50 Mr. J. B. N. Hennessey. [May 25, 



1881 series is half-hourly, and from 9 a.m. to 3 p.m. Taking sums of 

 the defects at the common hourly points, 9h. to 3h., we have — 



Defect before noon. 

 ,, after ,, . 



1879. 



70 

 107 



1881. 



58 

 79 



Radiation greater before noon 



37 



21 



where the two series agree in showing that the radiation before noon 

 is in excess, as pointed out before. And as respects differences 

 between defects from noon at corresponding hour angles, we have — 



Difference of defect. 



Comparing. 



Hour Z . 



1879. 



1881. 



Mean. 



8 and 4 



. . f 4 



. . 12 .. 





.. 12 



9 „ 3 f 



..3 



.. 17 .. 



.. 16 .. 



.. 16 



9. 5 „ 2.5 . 



. . 2. 5 . . 





9 .. 



9 



10 „ 2 . 



..2 



. . 11 . . 



3 f . 



.. 7 



10.5 „ 1. 5 . 



. . . 1. 5 





5 



5 



11 „ 1 • 



.. 1 



. f '9 .. 



.. 2 .. 



5 



where the value at 

 Z 



large an hour 



4 alone is contradictory, but the results at so 

 are difficult to observe in winter, and especially with 

 a Hodgkinson actinometer, which requires repeated " casts off " and 

 hasty manipulation. Otherwise these mean results, so far as they go, 

 increase with the sun's progress. 



12. T. 5 presents abstracts of the observations already published of 

 1869 and 1879, and now expressed in terms of No. 2 (unit 6°'01 F.) 

 and reduced to Z.D. 45°. 



13. T. 6, T. 7, and T. 8 present abstracts of daily results respec- 

 tively from T. 1, T. 2, and T. 3, i.e., for Autumn 1880, Summer 1881, 

 and Autumn 1881. [The results for the four autumns are exhibited 

 graphically in Plate 1.] 



14. With regard to T. 7, no reduction to Z.D. 45° has been 

 attempted, for circumstances unfortunately did not permit of 

 observing a long series at the time. In fact, observations in the 

 summer were not contemplated, for, as a rule, that season is highly 

 unfavourable for actinometrical work. As, however, the atmosphere 

 unexpectedly became fine and clear, advantage was taken of the 

 opportunity to do as much work as was practicable : for this purpose 

 none but highly favourable days were admitted, and it is a point to 

 be noticed that excepting on May 2, 6, and 9 the visible conditions were 

 decidedly good. On the dates just mentioned a dry (dust ?) haze 

 visibly enveloped the observer, and the observations then taken are 

 intended expressly to measure the effect of this haze. The results 

 from T. 7 are 



