“Good, again! The quick apprehension of these my little playmates,” said Mr. Twaddleton, as he turned towards Miss Villers, “is highly interesting; their minds, from well-regulated discipline, have acquired the faculty, if I may be allowed the use of the metaphor, of winnowing a subject, so as completely to separate the grain from the chaff.”
“It is my intention to proceed this morning with the consideration of those toys which have the property of producing sound,” said Mr. Seymour.
“I suppose you mean the whistle, whiz-gig, and humming-top,” observed Tom.
“Your papa, no doubt, alludes to those instruments,” said the vicar, “and I greatly approve of the arrangement; since our last lecture embraced the operations of the atmosphere, a subject with which the nature of sound is certainly intimately connected.”
“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.”