1880.] On Actinometrieal Observations, made in India. 163 



17. The facts of Table VII are exhibited in a form suitable for 

 comparison with the similar results of Table II of 1869.* Thus, for 

 the difference of radiation between Mussooree and Dehra, or M — D, 

 we have from the mean of all the results in each case 



M — D. 



For 1869, from Table II 77 



For 1879, from Table VII 74 



Difference 3 



where it will be remembered that M— D is independent of all varia- 

 tions which affected the two stations equally at each epoch. The 

 result shows no change of relative radiation in 1879 as compared with 

 1869. 



18. I next compare the radiation results at each station for the two 

 epochs, and these, no doubt, are subject to all the indicated causes of 

 change, some of which can be recognised, and even roughly estimated. 

 In the first instance, however, I accept the results as exhibited, and 

 compare them thus : — 



Means of all the Residfs. 



Mussooree. Dehra. 



In 1869, Table II 980 902 



In 1879, Table VII 954 877 



Difference on D p 26 . 25 



where the very close accordance of the two results suggests that they 

 represent intrinsic or residual causes solely. The magnitude of D x is, 

 however, liable to correction, from two causes,f because its components 

 are not in the same terms ; both the causes, however, affect the two 

 stations equally. 



Cause («.) In 1869 the time-range was ±1 hour. 



In 1879 „ „ + | hour. 



Difference .... ....... J hour. 



* "Proc. Roy. Soc," vol. 19, p. 229. 



f A third cause (c) may also be noticed, viz., that presented by a change in the 

 earth's radius vector ; reckoning the effect of this inversely as the squares of the 

 radius, there results a percentage of only 04 of the unit of radiation adopted in 

 this paper, i.e., one-tenth of a millimetre : this result is quite rejectaneous. It 

 would tend, however, to increase the magnitude of Dj. 



