NO. 6 SMITHSONIAN EXPLORATIONS, 1914 95 



distribution is variable from day to day. This variability appears 

 to be closely correlated with the variation of the total radiation of 

 the sun revealed by the solar constant investigations. It is confi- 

 dently hoped that further study of these two interesting phenomena 

 will throw light on the nature of the sun's radiating envelope. 



Sounding balloon work at Omaha. — In order to more thoroughly 

 confirm our determinations of the solar constant of radiation, 

 measurements were undertaken in connection with the U. S. Weather 

 Bureau at Omaha. Sounding balloons were sent up early in July, 

 1914, equipped with recording pyrheliometers (fig. 89). The work 

 was in the charge of Mr. L. B. Aldrich, on the part of the Smith- 

 sonian Institution, and of Dr. William R. Blair, on the part of the 

 Weather Bureau. Three instruments were sent up and all were 

 recovered. One of these was sent by night as a check on the accu- 

 racy of the work, and the other two by day, with the hope of 

 measuring the intensity of the sun's radiation at enormous altitudes. 

 The pyrheliometer was suspended by means of wire 22 meters below 

 three balloons each 1.25 meters in diameter, weighing with the 

 apparatus about 23 pounds. An altitude of 15 miles was reached 

 on July 11 when, as expected, two of the balloons burst by expan- 

 sion and the third balloon brought the pyrheliometer down in safety 

 near Carson, Iowa. 



One of the instruments made a very fine record of solar radiation 

 and fortunately was recovered entirely uninjured, and it has been 

 repeatedly tested and standardized at Washington. The tests are 

 not yet completely finished, but they indicate that three excellent 

 determinations of the solar radiation were made at heights so great 

 that the pressure of the air was extremely small, certainly much 

 less than one-twentieth of that which prevails at sea-level. The 

 results, when reduced to mean solar distance and corrected for all 

 known sources of error, come between 1.8 and 1.9 calories per sq. cm. 

 per minute, with a probable error of about 3 per cent. This result 

 is in close accord with the values of the solar constant of radiation 

 secured by spectrobolometric measurements in former years on Mt. 

 Wilson, Mt. Whitney, Bassour, Algeria, and at Washington. 



