The musical picture is not constant, but panoramic; we never hear a piece of music all at once, but as a panorama of successive sounds.

Tempo refers to the rate of speed with which the scroll passes before our minds.

Thus we speak of rapid tempo (_allegro_, and the like), or slow tempo (_adagio_), and so forth.
BEATS .-- The beats are the units in our System of Measurement,--as it were, the inches upon our yardstick of time; they are the particles of time that we mark when we "count," or that the conductor marks with the "beats" of his baton.

Broadly speaking, the ordinary beat (in moderate tempo) is about equivalent to a second of time; to less or more than this, of course, in rapid or slow tempo.

Most commonly, the beat is represented in written music by the quarter-note, as in 2-4, 3-4, 4-4, 6-4 measure.