Chemical and Physical Notes 27 



layers to indicate the gradient which separates the cold 

 bottom water from the comparatively warm intermediate 

 water. The recommendation, therefore, which the writer 

 would make is, that in these regions temperature observations 

 in the deeper layers should not be spared, and where there is 

 water of exceptional coldness at the bottom, the position and 

 steepness of the gradient which separates it from the overlying 

 water should be accurately determined. Further, as the whole 

 range of temperature to be dealt with in Antarctic water is 

 at the most from 28 to 35 or 36 F., and therefore small 

 differences of temperature are relatively of great importance, 

 it is well to have the thermometers constructed specially for 

 this work, the scale containing few degrees, but these wide 

 apart. In the survey of the Gulf of Guinea in the " Buccaneer," 

 the writer had such thermometers, and he regularly sounded 

 with one thermometer at the end of the wire, and another 

 usually 250 fathoms above it. 



It is also of great importance to ascertain the density of 

 this exceptionally cold bottom water. Near the coast of 

 South America it was found in the " Challenger " to be very 

 high, and this was confirmed by an observation of the 

 " Gazelle " in the same locality. It is this density at constant 

 temperature which decides whether a water can carry its 

 surface temperature down to great depths, or whether it shall 

 remain at the surface, and it is the annual range of tempera- 

 ture of such water which gives it its penetrating power. This 

 was clearly set forth in a paper sent home during the first 

 year of the voyage of the " Challenger," and published in the 

 Proceedings of the Royal Society*. The highest surface densities 

 are found in the North Atlantic, in the Trade-wind regions, 

 and there the surface water has a higher density than any 

 layer below it; consequently, when it is cooled in winter to the 

 same temperature as the water immediately below it, it sinks 

 through it, and in this way a high temperature is disseminated 

 through the whole thickness of the water of the North Atlantic. 

 In the eastern part of this ocean, the density and temperature 



1 ' Note on the Vertical Distribution of Temperature in the Ocean,' Proc. R. S. 

 (1875), vol. clvii. p. 123. 



