for him this Medal, which is the second that has been given to hira 

 by the Royal Society. In awarding it to his researches on the law 

 of extinction of the solar rays, the Council have not alone been 

 guided by their own sense of the author's merits, but also by a de- 

 tailed report with which Sir John Herschel has favoured them*. As 



* Sir John Herschel's report, above referred to, is as follows : — " Mr. 

 Forbes's paper, as far as my knowledge extends, records the first at- 

 tempt, and that in no slight degree successful and satisfactory (consider- 

 ing the very great complexity of the physical considerations it involves 

 and the difficulty of the experiments themselves), to obtain a positive 

 measure of the extinction of solar heat (as distinguished from light), in 

 traversing a measured portion of the atmosphere, and that one, of which 

 the meteorological conditions have been carefully ascertained at the two 

 extremes, and therefore in which the nature and density of the medium 

 traversed within their limits have been determined by direct observation as 

 well as such data can be ascertained at all on the great scale. For this 

 purpose, simultaneous observations are indispensable ; and in the choice of 

 a coadjutor, Mr. Forbes must be allowed to have been highly fortunate. 

 The rarity of opportunities for such observations to be made with any 

 prospect of a dependable result, I am satisfied, from my own experience, 

 is by no means overstated by Mr. Forbes. That of which he availed 

 himself seems to have been as nearly unexceptionable as could have oc- 

 curred, and to have been used with all regard to the obtaining a precise 

 knowledge of the meteorological particulars capable of influencing the 

 result. In such cases a single series under unexceptionable circumstances 

 thoroughly worked out may and must afford results far more valuable than 

 any number of series obtained on less select occasions. 



"The mode in which Mr. Forbes has analysed his own and M. Kamtz's 

 observations on the Faulhorn and at Brientz, as well as the conclusions 

 he deduces from them, are both in many respects remarkable. The method 

 of graphical interpolation is resorted to throughout, and curves so deduced 

 expressing at once to the eye and to the reason the simultaneous variations 

 of all the meteorological elements at both stations, as well as the march 

 of the actinometer. The comparison of these last curves leaves no room 

 to doubt either the practical efficiency of the method of observation pur- 

 sued, or the nature of the causes in action which give rise to the many 

 remarkable and corresponding peculiarities which their forms exhibit, and 

 which, as general affections of the actinometric curve, Mr. Forbes has 

 examined and traced up to their origin in the combination of the sun's 

 varying altitude, and the hygrometric changes induced on the column of 

 air traversed by his rays, by the heat already developed. It is a curious 

 and complex case of causation in which the direct and immediate effect of 

 the cause is modified by an indirect one of a cumulative kind, resulting 

 from the totality of its action from its commencement to the time of 

 observation. 



" The comparison of the hygrometric curves with the actinometric leads 

 to no very distinct conclusion, though this is a point on which Mr. Forbes 

 has bestowed great attention. A general but not very precise analogy is 

 pointed out between the curve of mean dampness and that of relative 

 extinction ; but on the whole, no distinct relation is pointed out between 

 that dampness u-Jiich affects the hygrometer and that which disturbs the 

 merely aerial extinction of solar heat — (if indeed simple dampness, as such, 

 be the only or the principal disturbing cause). 



On the hypothesis of ' uniform opacity,' or that in which the extinction 



