminishes from the surface down to a depth of 
800 or 1000 fathoms, and then increases towards the bottom, where, in the Pacific, 
the density varies from 1'0254 to 1'0257. In the South Atlantic the bottom density 
is 1*0258 to 1*0.260, and increases to as much as 1*0265 in the North Atlantic. Here, 
however, is found the greatest collection of dense water to be found in any open 
ocean, and the basin of the eastern portion of the North Atlantic receives the overflow of 
very dense water from the Mediterranean, which finds its way out along the bottom of 
the Strait of Gibraltar. This water having a high temperature (55° F.) and high density 
(l '028 to 1'029) affects the temperature as well as the density of the bottom water. 
In the trade wind regions, the density decreases from a maximum at the surface to a 
minimum about 1000 fathoms, and then slowly increases again. In the regions of the 
equatorial calms and rains the specific gravity most commonly increases from a minimum 
at the surface to a maximum at a depth of from 50 to 150 fathoms, from which point, 
downwards, it follows the same law as in the trade winds. The existence of the sub- 
surface maximum is probably due to the fact that the water, concentrated on both 
sides of the Equator, is driven by the wind towards the Equator, where there is a 
constant supply of fresh water of high temperature, beneath which it is forced to dip. 
Starting from the source of the trade wind, it is found that while it is concentrating the 
surface water it is always forcing it farther into warmer latitudes, where, owing to the 
rise of temperature, the water, though it has become salter, has at the same time become 
lighter. As the Equator, however, is approached, the rise of temperature with decreasing 
latitude diminishes, and the water thus becomes liable to sink of itself, even although it 
were not covered over by the tropical rains. A large quantity of water forced north- 
wards towards the Equator passes into the North Atlantic, owing to the preponderating 
force of the southeast trade. Here it follows the course of the Equatorial Current into 
the Caribbean Sea, reappearing as the Gulf Stream, and ultimately forming part of 
the great lake of warm and dense water which occupies the basin of the North 
Atlantic. The only outlet for this water is into polar regions, and a portion of the water 
is driven in this direction and keeps the sea free from ice far into Arctic latitudes. The 
bulk of it, however, remains in the lake-like basin of the ocean, the central portion of 
which is known as the Sargasso Sea. In the centre of the northeast trade wind, the 
evaporation which goes on is very great, whilst at the same time a not insignificant 
yearly oscillation of temperature takes place ; these two causes combined materially assist 
the propagation downwards both of heat and saltness, and in point of fact in both these 
respects the waters of this region exceed those of any other part of the ocean. This 
