308 Royal Society : — Rev. G. C. Hoclgkmson's Actinometrical 



English ft. English ft. 



Becca di Nona 10384 1 -p..™ _^ C/Mfi 



Aosta 1969/ Difference 8415 



and should the Piz Stella prove readily accessible, 



Piz Stella 1 1 1 75 1 -n- tf , MM nQr a 



r ,, . ioin r JDifierence 9850 



Cmanenna 1,319 J 



While simultaneous observations at several adjacent stations of 

 progressive heights are much to be desired, it should not be forgotten 

 how largely the condition of simultaneousness at even only two sta- 

 tions adds to the difficulty of the work. And the question arises, 

 whether detached readings of the actinometer (with the accompany- 

 ing meteorological facts) taken at various points, as opportunity 

 offers, may not be encouraged with advantage. An accumulation of 

 these, carefully reduced and tabulated, could hardly fail to be valu- 

 able ; and they may be obtained with comparative facility. It would 

 indeed only be prosecuting these observations as we do those of at- 

 mospheric 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 ob- 

 servations 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 de- 

 partment 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 heen preserved ; to recover it, a careful set of obser- 

 vations under a vertical sun would be necessary ; and since an arbi- 

 trary 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 

 Committee for their kind encouragement, an earnest recommendation 

 that measures 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, 1860. 



