NO. 9 SOLAR ECLIPSE I918 ALDRICH 3 



Our location being settled, necessary equipment was obtained from 

 Lakin by auto truck, and with the help of a local carpenter, the barn 

 was altered to suit our needs, piers erected, and the instruments 

 mounted as rapidly as possible. 



From the time we arrived on Monday morning, up to an hour 

 before the eclipse began on Saturday afternoon, the weather was 

 very discouraging. Almost continuous cloudiness existed during this 

 period. Thursday and Friday were completely overcast, with much 

 lightning and thunder. We were unable to obtain any focus plates 

 or to correct the rate of the driving mechanism of the photographic 

 apparatus. Saturday morning, the day of the eclipse, the sky was 

 more densely overcast than ever and the success of the expedition 

 looked hopeless. At noon a few rifts appeared and by an hour 

 before the eclipse began, the cumulus clouds had practically dis- 

 appeared, leaving the sky covered with streaks of thin cirrus cloud. 

 This condition continued the remainder of the day. While the sky 

 was not ideal for our work, it enabled us to carry out the complete 

 program with success. 



APPARATUS 



For the radiation work, Pyranometer A. P. O. No. 5 ' was used. 

 To meet more adequately the eclipse requirements, it was partially 

 rebuilt, as follows: A new thermopile, consisting of four tellurium- 

 platinum thermo-elements, was inserted beneath the blackened man- 

 ganin strip. (Pyranometer A. P. O. No. 5 is a single-strip type of 

 instrument.) This made it more sensitive than any pyranometer yet 

 constructed, and made easily measurable a radiation absorbed by 

 the strip as small as .0005 calory (per square centimeter per minute). 

 To avoid a small galvanometer drift found in measuring radiation 

 from the sky alone, the sun shade ' was increased in size so as to 

 shade from direct sun rays the whole copper disk surrounding the 

 absorbing strip. It was also raised to a distance of about 35 centi- 

 meters from the strip so as net to intercept too large a sky area. 



1 See Smithsonian Misc. Coll., Vol. 66, No. 7, p. 7. 



" With the small sun shade, this galvanometer drift occurs in the single-strip 

 type of instrument because, since the cold junctions of the thermopile are 

 buried in the copper disk, part of which is exposed to direct solar radiation 

 during the sky-alone measurements, the temperature of these junctions tends 

 to increase. In the two-strip form of instrument this difficulty is obviated 

 by placing the cold junctions beneath the second absorbing strip. The angular 

 radius for the large shade was 7 34', the small strip shade, 3 10', and for the 

 sun, 0° 16'. The corresponding solid angles subtended were, for the large 

 shade .0547, small shade .00962, and sun .000068. 



