NO. 2 SMITHSONIAN EXPLORATIONS, I918 27 



rocks in one of the open pits with the ore bed (O) at the bottom, 

 above this the Early Paleozoic limestone (P), and capping- the lime- 

 stone, the red beds of Mesozoic age (M). 



FIELD-WORK OF THE SMITHSONIAN ASTROPHYSICAL 

 OBSERVATORY 



As usual, for some years past, the Astrophysical Observatory 

 maintained its observing- station on Mount Wilson and the work was 

 in the hands of Mr. L. B. Aldrich. As heretofore, the principal 

 object was to follow by accurate measurements the variations in the 

 radiation of the sun as that would be found if one were on the moon, 

 for example, outside the earth's atmosphere. The season did not 

 jirove particularly favorable for this work on account of unusual 

 cloudiness. Nevertheless, Mr. Aldrich made many solar-constant 

 observations that will be unusually valuable on account of the possi- 

 bility of comparing- them with similar observations made in South 

 America, which will be related below. 



It happened that a station of the U. S. Aviation Service was 

 located near Mt. Wilson, at Arcadia, and military balloons not 

 infrequently passed up through the layer of fog" which often covers 

 the San Gabriel \'alley, lying between Mount Wilson and the sea. 

 It occurred to Mr. Aldrich to take advantage of this condition of 

 attairs to make a measurement of the reflecting power of such a great 

 layer of fog with a view to the applicability of such measurements 

 to a consideration of the temperatvu"e of the planets Earth and \'enus, 

 both of which are to a large degree covered with clouds. We have 

 at the Astrophysical Observatory an instrument called the pyra- 

 nometer, devised by Messrs. Abbot and Aldrich for the purpose of 

 measuring the heating effect of radiation received from a whole 

 hemisphere. For example, the heat from the sun and sky combined, 

 or from the sun alone, or from the sky alone, as it falls upon a 

 horizontal surface may be determined by this instrument. 

 Mr. Aldrich's plan, therefore, was to expose the pyranometer 

 upright to the sun and sky combined, and inverted to the radia- 

 tion coming up from the layer of fog. For this purpose he needed 

 a support for the pyranometer above the fog, and such a support 

 he thought might be furnished by a military balloon. 



With the approval of General Kenley the investigation was made 

 on a favorable day in September, when the upper and lower sur- 

 faces of the fog lay respectively about 2,800 feet and 1,000 feet 

 above the ground. Two officers and 50 men being detailed to aid 



