from rythmos, and métron, an instrument contrived to measure equal portions of sounds in musical performance, and to mark the time, at longer or shorter intervals, according to adjustment of the mechanism. Maelzel's metronome is the instrument of this kind most generally in use, although one more simple and less costly might easily be made. Diderot, in the fourth memoir of his mathematical works, suggested a musical chronometer, but added, "Le seul bon chronomètre que l'on puisse avoir, c'est un habile musicien qui ait du goût, qui ait bien lu la musique qu'il doit faire exécuter, et qui sache en battre la mesure." The chief utility of a rhythmometer consists in the power of fixing the duration of time which a composer wishes to give to the equalized portions of his composition, as indicated by the vibrations of a pendulum, and as marked by relative numerals affixed to characters of musical notation. Almost all pieces of music of classical importance are now printed with indications of the metronome measures of time; and this is a great improvement, inasmuch as before the present century we had nothing but tradition, and often no tradition at all, to guide us in the comparative slowness or quickness with which the notes of a musical composition were to be performed.