NO. lO SMITHSONIAN EXPLORATIONS, I923 23 



a groundwork for further studies of the relation of the variation of 

 the sun to the variation of the weather. As is well known, solar ob- 

 servations of this kind require the highest degree of cloudlessness and 

 uniformity of sky. After many inquiries, it was decided to occupy 

 a station near the city of Calama. on the edge of the Nitrate Desert 

 of Chile. This station was first set up in July, 1918, and continued 

 until July, 1920, when, by the advice and financial assistance of 

 Mr. John A. Roebling, it was removed to the top of Alt. Montezuma, 

 about ten miles south of the former location and high above the dust 

 and smoke which had hnulered to some extent the observations near 

 Calama. 



At the same time, also, by Mr. Roebling's assistance, the apparatus 

 which had hitherto been used on Mt. Wilson, Cal., was transferred 

 to the top of Mt. Harqua Hala, Ariz., selected after a long meteoro- 

 logical investigation conducted through the kindness of the Director 

 of the United States Weather Bureau. This station was first occupied 

 in October, 1920, and both stations have reported continuously from 

 their establishment until the present time. 



The method of solar observation invented by Langley and developed 

 by the Astrophysical Observatory requires a continuous imiform 

 transparency of the sky for several hours, either in the early morning 

 or the late afternoon. It also requires about twenty-five hours of 

 measurement and computing for each day of observation. In 1919, a 

 brief empirical method, based upon this longer and fundamental 

 method, was devised and applied first at Calama and later at Harqua 

 Hala and Alontezuma. In 1922, a still further abbreviation of the 

 methods of computing was devised and was introduced at both stations 

 in the spring of 1923. According to this newest method, the required 

 observations for determination of the intensity of the sun's heat as it 

 is outside the atmosphere can be secured in less than fifteen minutes, 

 and the results can be computed in less than an hour, so that it is now 

 possible and usual to make daily five independent determinations at 

 each station of the intensity of the solar heat as it is outside the atmos- 

 phere, reduce these observations by one or two o'clock in the after- 

 noon, and, again by Air. Roebling's financial assistance, communicate 

 them by telegraph, from the stations at Harqua Hala and Alontezuma, 

 to the Smithsonian Institution at Washington where they are received 

 early on the following morning. If it were essential, the matter might 

 be still further accelerated, so that telegraphic reports from these 

 distant observing stations could be had on the afternoon of the same 

 dav of the observation. 



