"Dies Iræ, Dies Illa."

Will any of your correspondents oblige me by answering the following Queries. Who was the author of the extremely beautiful hymn, commencing— "Dies iræ, dies illa, Solvet sœclum in favilla Teste David cum Sibylla." And in what book was it first printed? A copy of it is contained in a small tract in our library, entitled *Lyrica Sacra, excerpta ex Hymnis Ecclesiæ Antiquis. Privatim excusa Romæ*, 1818. At the end of the preface is subscribed "T. M. Anglus." And on the title page in MS., "For the Rev. Dr. Milner, Dean of Carlisle, Master of Queen's College, in the University of Cambridge, from T.…