xo. 8 SMITHSONIAN EXPLORATIONS, 1913 87 
All the balloons carrying pyrheliometers were fortunately re- 
covered, and in one instance the flight reached the altitude of about 
33.000 meters, or 108,000 feet. The registering pyrheliometers be- 
haved very well with the exception that their temperature sunk lower 
than was expected, so that in each case the mercury in the stem of the 
Fic. 86.—Observing station of Astrophysical Observatory 
on Mount Wilson with new tower telescope. Photograph by 
Abbott. 
thermometers was frozen at an altitude of from 40 to 50 thousand 
feet, and therefore their records did not extend as high as the flights 
of the balloons. Nevertheless these measurements are obtained at 
altitudes above the highest clouds, and where the water-vapor and 
dust of the atmosphere is almost inappreciable. The results reached 
do not differ from what would be expected in view of the value of 
