1866.] 



Intensities of Sunlight and Daylight ^ ^c. 



23 



Table V. — Ratio of Chemical Intensities of direct Sunlight to diffuse 



Light. 



Experiments. 



Sun's 



Calculated 



y 







Altitude. 



(Clausius). 



Heidelberg. 



Cheetham Hill. 



Owens College. 



20° 



0-491 



0-350 



0-19 



0-10 



25 



0-896 



0-480 



0-20 



0-11 



30 



1-320 



0-650 



0-23 





35 



1-690 



0-820 



0-26 





40 



2-Q32 



100 







50 



2-634 



1-37 







60 



3-129 



1-60 







These numbers show that whilst at 20° of altitude, according to theory, 

 the relation of the intensities of diffuse light to direct sunlight is as 100 to 

 49*1, the experiments at Heidelberg give a relation of 100 to 35 ; those 

 at Cheetham Hill 100 to 19, and those in Manchester 100 to 10. If we 

 compare the theoretical ratio for higher altitudes, we find that in our 

 latitudes the ratio even at 35° of altitude is only as 100 for diffuse light to 

 26 for sunlight, whereas theory gives the relation as 100 to 169. The 

 Heidelberg observations show indeed a more rapid rise in the intensity of 

 the direct sun's rays, the ratio reaching 100 to 82 for 35° of altitude. 

 The great difference between these and the other experimental results must 

 doubtless be ascribed to the considerable elevation (1900 feet above the 

 sea) at which these observations were made. 



Even at Heidelberg, however, no less than eight observations show that 

 at low elevations the chemical action of the sun becomes altogether inap- 

 preciable, whilst that of the diffuse light is still considerable ; and the same 

 inactive condition of direct sunlight at low altitudes has been frequently 

 observed both at Owens College and Cheetham Hill. In these cases the 

 intensity of the sun's direct visual rays was considerable, and a strong 

 shadow was cast ; but the more highly refrangible rays were altogether 

 absent, and the ratio became infinite. 



Heidelberg Observations. 



Altitude. 



Direct Sun. 



Diffuse Light. 



0° 34' 



0-000 



026 



1 32 



0000 



0024 



2 29 



000 



038 



3 27 



0000 



0028 



6 



0000 



030 



10 40 



0-000 



083 



11 61 



0000 



0-079 



12 58 



0-000 



080 



In some of the experiments made at Cheetham Hill the shadow of a 

 small disk was thrown on a horizontal surface of white paper, and careful 

 estimations made of the relative brightness of the shaded and unshaded 

 portions of the surface. A comparison of these results with those obtained 

 at the same time for the chemical rays showed that with the sun at a mean 



