1821.] Causes of Calorific Capacity, Latent Heat, %c. 387 



From this table it will be seen, that the temperature of the 

 mercury differed at a medium so little from that of the atmosphere 

 as not to exceed -pg-^ of a degree. The mercury, it would seem, 

 was about this quantity higher than the atmosphere ; but the 

 difficulty of making the observation to this nicety was so great, 

 the fluctuations of the temperature so considerable, and the 

 difference itself so minute, that it could not be attributed to any 

 other cause than the inappreciable errors of the observations, 

 unless it be true, as Dr. Wells has sagaciously suggested, that a 

 naked thermometer does not accurately indicate the temperature 

 of the air in which it is placed, but a temperature commonly 

 somewhat lower. Between the temperatures of the water and 

 atmosphere, the difference was much more marked; at a medium 

 it amounted to near three quarters of a degree. Such a differ- 

 ence as this is much too great to be laid to the errors of the 

 observations, and can only be explained on the principle of 

 evaporation, as I had anticipated from theory. I have not tried 

 it in other fluids, but there seems no doubt but the same cause 

 which produces an inferiority of temperature to that of the atmo- 

 sphere in one fluid operates to the same effect in all those which 

 evaporate at low temperatures. In general I expect the inferio- 

 rity will be greater the greater the evaporation, and, therefore, I 

 should think it would be more marked in ether than in alcohol, 

 and more in alcohol than in water, under the same circumstances. 

 Whether the effect would be materially different in winter it is 

 impossible, from these experiments, to say. We should certainly 

 expect it would be much less in cold damp weather than in warm 

 and dry ; but when I once set the window open, for two or three 

 hours during the rain, I could not perceive any sensible difference. 

 In fact, in the last set of observations, in which one would 

 have expected the damp to have had the greatest influence, and 



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