2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 7I 



STATION AND APPARATUS 



The station was located at an elevation of 4,800 feet above sea 

 level, on the eastern slope of Hump Mountain. The very heavy 

 winds prevailing at certain seasons made it impossible to select a 

 site near the summit, which rises to an altitude of about 5,500 feet. 

 The latitude was 36° 08' N. and longitude 82° o' W. 



Most of the sky radiation observations were made with the instru- 

 ment resting on a level platform erected on the roof of the obser- 

 vatory, in order to eliminate the building as an obstruction. Being 

 located below the summit of the mountain, a certain small per- 

 centage of the sky was cut ofif, but this was accurately measured by 

 means of a theodolite and found to be 5.1 per cent of the total hemi- 

 sphere. The mountain cut ofif an angle of 13.0° measured vertically 

 at its highest point. 



The galvanometer, ammeter, resistances and battery were located 

 in the room beneath, the first two mentioned being mounted on a 

 pier made of large boxes filled with stones. 



A brief description of the pyranometer and the auxiliary apparatus 

 may be of interest here. The pyranometer consists essentially of 

 two blackened strips of manganin of exactly the same horizontal 

 dimensions, but one strip about ten times as thick as the other. 

 These strips, which are exposed to an entire hemisphere of radia- 

 tion, are insulated and carefully soldered to copper blocks at each 

 end. Very sensitive tellurium-platinum thermo-elements are joined 

 in series with each other and are set across and underneath the two 

 strips and separated from them by thin waxed paper. The thermo- 

 element leads pass to the galvanometer, but for convenience in 

 obtaining suitable deflections, three different resistance combinations 

 are included in series. The manganin strips attached to the copper 

 bars are connected in parallel, but with proper resistances included, 

 so that the heating current applied divides in exactly the proper 

 proportion to cause the same heating efifect in both strips. When 

 the shutter is opened to admit radiation, since the blackened strips 

 have the same horizontal dimensions, the same quantity of heat is 

 imparted to each, but the thicker strip conducts its heat to the ends 

 the more readily and so a difiference in temperature at the thermo- 

 elements produces an electric current which deflects the galva- 

 nometer. When the shutter is closed a current is sent through the 

 strips, so as to cause about an equal deflection of the galvanometer 

 thus indicating a heating effect equal to that produced by the radia- 

 tion. This current is measured in a carefully calibrated milli-am- 



