SUN'S HEAT ABBOT. 159 



altitude of the sun, and thereby the mass of air traversed by its 

 beam, instead of to determine this by time observations as had 

 always been the case at Mount Wilson. Nevertheless, the work of 

 determining the " solar constant " on the same day as the observation 

 was extremely arduous and tedious to the two observers. Their en- 

 thusiasm was naturally extremely aroused by the favorable reports 

 which were found in the conference of Messrs. Moore and Abbot with 

 Messrs. Wiggin and Clayton in Argentina. 



On their return to Calama the writer and Mr. Moore discussed the 

 possibility of determining the "solar constant" by a short method, 

 and after a considerable computation and trial such a method was ob- 

 tained. It depends upon the fact that the transparency of the sky 

 is closely related to its brightness. It is easy to see that when the 

 sky is hazy the transparency will be diminished and the brightness 

 near the sun greatly increased. The amount of haze depends upon 

 the humidity in the air and also upon the amount of dry dust which 

 has been carried up by the wind, or by volcanic eruptions, or other- 

 wise. It was possible to effect a combination of the measurements of 

 the brightness around the sun by the pyranometer, and the humidity 

 of the air determined by Fowle's spectroscopic method by a single 

 holograph, so as to obtain a function which could give the coefficient 

 of transparency with a high degree of accuracy. This short method 

 was thereupon introduced at Calama and proved in extensive prac- 

 tice to be highly satisfactory. It is possible thus to obtain the 

 "solar constant" several times in each day, where one observation 

 before had been all that was usual. This new method is continually 

 checked against the older and fundamental one, and up to the present 

 time has shown very satisfactory and complete agreement, except 

 that the new method with its several observations is regarded to be 

 the more accurate of the two. 



TRANSFER OF MOUNT WILSON AND CALAMA STATIONS TO BETTER SITES. 



Early in the year 1920 the writer had an extended conference 

 verbally and by correspondence with Professor Marvin, Chief of 

 the United States Weather Bureau, as to the applicability of solar 

 radiation measurements for forecasting purposes in general, and in 

 the United States in particular. Mr. Marvin felt that the experi- 

 mental basis so far available from the results at Mount Wilson 

 and at Calama was not adequate to warrant much investigation of 

 this question. Feeling strongly the justice of this view, the writer 

 urged upon Congress at the hearing of the Smithsonian Institution 

 before the Appropriations Committee in February, 1920, that a suit- 

 able appropriation should be made to erect on an isolated mountain 

 in the most cloudless region of the United States a special observing 



