162 ANNUAL, REPORTS OF DEPARTMENT OF AGRICULTURE. 



observing stations. One of these, Madison, Wis., has been in opera- 

 tion since July 22, 1910, and others will be equipped as soon as appa- 

 ratus already ordered is received. Pyrheliometric obsei-vations have 

 been maintained throughout the year at Washington, D. C, and were 

 resumed at Mount Weather in May. 



The observations at the western stations showed radiation intensi- 

 ties in excess of the five-year averages for Washington, the excess 

 ranging from 4 per cent in August, at Lincoln, Nebr., to 22 per cent 

 in September, at Flagstaff, Ariz. 



The most striking features of the year have been the high value of 

 the radiation in February and March on the front of marled high 

 barometric areas, and the low value during the protracted hot wave 

 in May. At Madison, on February 23, and again on March 4, the 

 radiation intensity with the sun shining through an air mass 1.5 

 (zenith distance of the sun 48°) was 1.67 calories per square centi- 

 meter per minute, which is as high as any measurement obtained by 

 the Smithsonian Institution on Mount Wilson during the summers of 

 1905 and 1906. At Washington the corresponding maximum inten- 

 sity during this period was 1.47 calories, or 12 per cent less than at 

 Madison. 



During the hot wave of May, 1911, the maximum intensity of solar 

 radiation measured at Mount Weather, with the sun at zenith distance 

 of 48°, was 1.20 calories per square centimeter per minute, and the 

 average was little over 1.00 calorie. 



Measurements of the polarization of skylight, as well as othei' con- 

 siderations, indicate that during protracted hot periods a very con- 

 siderable percentage of the heat reaching the lower layers of the 

 atmosphere is received diffusely from the sky. A Callendar recording 

 pyrheliometer, capable of measuring the heat thus received, has been 

 m continuous operation at Washington throughout the year; but 

 quantitative resvilts can not be given until this instrument has been 

 carefully compared with a Marvin pyrheliometer, which will be done 

 as soon as a new Callendar instrument provided with an improved 

 form of recorder is received. 



In response to the request of certain European investigators, a 

 series of special observations on the positions of the neutral points of 

 Arago and Babinet was made by Prof. Kimball while on field duty. 

 These are now being continued at Mount Weather in connection with 

 the measurements of the percentage of polarization of skylight, made 

 as in previous years. 



The five-year averages of solar radiation intensities for Washington 

 were published in the Bulletin of the Mount Weather Observatory, 

 Volume III, part 2. In response to a special request, a resume of 

 that part of the above paper which treats of sky polarization, together 

 with a summary of the polarization observations obtained while on 

 field duty, was prepared by Prof. Kimball for pubhcation in the 

 Journal of the Franklin Institute for April, 1911. 



The constants to equation 20, Bulletin of the Mount Weather Obser- 

 vatory, Volume I, part 4, are being recomputed from data recently 

 furnished by the Smithsonian Institution. Prof. Humphrey's recent 

 computation of the distribution of aqueous vapor in the atmosphere 

 when the sky is cloudless will also be utilized. New tables for facili- 

 tating solar constant computations will be prepared from this revised 



