Daniell on Meteorological Observations: 101 



that the point of deposition is often three or four degrees above 

 the existing temperature of the air ; proving thereby that parti- 

 cles of water, though invisible, are floating in the atmosphere. 

 The ball of the instrument, being hotter than the surrounding 

 medium, forms an atmosphere of vapour, from these round 

 itself, of greater density than the general one, and which, con- 

 sequently, is precipitated at a higher degree. The notice of 

 such an effect is perpetually recurring in my register. Thus 

 then we are justified in adding another observation of the 

 hygrometer, though indirectly obtained, to the three already 

 recorded, by assuming the minimum temperature of the night 

 as the minimum temperature of the vapour. The mean of 

 these four observations form, I have no doubt, a very near 

 approximation to the mean of the whole day. I have calculated 

 these results for the last year, and have included the pressure 

 of the vapour so corrected in the fifth column of Table 1 . The 

 constituent temperature of the vapour, corrected for both 

 years, is included in the seventh column of Table 3. The dif- 

 ference is not so great as might at first be supposed. 



The mean degrees of dryness require correction upon the 

 same principle, but in a somewhat different manner. The same 

 fourth observation, so obtained, is available for this purpose ; but 

 the register-thermometer furnishes us with another datum of im- 

 portance to this correction, namely the maximum of heat. I 

 have made this the basis of another observation, by assuming the 

 medium of the constituent temperature of the vapour found 

 by the fore and afternoon experiments, and taking the dif- 

 ference of that and the greatest temperature. The degrees of 

 dryness, so corrected, are included in the sixth column of 

 Table I. It will be observed that they scarcely differ from 

 the former; so that half the dryness of the day, at its ma?(imum 

 heat, may be considered as the average degree of the twenty- 

 four hours. 



The same remarks apply, in a great degree, to the amount of 

 evaporation contained in the seventh column ; but the correction 

 in tills case would be so exceedingly small that 1 have not 

 tliought it worth while to calculate it. The method of csti- 



