The pyrheliometric and circumsolar sky radiation measurements of the Smithsonian Astrophysical Observatory (APO) are examined from 1923 to 1954. The measurements at Mt Montezuma, Chile, in the Southern Hemisphere and Table Mountain, California, in the Northern Hemisphere are examined in most detail. It is found that Paluweh (8° 19′S, 121°42′W), which erupted in 1928, had a dust veil index of 100 using Lamb's (1970) definition of the dust veil index. Quizapu (35°39′S, 70°46′W), which erupted in 1932, has a dust veil index of 35. Both of these volcanoes are climatologically significant. Between April 1932 and 1950 there are no detectable volcanic eruptions in either hemisphere. Local pollution at both stations after 1950 degrades the observations at these sites. There was a change of 0.6–0.7 % in the radiation scale at Mt Montezuma about 1940. No evidence for an 11- or 22-year cycle is found in the pyrheliometer data and any variation in the solar constant on this time scale seems to be a fraction of a tenth of a percent. The pyrheliometer and pyranometer serve complementary purposes and confirm the reality of changes in the atmospheric dust loading.