Later in life, the major impetus to fugal writing for Mozart was the influence of Baron Gottfried van Swieten in Vienna around 1782. Van Swieten, during diplomatic service in Berlin, had taken the opportunity to collect as many manuscripts by Bach and Handel as he could, and he invited Mozart to study his collection and encouraged him to transcribe various works for other combinations of instruments. Mozart was evidently fascinated by these works and wrote a set of transcriptions for string trio of fugues from Bach's "Well-Tempered Clavier", introducing them with preludes of his own. In a letter to his sister Nannerl Mozart, dated in Vienna on 20 April 1782, Mozart recognizes that he had not written anything in this form, but moved by his wife's interest he composed one piece, which is sent with the letter. He begs her not to let anybody see the fugue and manifests the hope to write five more and then present them to Baron van Swieten. Regarding the piece, he said "I have taken particular care to write "andante maestoso" upon it, so that it should not be played fast – for if a fugue is not played slowly the ear cannot clearly distinguish the new subject as it is introduced and the effect is missed". Mozart then set to writing fugues on his own, mimicking the Baroque style. These included the fugues for String Quartet, K. 405 (1782) and a Fugue in C Minor K. 426 for two pianos (1783). Later, Mozart incorporated fugal writing into his opera "Die Zauberflöte" and the finale of his Symphony No. 41.