2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 89 



Geographic Society. Later a friend of the Institution furnished the 

 funds for its operation. Unfortunately, the Mount Brukkaros site did 

 not prove as good as had been anticipated, owing to the cloudiness, 

 haziness, and high winds prevailing there at certain seasons. The 

 elevation was considerably less than the other two stations, being 

 only about 5,200 feet above sea-level. 



Through the generosity of the same friend of the Institution, it was 

 planned to send an expedition to Southwest Africa early in 193 1 to 

 investigate several widely separated mountain peaks in that country, 

 all of them considerably higher than Mount Brukkaros, the idea being, 

 if possible, to get above the haze that was causing so much trouble 

 there. The writer was chosen to head this expedition and it was 

 arranged that Mrs. Moore should accompany him. 



From past experience it had been learned that visual observations 

 are not sufficient for selecting a site of a solar-radiation station, and 

 also that a few days' visit to a place, even by an experienced observer, 

 would not suffice to judge its qualifications. Hence it was planned to 

 equip the expedition with various portable instruments to measure 

 the meteorological conditions prevailing on the peaks under investi- 

 gation, and it was also arranged for the expedition to visit the sites 

 at different seasons. 



The prime meteorological requisites of a good observing location 

 are: i, that it have skies which, for a large percentage of the days 

 throughout the year, are free of clouds, haze, dust, smoke, etc., and 

 with little wind ; 2, that the sky conditions remain nearly constant, 

 especially from sunrise until an hour or two before noon ; 3, that the 

 site be at an elevation of at least 7,000 feet above sea-level in order 

 to rise above the low-lying haze, dust, and smoke. 



A brief description will now be given of the various instruments 

 carried and their purpose. In order to get an accurate measure of 

 the uniformity of the transparency of the atmosphere over a peak, the 

 expedition was equipped with an Angstrom pyrheliometer for the 

 purpose of measuring the radiation of the sun at the earth's surface. 

 This instrument consists of two blackened strips of manganin foil, 

 each provided with an electric thermocouple in contact with the lower 

 surface of the strip. It is also arranged that an electric heating cur- 

 rent can be passed through each strip at will. The thermocouples are 

 connected so that the electromotive forces generated when the strips 

 are heated oppose one another. The remaining terminals are connected 

 to a fairly sensitive moving-coil galvanometer. The strips are alter- 

 nately exposed to the radiation. A measured current is passed through 

 the other strip such that its temperature is raised to exactly that of the 



