46 



column of air through which the rays pass. If we suppose the 

 atmosphere to be throughout of the density corresponding to a 

 pressure 0'76 and a temperature 0, and consider it as a horizontal 

 layer, and if A represent the action effected before entrance into the 

 atmosphere, the action, when the ray has passed through a thickness 

 of atmosphere = I, is represented by 



W =A10~ a/ > 

 where - signifies the depth of atmosphere through which the ray 



has to pass to be reduced to J^th of its original intensity, and where 

 I is dependent on the atmosphere's perpendicular height =A, and 

 the sun's zenith distance 0. The numerical values of A a and I may 

 be calculated from the direct observations, and hence the action W a 

 effected at any other zenith distance t , and under a pressure P^ is 

 found from the equation 



-<*h PI 



W 1= A10 C08 ^ p o, 



where P represents the atmospheric pressure under which A and a 

 are calculated. A comparison between the actions W x thus obtained, 

 and those, W , found by experiment, shows as close an agreement as 

 could be expected where the observational errors are necessarily so 

 large. 



From these experiments it is seen, that if the sun's rays were not 

 weakened by passage through the atmosphere, they would produce 

 an illumination represented by 318 degrees of light; or they would 

 effect a combination in one minute on a surface on which they fell 

 perpendicularly, of a column of hydrochloric acid 35 '3 metres in 

 height, assuming that the rays are extinguished by passing through 

 an infinitely extended atmosphere of chlorine and hydrogen. By 

 help of the above formula, it is also found that the sun's rays, after 

 they have passed in a perpendicular direction through the atmo- 

 sphere to the sea's level, under a mean pressure of 0*76 metre, only 

 effect an action of 14*4 light-metres, or that under these conditions 

 nearly two-thirds of their chemical activity have been lost by extinc- 

 tion and dispersion in the atmosphere. The total chemical action 

 emanating from the sun during each minute is therefore represented 

 by a column of hydrochloric acid 35 metres in height, and having 

 an area equal to the surface of a sphere whose diameter is the mean 



