Soon after the birth of the child,[4] however, at the end of July, 1783, they actually set out. Mozart and some of his friends had misgivings lest the Archbishop should seek to detain him in Salzburg, because he had never received any formal dismissal from service--"for a priest is capable of anything." With this idea, he proposed a meeting in Munich, but his father appears to have reassured him.[5] Before Mozart was married, he had "made a vow in his heart" that, if he succeeded in bringing Constanze to Salzburg, he would compose a mass to be performed there. "A proof of the sincerity of this vow," he wrote to his father (January 4, 1783), "is afforded by the score of the half of {VISIT TO SALZBURG, 1783.} (315) my mass, which is laying before me in full hope of completion." He took with him to Salzburg only the Kyrie, Gloria, Sanctus, and Benedictus, composed on a scale of great splendour (427 K.). The missing movements were probably supplied from an older mass, and the whole was rehearsed at the Kapellhaus on August 23, and performed at St. Peter's church (the Archbishop having apparently refused the cathedral) on August 25, Mozart's wife taking the soprano part.[6] Mozart was not by any means idle during this visit to Salzburg. The revival of Italian opera had suggested to him to look about for a libretto for an opera buffa, and even before he came to Salzburg he had entered into negotiations with Varesco through his father. Varesco declaring himself quite ready, it only needed the visit to Salzburg to concert the plan of the opera, "L'Oca del Cairo." Varesco prepared a detailed account of the plot, and carried out the first act in full; Mozart set himself with equal zeal to its composition, and took back to Vienna a sketch of part of the act. We shall have to do later with the fate of this opera. At the same time he found leisure for a service of love to Michael Haydn. Hadyn had been ordered by the Archbishop to compose some duets for violin and tenor, perhaps for his special use, but owing to a violent illness, which incapacitated him for work during a lengthened period, he was unable to finish them at the time appointed; the Archbishop thereupon threatened to deprive him of his salary. When Mozart heard of the difficulty he at once undertook the work, and, visiting Haydn daily, wrote by his bedside to such good purpose that the duets were soon completed and handed over to the Archbishop in Hadyn's name.[7] These two duets (423, 424, K.) show no signs of hasty composition, but are worked out with evident affection, partly no doubt from desire to do credit to himself and his friend, but partly also from the interest which the difficulties of the {MOZART'S FAMILY AND FRIENDS.} (316)