In spite of his desire to give up the writing of poetry and to devote himself to philosophy, Horace did not finish his career as a lyric poet with the completion of three books of odes. In 17 B. C. it was decided that the Sibylline books required the celebration of the _ludi sæculares_, which were supposed to recur at the end of every _sæculum_, or period of one hundred and ten years. An important part of the celebration was the singing of a hymn in honor of Apollo and Diana. This was to be sung by a chorus of boys and girls of pure Roman birth, both of whose parents were living, and whose mothers had married only once. Horace was asked by Augustus to compose this hymn, and could not refuse the honor, which distinguished him as the official poet laureate of the Roman Empire. [Sidenote: The Carmen Sæculare.] The hymn, called the _Carmen Sæculare_, is a somewhat formal poem, as is fitting for the solemn occasion at which it was first sung, but it shows real religious feeling, mingled with pride and confidence in the Roman greatness. It is the work of a masterly artist and an inspired poet. In addition to appointing him to write the _Carmen Sæculare_, Augustus demanded of Horace a song, or songs, in honor of his stepsons, Tiberius and Drusus. [Sidenote: The fourth book of Odes.] Horace could not refuse, and composed odes in honor of the victories of Drusus (IV, iv) and Tiberius (IV, xiv), to which he added thirteen other poems, making a fourth book of fifteen odes, written apparently in the years 17-13 B. C. The fourth book of _Odes_ is in no way inferior to its predecessors in variety of form or perfection of workmanship, and it contains a larger proportion of exalted, patriotic poems. The sixth ode, addressed to Apollo, seems to be a proœmium to the _Carmen Sæculare_, or at any rate to have some connection with the _ludi sæculares_. The fifth ode, to Augustus, urging his return to Rome, and the fifteenth, also to Augustus, on the restoration of peace, celebrate the greatness of Rome as well as its ruler. Horace, as well as Virgil, though in a different way, was a poet of the Roman Empire.