30 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. -/\ 



reached the ground. The extent of the work is limited, owing to 

 the pressure of other matters. 



On September 7 the pyranometer, with the auxiliary apparatus, 

 was taken down the mountain side north of the observatory into a 

 grove of beech and black birch. Observations were taken at five 

 spots : 



(a) In the shade of two large trees, no undergrowth within several 

 feet; 



(&) Under some rather thick saplings, partly shaded by higher 

 trees, thin undergrowth ; 



(c) In a bed of ferns which covered the instrument, but with an 

 open space directly above ; 



(c?) In open space; trees all around; ferns and other vegetation 

 somewhat above instrument toward south ; 



(^) Thick growth of saplings, considerable small vegetation on 

 ground. 



The sky, on this day, was cloudy and very variable. Observations 

 had been taken on the whole sky and sun two hours before the in- 

 strument was moved to the grove, but after that the sky became 

 nearly overcast. At the close, observations on the whole sky were 

 taken, but there was so much variation that the readings were of 

 little value. 



A few days later, Septeniiber 12, the observations were repeated, 

 with a cloudless sky. With better conditions the readings showed 

 far less variation among themselves than on the preceding days. 



On October 2 a third set of observations was taken. The instru- 

 ment was placed in the same five positions as before. This time the 

 leaves were beginning to fall, and many that remained on the trees 

 had turned yellow. The sky was cloudless and very clear. 



Readings were taken mostly in groups of four or five. Below are 

 the times for the middle of each group, the average calories for the 

 group, an estimation of the total sky and sun radiation, and the 

 ratio of the observed radiation under the trees to the estimated radia- 

 tion above them. The estimated whole sky radiation is based, for 

 September 7, on the observations taken on the tower, following those 

 in the grove ; for September 12, on solar observations by the pyrhelio- 

 meter and total sky observations taken before the pyranometer was 

 moved to the grove ; for October 2, on pyrheliometer measurements 

 taken at dififerent air masses during the afternoon and total sky obser- 

 vations taken in the morning with the pyrheliometer, the sum of the 

 two values at corresponding sun altitudes being used. 



