on the Northern Hemispliere at Summer Solstice. 355 



4 l J° X. lat., and the farther we get towards the north, the 

 more the diffused daylight will dominate. 



Table I. 



N. lat. 



Melville Island... 74° 47' 



Reykjavik 64° 8' 



St. Petersburg ... 59° 56' 



Manchester .">o D 20' 



Heidelburg 49° 24' 



Naples 40° 52' 



Cairo 30° 2' 



Bombay 10° 



Ceylon 10° 



Equator 0° 



w. 



W. 





Direct 



Diffused 



w+w r 



radiation. 



daylight. 





Light-units. 



Light-units. 



Light-units. 



1196 



10590 



11790 



5964 



15020 



20980 



8927 



16410 



25340 



11520 



18220 



32740 



18240 



19100 



37340 



23640 



20550 



47190 



36440 



21670 



58110 



43820 



22234 * 



66054 * 



47530 



22435 * 



69965 * 



48940 



22498 * 



71438 * 



The preference conferred upon the northern latitudes with 

 regard to their actinic illumination at the equinoxes will 

 increase with the declination of the sun, and reaches its 

 maximum value at the summer solstice. 



By means of the above-mentioned formulae of Bunsen and 

 Roscoe I have calculated the quantity of actinic light which 

 on the midsummer-day falls upon a horizontal element of 

 surface from sunrise to sunset for every tenth (or fifth) 

 degree of latitude. 



On the said day we shall have in formula (1) 



« = sin23° 27'sinp : /3=cos23° 27'cosjt>; 

 cos£ = tan23° 27'tanyy, 



and putting the constants 



0=31-99, £ = 417-6, r = 248-7, 



the integration will give 



I 



-t 



fit = 2 At + 2B sin t + C(sin t cos t + 1) 



>T |COS 2 1 sin / + 2 sin t v 2 eos 3 < sin t + 3 cos t sin /4 3 t 



+ - D 3 ' r ~ 4 



2 A 2 



