152 REPORTS OP INVESTIGATIONS AND PR0JE;CTS. 



to cast the disk was to be made early in July. It was decided to extend the 

 time of annealing over a very long period, so as to reduce the danger of 

 internal strain arising from too rapid cooling. In view of the special diffi- 

 culties of the case, it is probable that repeated trials will be necessary before 

 a satisfactory disk can be obtained. 



Mr. Ritchey has commenced work on a preliminary design for the mount- 

 ing of the 100-inch reflector in order that general estimates covering the 

 cost of this work can be prepared. Details of the design, however, can not 

 be completely worked out until the mounting of the 60-inch mirror has been 

 tested in practice. It is hoped that suitable tests of this nature can be made 

 in the erecting-house at Pasadena during the coming winter, by direct stellar 

 photography. 



RESEARCHES OF THE SMITHSONIAN EXPEDITION ON MT. WILSON. 



By C. G. Abbot. 



By invitation of the Director of the Solar Observatory of the Carnegie 

 Institution of Washington, a small expedition from the Smithsonian Astro- 

 physical Observatory was located on Mount Wilson from May to October, 

 inclusive, in 1905 and 1906, for the purpose of making measurements of the 

 energy of the solar radiation. The principal investigations made were : 



( 1 ) The determination of the value of the "solar constant of radiation," or 

 in other words, the intensity of the solar radiation in calories per square 

 centimeter per minute, outside the atmosphere and at the earth's mean dis- 

 tance from the sun. 



(2) The relative intensity of rays of the different wave-lengths in the 

 solar spectrum outside the atmosphere, and the probable temperature of the 

 sun as connected therewith. 



(3) The transmission of the earth's atmosphere for rays of different wave- 

 lengths. 



(4) The quantity and quality of the light scattered by the sky. 



(5) The reflecting power of clouds for all angles of incidence and reflec- 

 tion, and from this the determination of the amount of solar radiation v.hich 

 would be lost to the earth if the atmosphere were completely cloudy. 



A full account of the expeditions and results is included in volume ii of 

 the Annals of the Astrophysical Observatory of the Smithsonian Institution, 

 now in press. A brief summary is as follows : 



In 1905 two small observing shelters were erected and equipped with piers 

 and apparatus, the first of these containing the spectrobolographic appara- 

 tus required for observing the transmission of the atmosphere for nearly 

 monochromatic rays, the second containing a new form of standard pyrhelio- 

 meter adapted to register automatically and continuously the total intensity 

 of the sun's radiation at the earth's surface. Observations were begun with 

 secondary pyrheliometers in May, 1905, and complete series of observations 

 for determining the "solar constant of radiation" were made about twice or 

 thrice each week from June 5 to October 29, 1905. In 1906, "solar con- 

 stant" work was begun May 10, and continued until October 22. In all, 130 

 determinations of the "solar constant" were secured, all included between the 

 limits 1.93 and 2.14 calories \^zx square centimeter per minute, and giving a 



