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Photo by George C. Martin 

 ASH IN FOREST NEAR HEAD OF WOMAn'S BAY, NEAR UZINKI, AUGUST I, I912 



From this it would be possible to de- 

 termine how much the intensity of the 

 rays would have been increased had the 

 observation been made outside of the at- 

 mosphere altogether — as if it could have 

 been made, for instance, upon the moon. 

 From this result one could determine 

 how much the rays of each part of the 

 spectrum were diminished in intensity by 

 their passage through the atmosphere on 

 their way to the surface of the earth. 



AN INSTRUMENT THAT MEASURES ONE- 



MIEEIONTH PART OE A 



DEGREE OE HEAT 



No ordinary thermometer would be of 

 any value for this purpose ; but the bo- 

 lometer invented by Langley about 1881 

 is an electrical thermometer so sensitive 

 that a change of temperature of i one- 

 millionth part of a degree is observable 

 with it under ordinary conditions. 



We were equipped with such an ap- 

 paratus at Bassour, and Mr. Fowle had 

 one similar on Mount Wilson, and with 

 these, following the scheme of operations 

 which I have indicated above, we meas- 

 ured for all rays of the solar spectrum 

 the transparency of the atmosphere. 

 Similar measurements have been made 

 at Mount Wilson for many years, and 

 were made in Algeria in the year 191 1. 



The following table shows the decrease 

 in the transparency of the atmosphere, 

 first for the beam of the sun as a whole, 

 and then for the rays of different re- 

 gions of the solar spectrum : 



Percentage Decrease of Direct Solar Radiation 

 by Hase of 19 12 



Computed for Solar Zenith Distance 48° 



20 PER CENT OE SUN S HEAT LOST IN 1912 



From these results we see that the un- 

 common haziness of the sky during the 



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