3^4 



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



the results will not appear either disappointing or discouraging. The 

 season was extremely unfavourable for the further prosecution of the work. 

 Looking to the imperfection of the instrument employed by Principal 

 Forbes in his observations in 1832, it would seem to be highly desirable 

 that his experiments on the column of air between the Faulhorn and 

 Brienz should be repeated, and that other pairs of stations, intermediate 

 in character to that and the Chamonix pair, should be essayed. I have 

 selected, in the hope of future opportunities, the following among others : — 



English ft. . EngHsh ft. 



Becca di Nona 10384 | Diff,,ence 8415 



Aosta 1969 J 



and should the Piz Stella prove readily accessible, 



Pi^.S^^"^ lll^n Difference 9856 



Chianenna 1319J 



While simultaneous observations at several adjacent stations of progres- 

 sive heights are much to be desired, it should not be forgotten how largely 

 the condition of simultaneousness at even only two stations adds to the 

 difficulty of the work. And the question arises, whether detached readings 

 of the actinometer (with the accompanying meteorological facts) taken at 

 various points, as opportunity offers, may not be encouraged with advan- 

 tage. An accumulation of these, carefully reduced and tabulated, could 

 hardly fail to be valuable ; and they may be obtained with comparative 

 facility. It would indeed only be prosecuting these observations as we do 

 those of atmospheric temperature and pressure. In process of time we 

 might hope to obtain the mean actinic power at stations of various heights 

 and circumstance for different altitudes of the sun. 



Since the scale of each actinometer is empirical, in order that observa- 

 tions with different instruments may be comparable, a standard of reference 

 is necessary. If such a standard were kept at Kew, and each instrument 

 employed were marked with a factor of reduction, ascertained by careful 

 comparison, a great encouragement would be afforded to actinometry ; 

 nor can any material progress in that department of observation be looked 

 for until some such arrangement is made. The actine-standard of Sir J. 

 Herschel can hardly be said now to have been preserved ; to recover it, 

 a careful set of observations under a vertical sun would be necessary; 

 and since an arbitrary standard, which may be assigned without any such 

 trouble will answer every purpose, it seems best at once to resort to this. 



I would venture, in conclusion, to couple with my thanks to the Com- 

 mittee for their kind encouragement, an earnest recommendation that mea- 

 sures be taken to provide a standard actinometer accessible for comparison, 

 under such regulations as may seem best to them, 

 I have the honour to be. Sir, 



Your obedient Servant, 



George C. Hodgkinson. 



November 27, 1866. 



