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 
when 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, who 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. 
Angstrém, 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 pyrheliometry.—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. Angstrém’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. Aldrich 
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 lay in a field about six weeks 
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 
