1879.] Prof. B. Stewart. Inequalities of Declination. 241 



VI. During: twenty- two hours, evaporation was reduced from 2*123 

 grms. to "668 grm., or 68*5 per cent. 



VII. During twenty-four hours, the reduction was from 2*460 

 grms. to '180 grm., or 927 per cent. 



VIII. In a period of seventy-two hours, the reduction was from 

 7"638 grms. to -917 grm., or 88 per cent. 



IX. In seventy hours, the evaporation was diminished from 7' 732 

 grms. to 2*586 grms., or 66' 6 per cent. 



X. In forty-six Lours, the diminution was from 4*973 grms. to 

 1*647 grms., or 66*9 per cent. 



Experiments were also made with single drops of water suspended 

 in loops of fine platinum wire, and placed in the bell-jar filled with 

 dry air ; but it was found that the oily film had a strong tendency to 

 leave the drop and run up the platinum wire. In a comparative 

 experiment, in which one of the drops was protected by a coal-smoke 

 film, the unprotected drop lost 90 per cent, of its weight in two and 

 a half hours at 16°*6 C. ; whilst the protected drop lost only 37*8 

 percent, at 17°*8 C. in the same time. Another drop, protected by a 

 film of coal-tar, lost 37*6 per cent, of its weight in two- and a half hours, 

 the temperature being 14° C. in the bell- jar. 



It is highly probable that if globules of water without any solid 

 support (like those in cloud and fog) could have been operated upon, 

 the retardation of evaporation would have been still more marked, or 

 perhaps altogether arrested ; for in all the above experiments the oily 

 films manifested a tendency to break up and attach themselves to the 

 solid support of the water, leaving the surface of the- latter partially 

 unprotected. 



The results of these experiments point out a condition of very 

 common occurrence, competent to produce " dry fog," whilst they also 

 explain the frequency, persistency, and irritating character of those 

 fogs which afflict our large towns ; inasmuch as some of the 

 products of destructive distillation of coal are very irritating to 

 the respiratory organs, and a large proportion of them is scarcely if at 

 all volatile at ordinary temperatures. 



My thanks are due to my pupil, Mr. C. G. Matthews, for his assist- 

 ance in the foregoing quantitative determinations. 



IV. "Note on the Inequalities of the Diurnal Range of the De- 

 clination Magnet as recorded at the Kew Observatory." 

 By Balfour Stewart, F.R.S., Professor of Natural Philo- 

 sophy in Owens College, Manchester, and William Dodg- 

 SON, Esq. Received November 18, 1878. 



We are at present engaged in searching for the natural inequalities 



