166 MR. G. V. VERNON ON BLACK-BULB 



minima in January. The minima of the enclosed ther- 

 mometers read nearly all alike; the maxima of the 

 vacuo and carbonic-acid ones are nearly equal; and the 

 same remark applies to the hydrogen one and the one 

 filled with compressed air: the latter agrees with what 

 Tyndall points out, that hydrogen and atmospheric air 

 absorb heat equally. 



Table III. gives the difference of each monthly mean 

 referred to the reading of a freely exposed black-bulb 

 thermometer. 



In volume v. page 169, of Symons's f Meteorological 

 Magazine/ there is a paper by Mr. Francis Nunes, giving 

 comparisons of carefully made black-bulb thermometers 

 by Pastorelli, showing a considerable difference between 

 the thermometer in vacuo and the one partially exhausted ; 

 his observations were made in October, and show a differ- 

 ence of i°*2 to ii°'5, the vacuo thermometer being the 

 highest of the two. Mr. Nunes also states that an en- 

 closed thermometer without any exhaustion reads still 

 lower, being from o°*8 to I2°*8 below the vacuo ther- 

 mometer. 



From my observations the difference between the vacuo 

 and condensed-air thermometers is never very large, 

 amounting rarely in individual cases to 5°*o to 6°*o, but 

 in July 1865 reached occasionally io°*o, the mean differ- 

 ence in July only reaching 4°*3. 



I am not aware of any similar series of observations to 

 be found anywhere else, and thought it might be desirable 

 to tabulate the values for comparison with any subsequent 

 series that may be made. 



