When the Philharmonic joined together with the National Symphony in 1921, they had the Dutch conductor Willem Mengelberg. He stayed there for nine years, although other conductors, including Bruno Walter, Wilhelm Furtwängler, Igor Stravinsky, and Arturo Toscanini, led about half the concerts. They started to have an outdoor symphony series, playing low-priced summer concerts at Lewisohn Stadium in upper Manhattan. In 1920 the orchestra hired Henry Hadley as "associate conductor" to perform concerts which included works by American composers.