84 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1918. 



will be made below. The results on liquid water show it to be com- 

 pletely opaque in layers of 1 centimeter or more thickness to all rays 

 of the above described 100° C. radiation. The reflecting power of 

 water surfaces to these rays varies with the angle of incidence as fol- 

 lows: 



Incidence 0° 30° 55° 63° 70° 72° 



Reflection o/o 2 3 7 10 17 22 



From this it follows that though perfectly opaque in layers ex- 

 ceeding 1 cm. thickness, water is not a perfect absorber or a per- 

 fect radiator for long-wave rays. It may be regarded as emitting 

 about 90 per cent as much radiation as the perfect radiator at 

 temperatures from 0° to 100° C. 



Lampblack paint proved partially transparent and partially re- 

 flecting. Further experiments are required before publishing defi- 

 nite results, but evidently those who employ lampblacked surfaces 

 in experiments with long-wave rays should consider these imperfec- 

 tions of radiating and absorbing power. 



An investigation was made on possible regularities of periodicity 

 in the short-interval variability of the sun 1 observed at our Mount 

 Wilson station. Dr. H. H. Clayton had made such an investigation 

 for the year 1913 and found indistinct tendencies toward a repetition 

 of solar constant conditions after intervals of 12 and 22 days. Com- 

 putations were made here to extend the investigation to the other 

 years from 1908 to 1916, excepting 1912. 



Well marked relatively hot and cold hemispheres of the sun seem 

 to have prevailed for several months in 1915, giving a " solar con- 

 stant" periodicity of about 27 days. In 1916 an extraordinary 

 regular periodicity of 3| days seemed to be indicated. In other 

 years tendencies to periodicities of other intervals were found, and 

 generally more marked than in 1913, but not as prominently seen 

 as in 1915 and 1916. On the whole the irregularity of period of the 

 fluctuations of solar radiation would seem to be the most outstanding 

 result of the inquiry. 



In accordance with the wish expressed by Secretary Walcott, the 

 facilities of the observatory have been employed whenever possible 

 to assist in military investigations. This is not the time to detail 

 the results of this effort further than to say that a large part of the 

 work of the director and of Mr. Aldrich has been devoted to several 

 such investigations, and with highly appreciated results. Naturally 

 this has diminished the astrophysical output of the observatory. 



Chilean expedition. — Preparations and arrangements for a South 

 American solar-radiation expedition occupied much of the time of 

 the director and that of the instrument maker. As stated in last 



iOn Periodicity in Solar Variation. Smith. Misc.. Coll., vol. 69, No. 6, 1918. 



