158 



ANNUAL KEPOET SMITHSONIAN INSTITUTION, 1912. 



figure 4, which, may be standardized against the water-flow instru- 

 ment and in which the measurement is made by reading a thermom- 

 eter at stated intervals. 



In measuring the intensity of solar radiation as it would be outside 

 the earth's atmosphere at mean solar distance (generally called the 

 solar constant of radiation) it is important to select a station 

 where variability of atmospheric conditions is slight, and where 

 the quantity of air traversed by the solar rays is small. 

 Owing to smoke and clouds, Washington is a poor locality 

 for the purpose, and so in 1905 an expedition in my 

 charge was sent by invitation of Director Hale, of the 

 Mount Wilson Solar Observatory, to take station 

 on Mount Wilson. Plate 3, figure 1, shows some 

 of the gigantic apparatus erected by Director 

 Hale at that fine site. With this remarkable 

 outfit the work done by the Mount Wilson 

 Solar Observatory staff has been wonder- 

 fully productive. The Smithsonian Ob- 

 servatory on Mount Wilson, a Httle 

 affair comparatively, is shown in 

 plate 3, figure 2. It was built in 

 1908 on a small plot of ground 

 leased from the Solar Ob- 

 servatory. A cottage has 

 since been built close by for 

 observers' quarters. 



In order to test more 

 thoroughly whether we can 

 indeed truly estimate the 

 losses of solar rays in our 

 atmosphere, work was done 

 in 1909 and 1910 under my 

 charge at Mount Whitney 

 (4,420 meters), the highest 

 mountain in the United 

 States. To further 

 the work of this and 

 other scientific expe- 

 ditions the Institu- 

 tionerectedonMount 

 Whitney in 1909 the stone and steel shelter shown in plate 4, figure 1. 

 My apparatus is shown in plate 4, figure 2. With Mr. Marsh, of Lone 

 Pine, I remained two weeks on the summit in 1909 and again in 1910, 

 and made measurements of the solar constant of radiation there, while 

 my colleagues made similar measurements at Mount Wilson. 



Fig. 4.— The silveh-disk pyeheliosieteb. 



