XO. 2 SMITHSONIAN EXPLORATIONS, I918 2.'] 



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 ^lesozoic age ( M ) . 



FIELD-WORK OF THE SAIITHSONL\N 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 

 prove particularly favorable for this work on account of unusual 

 cloudiness. Xevertheless. 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 

 infrequentl}- passed up through the layer of fog which often covers 

 the San Gabriel A'alley, lying between [Mount \\ ilson and the sea. 

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

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

 layer of fog with a view to the a])plicaljility of such measurements 

 to a consideration of the temperature of the ]3lanets Earth and A'enus. 

 both of which are to a large degree covered with clouds. \A'c have 

 at the Astrojjhysical Observatory an instrument called the pyra- 

 nometer. dexiscd Ijy Messrs. Al)l)ot and Aldrich for the purpose of 

 measuring llie heating effect of radiati(jn rcceiveil trom a whole 

 hemisjjhere. \'ox exami)le, the heat from the sun and sky comljined, 

 or from the sun alone, or from tlie sky alone, as it falls u])on a 

 horizontal surface may l)e determined 1)\- this instiiumiit. 

 .Mr. .Mdrich's plan, therefore, was to expose the ]i\ranometer 

 upright to the sim anrl sky combined, 'a\m\ inverted to the radia- 

 tion coming up from the la\er ol fog. I'or this ])iir])ose he needed 

 a support for the i)_\ranometer aboxr the fog, .and sticli ;i sii]iiiorl 

 he thought might he furnished by .a military h.alloon. 



With the aj)])ro\-al ot ( icneral Kfnlc\- the iiuesi ig.alion \\;i< made 

 on a favorable day in Se])tcmber, when the u])pL'r .ind Idwer sur- 

 faces of the fog lay respectively .ahnul j.S(m) feet .md i,(K)() feet 

 above the ground. I \\(; officers and 50 men hfing dei.iiled to .aid 



