62 CARNEGIE INSTITUTION 



The Heat Radiation of the Sun. 



The advantages of studying the heat radiation of the sun at high 

 altitudes are well illustrated by the results obtained by Langley 

 from the summit of Mount Whitney in 1881. The dryness and 

 purity of the air at this elevation in the Sierra Nevada are perhaps 

 unsurpassed at any other mountain station. Over one half of the 

 atmosphere lies below the summit of the peak, and this comprises 

 the denser and more variable strata, which interfere most with 

 determinations of solar radiation. No sooner had Langley' s deli- 

 cate apparatus been set at work than a new class of solar rays 

 previously unknown was discovered. Previous estimates of the 

 absorptive effect of the lower regions of the atmosphere were found 

 to be far below the truth, and the value of the solar constant, which 

 measures the amount of heat received by the earth from the sun, 

 was immediately increased by about one half. The new radiations 

 which were found to possess so great importance cannot pass through 

 glass, and prisms of rock salt must be employed in studying them. 



The Mount Whitney observations extended over only a few 

 weeks, and were made with apparatus which, from his present point 

 of view, Professor Langley would consider extremely imperfect. 

 In spite of the unfavorable conditions that prevail at Washington, 

 Langley has continued his investigations at the Smithsonian Insti- 

 tution and developed his methods to an extraordinary degree of 

 perfection. Slow and inaccurate observations with insensitive in- 

 struments have given place to automatic records of the highest 

 precision, secured by the aid of photography with instruments so 

 sensitive that differences of temperature of less than one ten-mil- 

 lionth of a degree centigrade can be detected. 



The problems which should be attacked from the summit of Mount 

 Whitney, or some equally good station, with the refined instruments 

 now available, cover a wide range. The most important single ques- 

 tion concerns the intensity of the solar radiation. Is this constant, 

 or does it vary during that well defined period of about eleven years 

 in which solar phenomena are known to pass from a state of com- 

 parative calm to one of violent activity, and then again to subside to 

 their former condition? At times of sun spot minimum the sun's 

 surface for months together is wholly devoid of spots. Faculse and 

 prominences are few and inconspicuous, and the spectroscope shows 

 little, if any, evidence of disturbances of any kind. Gradually, 

 however, spots begin to appear, and then rapidly multiply in number. 



