CHAP, v.] GENERAL CONCLUSIONS. 261 



fathoms, they gave, broadly, a uiiifonn error of l°-4 F. in ex- 

 cess, and that this correction might be applied proportionally to 

 the depth at which the observation is taken, i. e., 0°'7 F. for ev- 

 ery 100 fathoms. This may probably hold as a rough rule for 

 ordinary instruments, where absolute accuracy is not required. 



On reconsidering this matter since our return home, a doubt 

 has arisen wdiether we were justiiied in apjDlying to the mini- 

 mum side of the thermometer these corrections on the scale 

 prepared by Captain Davis, and a new set of experiments has 

 been commenced at pressures up to three to four tons on the 

 square inch. 



This last class of errors may seem very trivial, but there are 

 cases, where questions of special delicacy arise, in which they 

 may assume considerable importance. Throughout the ocean 

 generally, at all events between the two polar circles, the tem- 

 perature of the ocean may be said as a rule to sink from the 

 surface to the bottom. There are many places, however, where 

 this gradual sinking appears to be arrested at a certain point, 

 from which the temperature remains uniform to the bottom. 

 Frequently the temperature as recorded by the thermometer 

 reaches a minimum at a depth of 1800 or 2000 fathoms: this 

 is the case, for example, throughout the greater part of the 

 Atlantic, and there is little doul)t that the result is in the main 

 correct, and can be accounted for by the action of a very sim- 

 ple law ; but if the temperature remained exactly the same, 

 the application of this ultimate correction to depths from 2000 

 down to 3000 fathoms would cause the thermometer to appear 

 to rise sensibly. This certainly is not generally the case, or it 

 would have come out in the large number of observations 

 which have been made under circumstances where such a re- 

 sult might have been expected ; and therefore I think we must 

 conclude that in all the great ocean basins, from some cause 

 or other, there is a very slight fall of temperature to the very 

 bottom. 



