NO. I OBSERVING THE SUN AT 19,300 FEET — BUTLER 3 



north of the Tropic of Capricorn. We all wore heavy clothing, stock- 

 ing caps, mittens, and heavy shoes. Despite the cold, one burned 

 severely in a few hours without the protection of a sunshade. The 

 natives are ordinarily a light brown color, but the men who work the 

 mine were burned almost black. The data show that the amount of 



lO 20 30 40 50 60 70 SO 90 

 CM H6. 



Fig. I. — Pyrheliometry at high altitudes. 



solar radiation here is greater than ever experienced at sea level, 

 but the only way this could be felt was in a feverish feeling about the 

 nose, cheeks, and neck. 



At air-mass 1.40 the pyrheliometers gave a mean value of 1.663 

 calories per square centimeter per minute. A reduction of this to what 

 it would be at vertical sun and at the earth's mean distance from the 

 sun becomes 1.753 and shows that the value lies nearly in line with 

 the plot in figure 58 in the Annals of the Astrophysical Observatory, 

 volume 4. 



