1866.] Intensities of Sunlight and Daylight J ^c. 21 



tides of air or solid floating material, but from the minute vesicles of water 

 which are supposed to be always contained in large quantities in the atmo- 

 sphere. According to this hypothesis, Clausius obtained the following 

 numbers as expressing the intensities of direct sunlight and diffused day- 

 light for altitudes varying from 20° to 60° 



Calculated Intensities of 



Sun's Altitudes. Total Daylight. Diffuse Light. Direct Sunlight. 



20° 0-10049 006736 003313 



25° 017808 0-09291 008517 



30° 0-25933 011184 014749 



35° 034049 012654 021395 



40° 0-41957 013832 0-28125 



,50° 0-56686 0-15599 0-41087 



160° 0-69442 016822 052620 



(The intensity of sunlight at an altitude of 90°, vinweakened by atmospheric absorption, 



is taken as the unit.) 



The measurement of the relative chemical intensities were made at three 

 localities: (1) Owens College, Manchester, 53° 2<)' N., and 0'^ 9"^0^W.; 

 (2) the Observatory, Cheetham Hill, near Manchester; and (3) the 

 summit of the Konigstuhl, near Heidelberg, 1900 feet above the sea, in 

 49° 24' N., and 34'" 48" E. We are indebted for the latter observations 

 to Dr. Wolkoff, who kindly forwarded us iiis results through Professor 

 Bunsen. 



The following experimental numbers, obtained at Owens College, may 

 serve to illustrate the method adopted ; in most cases several (four or five) 

 observations of the intensities of the total and diffuse light were made 

 quickly one after the other, and the mean of all the readings taken. 



Table I. — Observations at Owens College, Manchester, 

 53° 29' N. 0'^ 9"^ 0^ W. 



Date. 



Greenwicli 

 Mean Time 

 of Observa- 

 tion. 



Sun's 

 Hour 

 Angle. 



Sun's 

 Alti- 

 tude. 



Intensity 

 of total 

 Daylight. 



Number 

 of 



Observa- 

 tions. 



Intensity 



of 

 diffused 

 Light. 



Number 

 of 



Observa- 

 tions. 



Intensity 

 of 

 direct 

 Sunlight. 



1865. 



h m 





31 17 













Oct. 6. 



12 



44W. 



•073 



o 

 O 



•068 



4 



•005 



7. 



9 30 



36 42 E. 



23 23 



•060 



1 



•056 



1 



•004 





12 



48 W. 



30 54 



•063 



1 



•057 



1 



•006 



18. 



11 25 



7 18 E. 



26 30 



•075 



2 



-056 



2 



■001 





11 45 



2 17 E. 



26 46 



•111 



2 



•089 



2 



•022 





12 30 



8 58 W. 



26 20 



•088 



4 



•087 



4 



•001 





1 19 



21 13 W. 



24 15 



•071 



4 



•007 



5 



•004 





2 45 



42 43 W. 



17 8 



•062 



2 • 



•053 



2 



•009 



24. 



45 



12 51 W. 



23 42 



•139 



3 



•113 



5 



•026 





1 20 



21 41 W. 



22 4 



•123 



5 



•115 



4 



•008 



Nov. 15 



12 



1 33 W. 



17 55 



•101 



5 



•082 



4 



•019 





12 40 



11 33 W. 



17 15 



•065 



4 



•063 



5 



•002 





1 15 



20 18 W. 



15 50 



•063 



4 



•058 



5 



■005 



21. 



12 10 



3 43 W. 



16 27 



-056 



5 



•055 



4 



•001 





12 30 



8 43 W. 



16 8 



•066 



4 



•058 



5 



■008 





12 45 



12 28 W. 



15 44 



•058 



4 



•050 



5 



•008 



