322 



Rev. G. C. Hodgkinson's Actinometrical [Jan. 17^ 



limited for the wet-bulb readings. Thus the meteorological observations 

 at the upper station are of the scantiest. Neither above nor below were 

 the actinometrical readings so continuous as I had wished to make them. I 

 had no one with me on the summit capable of rendering me the smallest 

 assistance ; but it is some consolation to think that, even had this been 

 otherwise, the results could not, under the circumstances, have been mate- 

 rially enhanced. 



There either did not exist, or I failed to detect, as the sun's altitude 

 increased, anything like a uniform progression of actinic power at either 

 station during the limited time in which the observations were continued. 



The results do little more than determine the ratio of the average inten- 

 sity at the two stations for a portion of the forenoon. This indeed was the 

 main object which I had in view. For looking at the experience of Prin- 

 cipal Forbes under easier conditions, when the continuance of the observa- 

 tions, as long as the clearness of the sky might last, presented no difficulty, 

 I did not at all anticipate being able to trace a dependence of the actinic 

 power on the hygrometric 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 suffi- 

 cient to show the kind of dependence which the opacity has on the damp- 

 ness, and that the values of the coefficient of extinction do not present any 

 correspondence 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^ to 10^ 11™ apparent 

 time, presents, with a single exception, a gradual decrease from r244 to 

 1*206. The interest of a comparison of these results with those which 

 Principal Forbes obtained between the Faulhorn and Brienz is unfortu- 

 nately diminished by the fact that his actinometer was not furnished with 

 an internal thermometer for ascertaining the temperature of the liquid 

 employed. This was ammonio- sulphate of copper, which has a coefficient 

 of dilatation varying from 1 at 60° P., 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 : — 



At 10*^ on the Faulhorn the ratio seems to have been rapidly increasing ; 

 on Mont Blanc it was slowly diminishing. The actual amount of the 

 ratio at 10^ is almost exactly coincident in the two cases ; but at 1 1^ on the 



Hour. 



Ratio. 

 M41 

 1-214 

 1-345 

 1-219 

 1-078 

 1-207 

 1-217 



9 

 10 

 11 

 12 

 1 

 2 

 3 



