“We have lately considered the phenomenon of wind, as produced by the motions of the atmosphere, and I now propose to investigate another species of agitation of which the air is susceptible, a kind of vibratory or tremulous motion, which, striking on the drum of the ear, produces SOUND.” “Is it the air which produces sound!” said Louisa, with much surprise; “I thought it was always occasioned by the vibrations of solid bodies. Well do I remember, when Tom struck the finger-glass, that you immediately silenced the sound by placing your hand upon it, and which you told us stopped the vibration of the glass, and so destroyed the sound.” “You speak the truth, but not the whole truth,” replied her father. “Sound is undoubtedly the result of certain motions, or vibrations, produced in sonorous bodies, but these vibrations are communicated to the air, and from thence to the ear, in a manner which I shall presently explain.” “Do you mean to say, papa, that, if air were entirely excluded, bodies would be incapable of producing sound when struck?” “Not exactly. Air is the usual conductor of sound, and unless some other medium be substituted, the removal of it would prevent a sonorous body from communicating any sensation to the ear. Liquids, however, are capable of conveying the vibratory motion to the organ of hearing; for sound can be heard under water. Solid bodies will also convey it, and in a much more perfect and rapid manner(44); thus the slightest scratch with a pin, upon one end of a long piece of timber, will be distinctly heard on applying the ear to its opposite extremity. The tramping of a horse is to be perceived at a greater distance by listening with the ear in contact with the ground, than by attending to the sound conveyed through the air; and hence, amongst many eastern tribes, it is a common practice to ascertain the approach of an enemy, by applying the ear to the ground. Upon the same principle, if we place our ear against a long brick wall, and desire a person at a considerable distance to strike it _once_ with a hammer, it will be heard _twice_, the first sound travelling along the wall, the second through the air.” “I thank you for that hint,” said the vicar. “I now understand the principle of a new instrument which Dr. Doseall employs for examining the pulsations of the heart. He places the end of a wooden rod upon the breast, and applying the other extremity to his ear, declares that the sounds, thus conveyed to it, enable him to form the most accurate opinion in cases of diseased chest.” “In the same manner,” observed Mrs. Seymour, “that you may hear the boiling of the tea-kettle, by placing the end of the poker on the vessel, and applying your ear to the handle.” “I do not exactly understand what you mean by a _sonorous_ body. Will not every body produce a sound when struck?” asked Fanny.