1879.] Prof. E. Frankland on Dry Fog. 239 



liampton, on July 17, 1862, at an altitude of 9,882 feet, and when 

 passing thro ugh a cloud so dense that the balloon could not be seen 

 from the car, the dry bulb thermometer read 37°'8 F. and the wet 

 30*°2, indicating a dew point 17° '9 below the air temperature. Again, 

 on the 30th July in the same year, when at an altitude of 6,466 feet, 

 between the Crystal Palace and Gravesend, and whilst the balloon 

 was passing through a " great mist," the dew point was 12 0, 7 F. 

 below the temperature of the air. The following is a tabulated state- 

 ment of other instances in which there was comparative dryness of the 

 air in the midst of cloud or fog : — 



Date* 



Place of ascent. 



Altitude 

 in feet. 



Temperature 

 of air. 



"TV £ 



Degree or 

 humidity. 







100 = saturation. 



1862. 







53 5 F. 





August 18th 



Wolverhampton 



5,922 



61 



1863. 











March 31st 



Crystal Palace 



5) 55 



3,698 



38-5 „ 



62 



April 18th 

 jj ?> 



9,000 



32 5 ., 



52 



55 55 



8,000 



34-9 „ 



73 





55 55 



7,000 



37-8 „ 



87 



5) J) 



55 55 



6,000 



41-0 „ 



76 



55 55 



June 26th 



55 55 



Wolrerton 



5,000 



45-0 „ 



67 



11,000 



30-0 „ 



68 



July 11th 





10,000 



315 „ 



46 



Crystal Palace 



3,200 



65-2 „ 



57 



» jj 



55 55 



55 J) 

 55 55 



2,600 

 1,600 





53 

 50 



55 5) 



1864. 



55 55 



1,000 



*64-7 



53 











April 6th 



Woolwich 



6,000 



44-0 „ 



64 



jj >) 





1,000 



41-7 „ 



75 



1865. 











Feb. 27th 



55 



4,400 



42-0 „ 



52 



It is thus evident that the air closely surrounding the spherules of 

 water in a mist, cloud or fog, is sometimes far from saturated with 

 moisture ; although, as is well known to persons occupied with gas 

 analysis, when a perfectly dry gas is admitted into a moist eudiometer 

 it very rapidly assumes the volume indicating saturation, notwith- 

 standing that the proportion of water surface to volume of gas is 

 obviously far less than that afforded to the interstitial air of a fog. 



In a special experiment of this kind, it was found that air dried 

 over calcic chloride became completely saturated with moisture in less 

 than one minute and fifty seconds, when it was passed into a moist 

 glass tube three-fourths of an inch in diameter. It appeared to me, 

 therefore, interesting to inquire under what condition the evaporation 

 from the surface of water can be so hindered as to cause this delay 

 in the saturation of the closely surrounding air. Many years ago I 

 became acquainted with the fact, first noticed I believe by Mr. P. 



