This is to the eye what such a hollow sound is to the ear.”[27] [24] Description of Volcanoes, p. 170. [25] _Edinburgh Journal of Science_, New Series, No. i., p. 124. [26] Considerations on Volcanoes, and _Edinburgh Journal of Science_, No. xx., p. 261, and No. xiv., p. 265. [27] Art. SOUND, _Encycl. Metrop._, § 110. It has been recently shown by M. Savart, that the human ear is so extremely sensible as to be capable of appreciating sounds which arise from about _twenty-four thousand_ vibrations in a second, and consequently that it can hear a sound which lasts only the twenty-four thousandth part of a second. Vibrations of such frequency afford only a shrill squeak or chirp; and Dr. Wollaston has shown that there are many individuals with their sense of hearing entire, who are altogether insensible to such acute sounds, though others are painfully affected by them. Nothing, as Sir John Herschell remarks, can be more surprising than to see two persons, neither of them deaf, the one complaining of the penetrating shrillness of a sound, while the other maintains there is no sound at all. Dr. Wollaston has also shown that this is true also of very grave sounds; so that the hearing or not hearing of musical notes at both extremities of the scale seems to depend wholly on the pitch or frequency of vibration constituting the note, and not upon the intensity or loudness of the noise. This affection of the ear sometimes appears in cases of common deafness, where a shrill tone of voice, such as that of women and children, is often better heard than the loud and deeper tone of men.