ARNO, or AQUILA, tenth bishop and first archbishop of Salzburg, was one of those learned churchmen whom Charles the Great gathered round him, and who did so much to frame and strengthen that alliance between the Emperor and the Pope which lay at the basis of the Holy Roman empire of the West. The occasion of his introduction to the emperor was the defeat of Thassilo, duke of Bavaria, who had made war upon Charles, and was com- pelled to sue for peace. He sent at the head of the embassy Arno, who was then bishop of Salzburg, and in whose talents and fidelity he had the utmost confidence. The embassy did not succeed, mainly because Thassilo refused to make the required concessions, and Bavaria was annexed to the empire. When this took place Charles secured the services of Arno, and got for him from the Pope the archbishopric of Salzburg in 798 A.D. From this time forward Arno was frequently at the court of Charles, and became the intimate associate of Alcuin and other scholars whom the emperor delighted to gather around him. In 799 he presided at a synod of the church held at Reisbach, and in 807 at the more important synod of Salzburg. The zeal which he evinced for the conversion of the pagans of Hungary and Bohemia commended him to his ecclesiastical superiors. He established a library in Salzburg formed on the model of the emperor's palace library, and did all he could to further the interests of learning within his diocese. Assisted by a deacon named Benedict, he published a catalogue of the church lands, proprietary rights, and so on, belonging to the church in Bavaria. This is of great value to the historical student, and goes by the name of the Congestum, or Indiculus Arnonis ; an edition with notes was published by Frederick Keinz, Munich, 1869. Arno also wrote De Donis Ducum Bavariae Salzbrugensi Ecclesiae datis, which is to be found in the Thesaurus Monumentorum ecclesiasticorum et historicorum (Antwerp, 1725). (T. M. L.)