DEC. 19, 1921 proceedings: philosophicae society 513 



PROCEEDINGS OF THE ACADEMY AND AFFILIATED 



SOCIETIES 

 PHILOSOPHICAI. SOCIETY 



854th meeting 



The 854th meeting of the Philosophical Society of Washington was held 

 in the Cosmos Club auditorium on October 22, 1921. It was called to order 

 at 8:15 p.m. by President Faris with 45 persons present. 



The first paper of the evening, on Sky brightness and daylight illumination 

 measurements, was presented by Mr. H. H. Kimball, and was illustrated. 

 It was discussed by Mr. Humphreys. 



The brightness of the sky has been measured almost daily at the American 

 University, Washington, D. C, between April 5 and July 14, inclusive, and 

 at Chicago, Illinois, between July 19 and August 15, inclusive, 1921. 



The illumination from sunlight and skylight combined, and from sky- 

 light alone, was measured on a horizontal surface, and also on a surface 

 normal to the incident solar rays; and at Washington, measurements were 

 obtained of the skylight illumination on vertical surfaces facing 0°, 45° 

 90°, 135°, and 180° in azimuth from the sun. 



About half the Chicago measurements were made on top of the dome of 

 the Federal Building, in the Loop district, one of the smokiest sections of 

 the city. The remainder were made at the University of Chicago, which 

 in summer is comparatively free from smoke when the wind blows from the 

 lake. Southeast and southwest winds, however, bring considerable smoke 

 from South Chicago and the Union Stockyards, respectively. 



There is little smoke in the atmosphere at the American University, D. C. 



A comparison of the Washington and Chicago measurements shows that 

 towards the sun on cloudless days the sky brightness does not differ mater- 

 ially at the two places, but opposite the sun the horizon in Chicago is dark- 

 ened by smoke, especially in the Loop district. 



With a cloudless sky the direct solar illumination at Chicago is notice- 

 ably weaker than at Washington. In the Loop district, with the sun not 

 more than 40° above the horizon, it averages only 60 per cent as intense. 

 The illumination on a vertical surface facing 180° in azimuth from the sun, 

 computed from the sky brightness measurements, averages only about 

 two-thirds as intense as the illumination computed from similar measure- 

 ments for Washington. 



A method is given of combining computations of illumination on hor- 

 izontal and vertical surfaces from sky brightness measurements with deter- 

 minations of the shading effect of buildings or other objects. This makes 

 it possible to compute for average sky conditions of the various types the 

 illumination that results from exposure to any given portion of the sky 

 at any hour of the day or season of the year. 



The second paper, on The mathematical equations for heat conduction in 

 the fins of air-cooled engines, by S. R. Parsons and D. R. Harper, 3d, was 

 presented by Mr. Harper. It was discussed by Mr. Hawkesworth. 



The communication included a very brief historical sketch and a discussion 

 of the assumptions, both physical and geometrical, which must be necessarily 

 made to bring the problem within possibility of solution. Then followed 



