SUN'S HEAT ABBOT. 151 



The outcome of his recommendations did not lead to the establish- 

 ment of this kind of work by the Carnegie Institution itself, but did 

 lead to the most cordial cooperation on the part of their new Mount 

 Wilson Solar Observatory through its director, Dr. George E. Hale, 

 with Doctor Langley in the furtherance of his favorite investigation. 

 On Doctor Hale's invitation, an expedition from the Smithsonian 

 Institution was prepared and went forward in charge of the writer in 

 1905 to Mount Wilson, California, where temporary buildings were 

 erected and occupied each year except 1907 from 1905 to 1910, when a 

 more suitable observatory was constructed of cement blocks. In 1913 

 a tower telescope was added to the equipment of the Smithsonian 

 observatory on Mount Wilson, so that the whole now appears as in the 

 illustration, plate 4. Just above the observing station is a cot- 

 tage occupied as quarters for the observers. The situation is remark- 

 able for its boldness, standing on the edge of an almost precipitous 

 ravine which falls away almost immediately nearly a thousand feet. 

 It overlooks the valley cities of Pasadena and Los Angeles and the 

 ocean on the one side, while to the east lie the Sierra Madre Ranges 

 with the 10,000-foot mountains San Antonio, San Bernardino, San 

 Gorgonio, and San Jacinto plainly visible on clear days. 



A little nearer the summit of the mountain lies the wonderful 

 Mount Wilson Solar Observatory of the Carnegie Institution where 

 apparatus of the greatest ingenuity, power, and extreme size has been 

 accumulated year by year, culminating in 1919 in the completion of 

 the 100-inch reflector with its dome 100 feet in diameter. As one con- 

 templates this collection of splendid astronomical instruments and 

 compares them with the little telescope with which Argelander made 

 his famous " Durchmusterung " of the northern heavens, it seems as 

 if some of the dinosaurs had come to life and were disporting among 

 the little lizards which snap up the flies in the sun on Mount Wilson. 



From 1905 until 1920, with the single exception of the year 190T, 

 measurements of the solar radiation were made on Mount Wilson 

 during the summer and autumn months by the Smithsonian observers. 

 This body of observations, published in the Annals of the Smithsonian 

 Astrophysical Observatory, Volumes II, III, and forthcoming Vol- 

 ume IV, is the basis of our knowledge of the radiation of the sun, its 

 variability and its relation to our atmosphere and to terrestrial tem- 

 peratures. 



A hint of the existence of variations in the sun's radiation had been 

 obtained in 1903 at Washington. Errors associated with the work in 

 such a cloudy and smoky atmosphere as that of this eastern city are 

 so large that the result could not be at all certain. Beginning with 

 the observations on Mount Wilson in 1905, every year has added some- 

 thing to the certainty of the variation of the sun as well as to the 

 accuracy of the methods of observation and the number of special 



