23 



Messrs. Roscoe and Baxendell on the relative [Feb. 22, 



As the altitudes here observed vary only from 15° 44' to 31° 47', we 

 thought it best to collect the results into two groups, containing the eight 

 highest and eight lowest observed altitudes. 



Table II. — Results of Observations at Owens College. 



* 



Number of 

 Observations. 



Mean 

 Altitude, 

 of Sun. 



Intensity 

 of Sky or 

 diffused 

 Daylight. 



Intensity 

 of direct 

 Sunlight. 



Eatio of 

 Sun to Sky. 



Sky. 



Sun. 



Group 1 

 2 



33 

 20 



34 

 24 



17° 8' 

 26 38 



•066 

 •074 



•007 

 •008 



0106 

 0108 



The determinations made at Cheetham Hill (53° 30' 50" N., and 

 0^ 8"" 56' W.) were sixty- three in number, in which the altitude varies 

 from 16° 8' to 46° 14', and these are divided into three groups, as follows : — ■ 



Table III. — Results of Observations at Cheetham Hill. 





Number of 

 Observations. 



Mean 

 Altitude 

 of Sun. 



Intensity 

 of Sky or 

 diffused 

 Dayliglit 



Intensity 

 of direct 

 Sunlight. 



Eatio of 

 Sun to Sky. 



Sky. 



Sun. 



Group 1 

 2 



3 



23 

 22 

 18 



24 

 22 

 17 



19° 30^ 

 25 31 

 34 8 



•064 

 •091 

 •104 



•012 

 •019 

 •026 



0187 

 0-208 

 0-250 



The range of altitude in the Heidelberg experiments being a much wider 

 one (viz. from 0° to 63° 49'), we have been able to arrange these (con- 

 taining ninety-nine observations) in five groups, as follows : — 



Table IV. — Results of Observations at Heidelberg. 





Number of 

 Observa- 

 tions. 



Range of 

 Altitude of 

 of Sun. 



Mean 

 Altitude 

 of Sun. 



Intensity 

 of Sky or 

 diffused 

 Daylight. 



Intensity 

 of direct 

 Sunlight. 



Eatio of 

 Sun to Sky. 



Group 1 



10 



0° to 15° 



7° 15' 



•048 



•002 



0041 



2 



19 



15 —30 



24 43 



•134 



-066 



0-472 



3 



31 



30 —45 



34 34 



•170 



•136 



0-800 



4 



22 



45 —60 



53 37 



•174 



•263 



1511 



5 



17 



above 60 



62 30 



•199 



•319 



1-603 



The curves on PI. I. fig. 1 are derived from the foregoing numbers, 

 the ordinates representing the intensities and the abscissae denoting the 

 corresponding altitudes. The curves marked 5, c give respectively the 

 observations at Heidelberg, Cheetham Hill, and Owens College ; the dotted 

 curves represent the intensities of the diffuse light, the black curves those 

 of the direct sunlight. The ratio of the sun and skylight for the same 

 places is represented by the curves a, h, and c, fig. 2. 



In the following Table the experimental ratios are compared with those 

 calculated by Clausius. 



