334 NATURE AND MAN. 



No part of the Challenger's work has been more thoroughly 

 and successfully carried out than the determination of the thermal 

 stratification, or vertical distribution of temperature in the different 

 parts of the Oceanic area; an inquiry first prosecuted with trust- 

 worthy thermometers ("protected" to resist pressure) in the 

 Porcupine expeditions of 1869 and 1870. This determination 

 was effected by "serial" temperature-soundings; thermometers 

 attached to a sounding-line being let down to depths progres- 

 sively increasing by 10 fathoms down to 200, and below this to 

 depths progressively increasing by 100 fathoms to the bottom. 

 It is in the upper stratum of 200 fathoms that the most rapid 

 reduction of temperature usually shows itself; the further re- 

 duction beneath this stratum taking place at a progressively 

 diminishing rate, until from 1500 fathoms downwards to the 

 bottom at any depth there is usually very little change. 



The Temperature-soundings of the Challenger, supplemented 

 by other more limited explorations of the same kind, have clearly 

 brought out this most unexpected result, that the low bottom- 

 temperatures previously observed represent, — not, as has been 

 supposed, the overflowing of the sea-bed by " Polar currents " 

 of limited breadth and inconsiderable thickness, overlaid by a 

 vast mass of comparatively warm water, — but the reduction of 

 nearly the whole body of oceanic water, in every basin except 

 that of the North Atlantic (to whose exceptional character I shall 

 presently advert), to a temperature which averages but a very few 

 degrees above 32° Fahr., that of its deepest stratum being some- 

 times even a degree or two below the freezing-point of fresh water; 

 while the heating influence of the solar rays is limited to a very 

 small depth beneath the surface. 



Thus in the South Atlantic, in which a sounding taken near 

 37° S. lat. gave a depth of 2900 fathoms and a bottom-tempera- 

 ture beneath 32° Fahr., the lowest stratum, consisting oi absolutely 

 glacial water, was found to have the enormous thickness of 1000 

 fathoms : this was overlaid by another stratum of 1000 fathoms, 

 in which the temperature rose slowly from 32° at its lower to 

 36^° at its upper surface ; and this, again, by another of about 

 500 fathoms, which showed a further rise at its upper surface to 

 40°, the rate of elevation from below upwards being no more 

 than about 0.7° for every 100 fathoms. Thus it is only in the 



