MK. \V. N. SHAW OX HYGHOMKTRIU MK I HMD- 



The values of the constant A of REONAULT'S formula lay between 0*00062 and 

 0-00233 for No. 1, and between O'OOOGO and 0'00199 for No. 2. 



CHISTONI has continued his work by making a series of comparisons between the 

 ventilated psychrometer and REGNAULT'S hygrometer readings for temperatures below 

 15 C. The observations published in the ' Annali della Meteorologia' for 1879 comprise 

 191 comparisons for temperatures mostly below 15 C C., when the wet bulb stood above 

 0C. ; 32 comparisons when the psychrometer was in the so-called critical state, i.e., 

 when the dry bulb stood above C. and the wet bulb below that temperature, and 

 32 comparisons when the dry and wet bulbs were both below zero. The observations 

 were made at Collio, where the mean barometric pressure for the six months over 

 which the observations lasted was 680 mm. 



The following table is taken from CHISTONI'S paper : 



The errors given are the actual errors in the computed humidity. I have not 

 calculated the percentage errors ; a rough idea of them may be got by taking the 

 humidity at 50, when the percentage errors would require each number given in the 

 columns of the table to be multiplied by 2. 



These errors are very serious ; they are only briefly discussed in the paper ; the 

 generalisation is made, namely, that the errors are positive and small when the 

 humidity is great, and they are negative and larger when the humidity is small. The 

 psychrometer observations were reduced, as before, by means of HAEQHENS'S ' Tavole 

 ad uso della Meteorologia. 1 



CHISTONI has also published* a paper discussing the various psychrometrical 



* ' Memorie e Notizie della Meteorologia Italiana,' 1878. 



