PHYSICAL OCEANOGRAPHY 



221 



temperature has by the author of the present chapter been Potential 

 called \\-\^ potential temperature, a term used in meteorology. temperature. 



From these and many similar observations it is seen that 

 the temperature in the deepest strata of the North Atlantic is 

 about 2^° C. (as a rule a little lower). The temperature of the 

 deepest strata below 2000 fathoms appears to remain almost 

 constant through long periods of time, the variations probably 

 not amounting to more than a few hundredths of a degree. 

 Very delicate instruments are necessary to detect them, and as 

 yet we have insufficient observations to enable us to study 

 the details. 



It is apparent from the tables that the temperature would 

 fall several tenths of a degree if the "deep-water" were raised 

 to the surface without being heated by mixing on the way. 

 This we have been able to prove in a direct way by means of 

 the insulating water-bottle, which we used at Station 91 at a 

 depth of 4750 metres, the temperature inside the water-bottle 

 after hauling up being only 2.00° C, whereas the water at that 

 depth was in reality several tenths of a degree warmer. When 

 ill situ the water has the temperature indicated by the reversing 

 thermometer, but when brought to the surface it has the 

 potential temperature nearly indicated by the thermometer 

 inside the insulating water-bottle. Granted that no other 

 change has taken place, the bottom-water must have had a 

 temperature of about 2° C. at the time when it began sinking 

 down from the surface ; as it sinks the temperature gradually 

 rises, and at Station to A, for instance, it was found to be 

 0.12° C. higher at 4500 metres than at 3000 metres. Some 

 such increase of temperature towards the bottom has long 

 been suspected as an effect of the internal heat of the earth ; 

 as early as about 1840 Aime looked for it, but his methods 



