306 Royal Society: — Rev. G. C. Hodgkin son's Actinometrical 



state of the atmosphere. He thus remarks (Bakerian Lecture, Phil. 

 Trans, part 2 for 1842, p. 253) of the experiments on the Faulhorn 

 and at Brienz, that " it cannot be affirmed they are sufficient to show 

 the kind of dependence which the opacity has on the dampness, and 

 that the values of the coefficient of extinction do not present any cor- 

 respondence with the hygrometric variations ;" and again, p. 268, 

 " It must be confessed that no evident relation to the hygrometric 

 condition of the air appears in the individual observations." 



From the experiments of the 14th of July the actinic ratio between 

 the summit of Mont Blanc and Chamonix, from 9 h 31 m to 10 h ll m 

 apparent time, presents, with a single exception, a gradual decrease 

 from 1*244 to 1*206. The interest of a comparison of these results 

 with those which Principal Forbes obtained between the Faulhorn 

 and Brienz is unfortunately diminished by the fact that his actino- 

 meter was not furnished with an internal thermometer for ascer- 

 taining the temperature of the liquid employed. This was ammonio- 

 sulphate of copper, which has a coefficient of dilatation varying from 

 1 at 60° F., to 2-562 at 32° F., and 0*626 at 100° F. His recorded 

 numbers for three hours before and three hours after apparent noon 

 derived from his freehand curve, are as follows : — 



Hour. Ratio. 



9 1*141 



10 1*214 



11 1*345 



12 1*219 



1 1*078 



2 1*207 



3 1-217 



At 10 h on the Faulhorn the ratio seems to have been rapidly in- 

 creasing; on Mont Blanc it was slowly diminishing. The actual 

 amount of the ratio at 10 h is almost exactly coincident in the two 

 cases ; but at 1 l h on the Faulhorn it was 1*345, a value much higher 

 than any which was obtained at any time on Mont Blanc, or seemed 

 likely to have been obtained at that hour had the observations been 

 continued so long. What share the greater depression of the lower 

 station in the experiments of 1832, the more complete isolation of 

 the upper station in those of 1866, or variable atmospheric conditions 

 in both sets may severally have had in contributing to this effect, re- 

 mains a matter for future investigation. The respective heights of 

 the stations are as follows : — 



Difference 6853 



English ft. English ft. 



Faulhorn 8799 



Brienz 1946 



Mont Blanc .... 157841 Difference 12359 

 Chamonix ...... 3425 J 



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