EEPORT OF THE SECRETARY. 83 



of the sun be, which is absorbed at once on the ^^■alls of the cliamber instead 

 of indirectly by air convection. 



The instrument is mounted equatorially and driven by clockworlv, and the 

 rate of flow of tlie water current, lapse of time and temperature change 

 caused by radiation, are all recorded photographically on a moving drum. 

 Hence a continuous automatic record of the solar radiation is. produced. In 

 use at Mount Wilson it is not easy to avoid considerable temperature change 

 of the apparatus, and this causes a slight " drift " of the record, but not 

 enough to prevent runs of several hours' duration without attention. 



A considerable part of the apparatus for the continuous pyrheliometer was 

 made by the International Instrument Company, of Cambridge, Mass. The 

 more refined parts for the receipt and measurement of the radiation were 

 constructed by Mr. Kramer at the Observatory shop. In this, and in a great 

 amount of other work during the year, Mr. Kramer has combined skill with 

 rare willingness and interest in a way which deserves commendation. 



(3) THE EXPEDITION TO MOUNT WILSON, IN CALIFOENIA. 



From the very beginning, in 1902, of experiments in Washington on the meas- 

 urement of the solar constant of radiation it has been your aim to continue 

 these measurements at a more favorable situation as regards cloudiness, and 

 particularly as regards elevation. You long ago showed that in optical quality 

 the lower air is far inferior to that lying above, and you have repeatedly stated 

 your conviction that exact determinations of the absorption of the atmosphere 

 are impossible at stations near sea level. Congress having approved of your 

 plan to conduct these observations at high altitudes, apparatus has been col- 

 lected during the past three years for an expedition to a favorable station for 

 the measurement of the solar constant. 



In the meantime the Carnegie Institution had been founded, and by invita- 

 tion of one of its officers you stated, in a communication to the Hon. C. D. 

 Walcott, dated February 28, 1902, your belief that the establishment by the 

 Carnegie Institution of an observatory to be situated at some high and cloud- 

 less point and engaged for at least a complete sun-spot cycle in the accurate 

 determination of the solar constant of radiation would be a worthy astronom- 

 ical undertaking and one most likely, in your judgment, to yield results of value 

 both from the standpoint of pure science and from that of practical utility 

 to mankind. 



In 1904 the Carnegie Institution began the establishment of a solar observa- 

 tory on Mount Wilson, in California, under the direction of Prof. G. E. Hale, 

 iind the objects of this observatory include the measurement of solar radiation, 

 which you had urged. Almost immediately after the installation at Mount 

 Wilson had been begun. Professor Hale, recognizing the large share you have 

 had from the first in promoting the establishment of such an observatory, and 

 knowing of the preparations making at the Smithsonian Astrophysieal Observa- 

 tory for an expedition to a high station to observe the solar constant, invited 

 you to send this expedition to Mount Wilson, and promised all possible coopera- 

 tion during its stay, and suggested that the work would, if desirable, be taken 

 up by the new solar observatory when the Smithsonian expedition should be 

 withdrawn. This invitation was accepted. The equipment, consisting of a full 

 spectro-bolographic and" pyrheliometric outfit, the equal and in some respects 

 the superior of that installed in Washington, was sent forward in April, 1905. 

 The observers, C. G. Abbot in charge, and L. R. Ingersoll, temporary assistant, 

 reached the ground about May 10. Two shelters for the spectro-bolometer and 

 the continuous pyrheliometer, respectively, already framed in the valley below, 



