300 THREE CRUISES OF THE " BLAKE." 



North Atlantic, in the heart of the region of the tradewinds, 

 and in the South Atlantic in the corresponding regions to the 

 east of the coast of Brazil. To determine from its density the 

 salinity of the water, the water specimens were reduced to its 

 value at a standard temperature, from the tables prepared by 

 Professor Hubbard of \yashmgton, and slightly corrected by 

 the physicists of the " Challenger." 



The salinity is affected by the fixing of the salts held in so- 

 lution, which, like lime and silica, are precipitated, as it were, 

 by organic substances. 



In the regions of great rainfall near the equator, the surface 

 water is frequently quite fresh ; and in the temperate zone there 

 is nearly an equilibrium between the rain and evaporation. To 

 these subdivisions of concentration and precipitation Thomson 

 assigned an important part in the formation of oceanic currents, 

 for these result from the constant interchange of vapor and 

 water between the air and the sea. 



It is interesting to compare the density of the Caribbean 

 and that of the Gulf of Mexico (below 1.0270), both enclosed 

 seas, but supplied with water from a region of low specific 

 gravity (between 1.0260 and 1.0270), with the high density of 

 seas, like the Red Sea and the Mediterranean (above 1.0280), 

 where evaporation goes on with immense acti\dty, although they 

 are both in communication with oceanic basins. In connection 

 with this we might also compare the density of Hudson's Bay 

 with that of the Baltic, and that of the Great Salt Lake with 

 that of the Dead Sea and the Caspian, representing climatic 

 conditions totally unlike. 



As has been well said by Moseley, we live in the depths of 

 the atmosphere, just as deep-sea creatures live in those of the 

 sea ; but our range, measured by the difference of temperature, 

 is far wider than that to which deep-sea animals, even when 

 subjected to the greatest exti-emes, are exposed. Terrestrial ani- 

 mals can withstand differences of heat and cold far better than 

 marine animals. The difference of temperature between an 

 arctic winter and a tropical summer, for instance, is from three 

 to four times as large as the widest range of temperature to 

 which any marine animal can be subjected. The lowest tem- 



