ON SOLAK RADIATION. 129 



kindness of the Rev. Robert Main, F.R.S., and the Trustees 

 of the Radcliffe Observatory, Oxford, of copies of the 

 volumes of Radcliffe Observations for the years 1858 to 

 1864, and finding that they contained a valuable series of 

 solar-radiation observations, I have been led to resume the 

 subject; and although the inquiry is still incomplete, I have 

 thought that some of the results already obtained are suf- 

 ficiently curious and remarkable to render it desirable to 

 bring them before the Society, in order that attention may 

 be drawn to a much neglected but highly interesting 

 branch of inquiry, and to the necessity of devising and 

 adopting a more reliable and systematic method of deter- 

 mining the intensity of solar radiation than the one at pre- 

 sent in use. It is much to be regretted that no regular and 

 long-continued series of observations has ever been made 

 with Sir John HerschePs actinometer, since there can hardly 

 be any doubt that such a series would yield much more ac- 

 curate and reliable results than can be obtained by the use of 

 either the ordinary or the vacuo black-bulb thermometer. 



The Oxford solar-radiation observations commenced on 

 the 2nd of January 1856, and were made with a thermo- 

 meter supplied by Negretti and Zambra. On the 3rd of No- 

 vember 1858, this thermometer was unfortunately broken ; 

 but a new thermometer being obtained, the observations 

 were resumed on the ist of December 1858, and continued 

 uninterruptedly till the nth of September 1864, when the 

 second thermometer was also broken : a third thermo- 

 meter was obtained and brought into use on the 1 7th of 

 September, and the observations continued with it to the 

 end of the year. The entire series of observations during 

 the nine years 1856- 1864 must therefore be regarded as 

 consisting of two distinct series, made with different ther- 

 mometers, and therefore not strictly comparable with each 

 other. I have, in consequence, thought it desirable to con- 

 fine the discussion, in the first instance, to the observations 



SER. III. VOL. IV. K 



