88 ANNUAL REPORTS OF DEPARTMENT OF AGRICULTURE. 



The above sumTimrv also has served as the basis for determinations 

 of the intensity of direct sohir ilhunination, and of the iUumination 

 from the sun and sky, which hitter is commonly called daylip;ht in- 

 tensity. These results are also published in the Review for Novem- 

 ber, 1919. in the form of diairrams, by means of which the illumina- 

 tion of surfaces normal to the incident solar ravs, of horizontal sur- 

 faces, and of vertical surfaces variously oriented, may be determined 

 for different parts of the ITnited States east of the Eocky Mountains, 

 at any hour of any day of the year, under averaf^e clear-sky or over- 

 cast-sky conditions. 



On account of the few points at which solar radiation measure- 

 ments liave been made, the above results must be considered as a first 

 api:>roximation only. With a view to providinfi; for additional ob- 

 servin<!: stations, two new Marvin pyrheliometers were constructed 

 and standardized during the year, and a few pyrheliometric meas- 

 urements were made at points in California and Oregon during Feb- 

 ruary to April, 1920. Owing to the lack of adequate funds, how- 

 ever, it has been necessary to defer the establishment of stations on 

 the Pacific and Gulf coasts, where they are urgently needed. 



The few measurements obtained indicate that during the spring 

 months, at least, solar radiation near sea level in the Pacific Coast 

 States has about the same intensity as in the Plains States. 



Measurements of the intensity of the heat radiated from different 

 types of orchard heaters and of the retardation of nocturnal cooling 

 by a smoke cover were made during the winter of 1919-20 in orchards 

 in California and Oregon. The results, which were published in 

 the Review for June, 1920, indicate that to protect orchards from 

 frost the attempt should be made to warm the air by the total com- 

 bustion of fuel rather than to retard cooling by the production of a 

 smudge through the imperfect combustion of fuel. 



A study of the relation between the solar constant values published 

 by the Smithsonian Institution and the weather conditions in the 

 United States was well under way at the end of the year. It prom- 

 ises interesting developments in the subject of temperature control. 



