Probably no composer has revised his manuscripts more carefully, and re-written whole pieces with the view of improving them, than has J. S. Bach. His forty-eight Preludes and Fugues, entitled 'Das wohltemperirte Clavier,' afford instructive examples of improvements, which may be traced by a comparison of the several editions of the work, and especially by an examination of the several manuscripts of these preludes and fugues in Bach's handwriting which have been preserved. The prelude in C major, in the first set, was originally longer than in subsequent revisions. The second half, which Bach has struck out, was a repetition of its first half. The prelude in C[#] major, in the first set, he has curtailed by striking out thirty-five bars. This he did evidently for the purpose of increasing the unity of this charming composition by discarding what was foreign to its character, as indicated by the theme. On the other hand, the beautiful prelude in D minor, in the same set, he has considerably enlarged. These few remarks must suffice to draw the reader's attention to the careful reconsideration given by Bach to 'Das wohltemperirte Clavier.' Beethoven generally kept his manuscripts a long time by him, and altered and polished them up gradually. This he did especially with the manuscripts of his earlier compositions. Gluck, in composing an opera, carried out in his mind the principal airs and choruses before he wrote down a note; so that, when he began to commit the music to paper, he considered his opera as almost finished. Mozart, too, had sometimes a whole new composition in his head before he commenced writing it down. The overture to 'Don Giovanni' he is recorded, by some of his biographers, to have composed a few hours before the first performance of the opera, so that the copied parts for the musicians were not yet dry when they were carried into the orchestra. Probably Mozart did not compose the overture when he committed it to paper, but had it ready in his head. He was often composing when otherwise occupied, and even while he was playing billiards.