NO. 4 SOLAR RADIATION — ABBOT, FOWLE, AND ALDRICH 33 



nary character. The results, such as they are, indicate radiation 

 values not exceeding 1.8 calories per cm. 2 per min. 



Early in the year 1914, we began to rebuild the instruments, which 

 had been injured in their flights. On February 18, the preparations 

 having been considerably advanced, Mr. Abbot wrote the following 

 letter to Mr. Very, which is self-explanatory : 



February 18, 1914. 

 Dear Mr. Very: 



As you know, we are interested in the value of the solar constant of 

 radiation. We know that you are also. In our view this quantity lies between 

 1.9 and 2.0 calories per sq. cm. per min. In yours it lies between 3.0 and 4.0 

 calories or possibly higher. All measurements made by us rest on the 

 " Smithsonian Revised Pyrheliometry of 1913." They are 3.5 per cent higher 

 than they would be on Angstrom's scale, as shown by numerous comparisons 

 made in America and Europe. In the interests of ascertaining the truth, 

 which I know to be your sole object, as it is ours, will you be so good as to 

 answer these questions : 



1. Do you consider the " Smithsonian Revised Pyrheliometry of 1913 " as 

 satisfactorily furnishing the standard scale of radiation? 



2. If not, why not? 



3. If in error, is it too high or too low, and how much? 



I assume that you are not likely to think its results as much as 5 per cent 

 too low, and that the discrepancy between your ideas of the solar constant 

 and ours lies mainly outside of our conclusions as to the realization of the 

 standard scale of radiation. In this posture of affairs, I propose to try the 

 following experiments, which I hope will be crucial : 



By cooperation with the United States Weather Bureau we propose to send 

 up with balloons five automatic-registering pyrheliometers in June or July 

 next. In preliminary experiments last summer the balloons generally reached 

 20 to 30 kilometers altitude, and in one case 33 kilometers. Mr. Blair expects 

 personally to attend to the balloons this year, and hopes to get them all above 

 30 kilometers, and some even to 40 kilometers. [This hope was disappointed, 

 probably because the balloons used in 1914 were a year old.] These elevations 

 are of course derived from barograph records, and it is not the elevation we 

 care about, but the pressure of atmosphere above. This is given directly by 

 the barographs, which will be calibrated, at the temperatures expected, by Mr. 

 Blair. [Calibrations were finally made at the Smithsonian Institution.] We 

 may expect the pressure reached will be less than 1 per cent of that at sea level. 

 It is designed to make the pressure record on the same drum as the pyrheli- 

 ometer record, so that there can be no error by differences of running of 

 independent clocks. 



I now come to a second group of questions. 



4. Do you think that the intensity of the solar radiation in free space at the 

 earth's solar distance is materially higher than that at a station within the 

 atmosphere of the earth, where the barometric pressure is less than 1 per cent 

 of that which prevails at sea level? 



5. If so, how much and why ? 



