92 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1914. 



very light electric battery for this purpose. At length a modification 

 of the Roberts cell was designed, in which individual cells weighing 

 20 grams (^ ounce) would furnish a constant potential of 1.3 volts 

 and yield a nearly constant current of about 0.5 ampere for nearly 

 two hours. The internal resistance of the cells was only about 0.3 

 ohms. Barometric elements were made to record on the same drum 

 that recorded radiation. One instrument was constructed to be sent 

 up at night, so as to show if any unexpected phenomena occurred 

 Avhen the instruments were being raised, apart from those due to the 

 sun. Many tests of the instruments were made at different tempera- 

 tures and pressures, and while immersed in descending air currents 

 comparable to those anticipated to attend the flights. The accom- 

 panying illustration shows one of the balloon pyrheliometers as 

 reconstructed. 



Silver-disk pyrheliometers. — As in former years, a number of 

 silver-disk pyrheliometers were standardized at the Observatory and 

 sent out by the Institution to several foreign Government observa- 

 tories. 



IN THE FIELD. 

 MOUNT WILSON EXPEDITION OF 1913. 



Mr. Aldrich went to Mount Wilson early in July, 1913, and car- 

 ried on there solar constant measurements until September when he 

 was joined and then relieved by Mr. Abbot, Avho continued the obser- 

 vations until November. An expedition at the charge of the private 

 funds of the Smithsonian, and under the direction of Mr. A. K. 

 Angstrom, was in California during July and August for the pur- 

 pose of measuring nocturnal radiation at different altitudes, ranging 

 from below sea level to the summit of Mount Whitney, 4,420 meters 

 (14,502 feet). Mr. Aldrich cooperated as far as possible with this 

 expedition. 



Balloon fyrTielioTnetry . — At the same time a cooperating expedi- 

 tion from the United States Weather Bureau made ascents of captive 

 and free balloons in order to determine the temperature, pressure, and 

 humidity at great elevations, for use in reducing Mr. Angstrom's 

 observations. Advantage was taken of the opportunity to send up 

 special pyrheliometers for measuring solar radiation at great alti- 

 tudes. These experiments, which were made jointly by Mr. Aldri \ 

 and Mr. Sherry of the Weather Bureau, were referred to by anticipa- 

 tion in last year's report. Five balloon pyrheliometers were sent up 

 from Santa Catalina Island. All were recovered, with readable 

 records. One instrument unfortunately la}'^ in a field about six week, 

 before recovery, and parts of its record referring to the higher 

 elevations were obliterated, but it yielded the best results of any up 

 to about 8,000 meters. Two of the instruments unfortunately were 



