470 On some Results of the Cruise of H.M.S. ' Challenger. 3 



a result can only be arrived at by a careful combination and comparison 

 of many observations, taking into full consideration the errors of the 

 thermometers arising from all sources. There is a like very slight de- 

 crease in the bottom-temperatures from east to west. 



I think we can scarcely doubt, looking at such a diagram as Plate 19, 

 that, like the similar mass of cold bottom-water in the Atlantic, the bottom- 

 water of the Pacific is an extremely slow indraught from the Southern 

 Sea. That it is moving, and moving from a cold source, is evident from 

 the fact that it is much colder than the mean winter temperature of the 

 area which it occupies, and colder than the mean temperature of the 

 crust of the earth ; that it is moving in one mass from the southward is 

 shown by the uniformity of its conditions, by the gradual rise of the 

 bottom-temperatures to the northward, by the fact that there is no 

 adequate northern source of such a body of water (Behring's Strait being 

 only 40 fathoms deep, and a considerable part of that area being occupied 

 by a warm current from the Pacific into the Arctic Sea), and by our 

 knowlege from observations that one or two trifling currents from the 

 Sea of Okotsk and the Behring Sea, which are readily detected and 

 localized, and are quite independent of the main mass of cold water, re- 

 present the only Arctic influx. During its progress northwards the 

 upper portion of the mass becomes slightly raised in temperature by 

 mixture with, and possibly by slow conduction from, the upper layers 

 which are affected by solar heat. At the end of the Gulf (that is to say 

 in the extreme north, furthest from the cold source) the temperature is, 

 as I have already pointed out, influenced to the very bottom ; and the 

 isothermobaths between 8° and 5° C. are obviously raised and pressed 

 together, probably by the accumulation of the cold water against the 

 land. The colder bottom-water to the westward might be expected, from 

 the lower initial velocity of the Antarctic water causing it to drag against 

 the west coast. 



I am every day more fully satisfied that this influx of cold water 

 into the Pacific and Atlantic oceans from the southward is to be re- 

 ferred to the simplest and most obvious of all causes, the excess of 

 evaporation over precipitation in the northern portion of the land hemi- 

 sphere, and the excess of precipitation over evaporation in the middle 

 and southern part of the water hemisphere. 



After what I have already said I need scarcely add that I have never 

 seen, whether in the Atlantic, the Southern Sea, or the Pacific, the 

 slightest ground for supposing that such a thing exists as a general ver- 

 tical circulation of the water of the ocean depending upon differences of 

 specific gravity. 



Valparaiso, 

 Dec. 5, 1875. 



