44 



taincd are sufficient to enable them to determine the relation accord- 

 ing to which the chemical energy proceeding from the sun is diffused 

 over the earth when the sky is unclouded. 



From a series of observations made on June 6, 1858, the relation 

 between the amount of light optically measured falling from the 

 whole sky, and the amount (taken as unity) which, at the same 

 time, falls from a portion of zenith sky equal to } fa O th of the whole 

 visible heavens, has been calculated for every degree of sun's zenith 

 distance from 20 to 90; the results being tabulated, and also 

 represented graphically. These numbers, multiplied by the che- 

 mical light proceeding from the same portion of zenith sky for the 

 same zenith distances, must give the chemical action effected by the 

 whole diffuse daylight. The amount of chemical light which falls 

 from the zenith portion of sky is, however, the chemical brightness 

 of that portion of sky. This chemical brightness has been deter- 

 mined, by the chlorine and hydrogen photometer, on various days, 

 and at different hours, when the sky was perfectly cloudless. A 

 table contains the chemical action, expressed in degrees of light, 

 which is effected on the earth's surface by .a portion of zenith sky 

 equal in area to 10 1 QQ th of the whole visible heavens, under the cor- 

 responding sun's zenith distances from 20 to 90. A curve repre- 

 senting the relation between the action and the height of the sun, 

 shows that although the single observations were made on different 

 years and at different times of the year and day, they all agree 

 closely amongst themselves, and hence another proof is gained of the 

 slight effect which variation in the air's transparency produces ; and 

 it is seen that the total chemical action effected by the diffuse light 

 of day may be represented as a function of the sun's zenith 

 distance. 



The numbers thus obtained have only to be multiplied by the 

 corresponding numbers of the former table, in order to give the 

 chemical action effected by the total diffuse light of day for zenith 

 distances from 20 to 90. A table and graphic representation of 

 these numbers is given. Knowing the relation between the sun's 

 altitude and the chemical action, the chemical illumination effected 

 each minute at any given locality at a given time may be calculated ; 

 this calculation has been made for a number of places for each hour 

 on the vernal equinox, tables and curves representing the alteration 



