45 



of luminous intensity with the height of the sun at these places being 

 given. 



From these data it is possible also to calculate the action produced 

 by the whole diffuse light, not only for each minute, but during any 

 given space of time. For the following places the amount of che- 

 mical illumination expressed in degrees of light which falls from sun- 

 rise to sunset on the vernal equinox, is 



Melville Island , 10590 



Reykiavik 15020 



St. Petersburg 1 6410 



Manchester 18220 



Heidelberg 19100 



Naples 20550 



Cairo 21670 



Experiment has shown that clouds exert the most powerful influ- 

 ence in reflecting the chemical rays ; when the sky is partially 

 covered by light white clouds, the chemical illumination is more than 

 four times as intense as when the sky is clear. Dark clouds and 

 mists, on the other hand, absorb almost all the chemically active 

 rays. 



The chemical action of the direct sunlight was determined by 

 allowing a known fractional portion of the solar rays to fall perpen- 

 dicularly on the insolation vessel of the chemical photometer. The 

 solar rays reflected from the mirror of a Silbermann's heliostat 

 were passed through a fine opening of known area into the dark 

 room, and a large number of reductions and corrections had to be 

 made in order to obtain, from the direct observations, the action, 

 expressed in degrees of light, which the sun shining directly upon 

 the apparatus would have produced if no disturbing influences had 

 interfered. This action of direct sunshine was determined on three 

 different cloudless days for various altitudes of the sun. As the sun 

 approached the zenith the observed action rapidly increased ; thus 

 at 7 h 9' A.M., on September 15, 1858, when the sun's zenith 

 distance was 76 30', the reduced action amounted to 5 '5 degrees of 

 light, whilst at 9 h 14' A.M. on the same day, the zenith distance 

 being 58 11', the action reached 67*6. This increase in the sun's 

 illuminating power is owing to the diminution in length of the 



