RADIATION OF THE SUN ABBOT. 



159 



The differences between the results obtained simultaneously at the 

 two stations were between 1 and 2 per cent. But considering that 

 the optical apparatus used on Mount Wilson comprised a silvered 

 glass mirror coelostat, an ultra-\4olet crown glass prism, and two sil- 

 vered glass mirrors, while that on Mount Wliitney comprised only a 

 quartz prism and two magnahum mirrors, and, furthermore, that the 

 pyrheliometers employed at the two stations were read at very differ- 

 ent temperatures, it is probable that the shght difference found 

 between the results may be due mainly to experimental differences 

 and imphes no discrepancy due to the difference of altitude between 

 the two stations. 



Tliis conclusion seems worth emphasizing. We liave now made 

 simultaneously solar-constant determinations at sea level (Wasliing- 

 ton), and at over a mile altitude (Mount Wilson) ; and again at Mount 

 Wilson, and at nearly 3 miles altitude (Mount Wliitney). Although 

 both the quantity and the quality of the solar radiation found at 

 these stations differ very much, neither the ''solar constant" nor 

 the distribution of the solar energy in the spectrum outside the 

 atmosphere, as fixed by the whoUy independent measurements at 

 these three stations, differs more than would be expected in view of 

 the unavoidable small errors of observation. We seem justified in 

 concluding that we do, in fact, eliminate the eft'ects of atmospheric 

 losses and actually determine the true quantity and quahty of the 

 sun's radiation outside the atmosphere as we might do if we could 

 observe in free space with no atmosphere at aU to hinder. 



Expeditions to Mount Wilson have now been made in 1905, 1906, 

 1908, 1909, 1910, 1911, and 1912, continuing from May until Novem- 

 ber. In the earlier years the observations were not made daily, but in 

 1908, 1909, 1910, and 1911 daily determinations of the solar constant 

 were made when possible. I give below a simimary of this work up 

 to the end of 191 1, and with it also the results obtained at Washington, 

 1902-1907. 



Other days of observation not yet ready. General mean, 1.932 calories (15° C.) per square centimeter 

 per minute. Number of determinations, 09G. 



We draw the conclusion that, for the epoch 1902 to 1911, an object 

 in outer space, at the earth's mean solar distance, would have received 

 radiation on surfaces at right angles to the solar beam at the mean 



