130 AjStnual reports of department of agriculture. 



tensity of solar radiation at normal incidence. Pyrheliometers of 

 this tj^pe are now in use at "Washington; one has been furnished to 

 the Chemical Warfare Service for experimental work at its arsenal, 

 Edgewood. Md., and two others are about ready to be installed at 

 the Weather Bureau observatories at the University of Chicago, 

 Chicago, 111., and in Central Park, New York. A description of this 

 pyrheliometer has been published in the Monthly Weather Review 

 for May, 1923, and in the Journal of the Optical Society of America, 

 and Review of Scientific Instruments for September, 1923. 



A convenient method has been devised for the determination of the 

 intensity of daylight that may be expected at window openings in the 

 countr}^, on city streets, or on open or closed courts. Briefly stated, 

 the method utilizes the measurements of sky brightness and daylight 

 illumination made by the Weather Bureau in recent years and deter- 

 minations by the Weather Bureau and others of the coefficient of dif- 

 fuse reflection of different materials to determine the brightness of the 

 various surfaces that are visible from a window, such as sky surface, 

 lawn or street surface, the walls of buildings, etc. A complete dem- 

 onstration of this method has been prepared for publication in an 

 early number of the Journal of the Illuminating Engineering Society. 



The Weather Bureau received 1 of the 12 Owens dust counters 

 which were distributed to observatories in 12 different countries by 

 the Bureau of Section (C), Meteorology, of the International Union 

 of Geodesy and Geophj^sics and is taking part in an international 

 investigation of the dust content of the atmosphere. Since Decem- 

 ber 7, 1922, a determination of the number of dust particles per cubic 

 centimeter has been made on each working day and the character of 

 the particles noted. Most of the determinations have been made in a 

 suburb of Washington, but on some days measurements have also 

 been made on the street near the center of the city and also at the 

 top of the Washington Monument, 500 feet above the street level. 

 Through the cooperation of the Army Air Service a few examina- 

 tions of atmospheric dust have been made during airplane flights up 

 to a height of 12,000 feet above sea level, and arrangements have been 

 made for more observations of this character. For this purpose a 

 modification of the Owens dust counter has been designed and con- 

 structed in the Weather Bureau machine shop. 



Besides city smoke and particles taken up from the surface of the 

 earth by the wind, occasionally particles are found that appear to be 

 of volcanic origin or even from some source other than the earth. 



The investigation has special reference to atmosiDheric pollution in 

 cities, and also to atmospheric transparency, which is of prime impor- 

 tance to aviators. 



In cooperation with the Chemical Warfare Service experiments 

 were conducted at the Edgewood Arsenal, Md., to determine the effi- 

 ciency of a smoke barrage, such as is employed by the Army to cover 

 the movements of troops, in protecting orchards and other forms of 

 vegetation from damage by frost. The results, which have been pre- 

 pared for publication, confirm our previous conclusions that the most 

 practical and economical method of combating frost is to heat the 

 surface layer of air by the combustion of some form of cheap fuel. 



