When our auditory nerves receive a succession of uniform impulses at equal intervals of time, the intervals being at the same time very small, the impressions become so interblended with each other as to produce an apparently continuous sound, even and equal, except as to variation in loudness. This sound we call a musical note, produced by the string of the violin, or piano; if the intervals between the vibrations be, comparatively speaking, long, the note is grave; but if within the same given time the vibrations be very numerous, the note is in proportion acute. Some notes harmonize with each, and thereby please the mind through the ear; others are discords, and offend. Grave or acute notes depend upon the length and the tension of the strings of an instrument, or the length of a wind instrument. In the piano, the low notes are produced by long, thick strings, the high notes by slender, short wires, in which the vibrations are excessively rapid. In the violin, which has strings of equal length, the thin strings for producing the high tones are screwed to a far greater degree of tension than those of the lower tones; and each string, as its length (that is, vibratory length) is shortened by the pressure of the finger at due intervals, produces high and higher tones in proportion. We have here said enough (for we aim not at a treatise on acoustics) to show what sound really is, and what are the principal laws by which it is governed. We have also shown, that it is our mind alone that takes cognizance of the impulses given by vibratory wavelets of the atmosphere to the auditory nerves; consequently, that as the mind sees, so the mind alone hears, for vibration cannot in and of itself be that sensation which we call sound. What is the knowledge gained through the ear by the appreciation of sound, or in common language, by hearing? Let us answer:--We commonly judge of the distance and position of objects producing sound by the sense of hearing, but in these points our accuracy greatly depends upon our attention and our experience. Yet are we liable to deception; we are thoroughly deceived by a skilful ventriloquist, who can so modulate his voice as to make it seem distant or near, or to proceed from one quarter of the room or house, or from another. The voice of the corn-crake (_Crax pratensis_) is, from some cause or other, very perplexing; sometimes it will appear as if the bird were only a few yards distant from us, and the next instant in some far part of the field. Perhaps instinct leads it to alter the pitch of its monotonous cry, in order to deceive the intruder. Again, echoes deceive the ear; not only may we mistake echo for the actual voice of the speaker, but it may appear as if close or distant, faint or loud, and thus in every point lead to erroneous conclusions.