“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.
“Those bodies are called sonorous, which produce clear, distinct, regular, and durable sounds, such as a bell, a drum, musical strings, wind instruments, and so on.”
“And upon what does this peculiar property depend?” enquired Tom.
“Before I answer that question, I must explain the supposed nature of those vibrations of the air, upon which sound depends; you will then readily perceive why one species of matter should be better calculated than another for exciting them. It is generally believed that sound is conveyed through air by a succession of pulsations similar to those which are occasioned on the surface of smooth water by throwing a pebble into it. This at first produces a small circular wave round the spot in which the stone falls; the wave spreads, and gradually communicates its motion to the adjacent waters, producing similar waves to a considerable extent. The same kind of waves are produced in the air by the motion of a sonorous body, which will of course be in the centre, and the waves or pulsations will diminish in strength as they recede from that centre, until at last they become too weak to produce any effect on the ear.”
“When I strike a bell, then do I produce exactly the same motion in the air, that I do in the water by throwing a stone into it?” asked Louisa.
“With this difference,” replied her father, “that as air is an elastic fluid, the motion does not consist of regularly extending waves, but of vibrations, which are composed of a motion forwards and backwards; the undulations of the air differ also from those of the water, in not being confined to a plane, but in diverging in all directions from the centre; or, in other words, the aërial undulations are spherical.”
“It is a very puzzling subject,” cried Tom.
“I cannot understand,” said Louisa, “how the motion of the air can extend so as to convey sound to a distance, if, as papa says, the air moves backwards as well as forwards.”
“I see your difficulty, and will endeavour to remove it; attend to me. The first set of undulations which are produced immediately around the sonorous body, by pressing against the contiguous air, condense it. The condensed air, though impelled forward by the pressure, re-acts on the first set of undulations, driving them back again. The second set which have been put in action, in their turn, communicate their motion, and are themselves driven back by reaction. Thus there is a succession of waves in the air, corresponding with the succession of waves in the water.”