He explained it as follows:--
“M shows the situation of the mouth of the speaker, and E that of the ear of the hearer. Now, since sound radiates in all directions, a part of it will proceed directly from M to E, while other rays of it will proceed from M to u, and from M to z, &c.; but the ray that impinges upon u will be reflected to E, while that which first touches z will be reflected to y, and from thence to E; and so of all intermediate rays, which are omitted in the figure, to avoid confusion. It is evident, therefore, that the sound at E will be much stronger than if it had proceeded immediately from M without the assistance of the dome; for, in that case, the rays at z and u would have proceeded in straight lines, and consequently could never have arrived at the point E.”
“I have understood that a similar effect may be witnessed in the stone recesses on Westminster-bridge,” said the vicar.
“That is the fact,” replied Mr. Seymour. “The recesses to which you allude are semi-domes; and if a person whispers in the focus of one of them, he will be distinctly heard by another stationed in the focus of the opposite one. Two inanimate busts may be thus made to appear as if holding a conversation, by placing them in the foci of two large concave mirrors constructed of pasteboard, and arranged opposite to each other; when a whisper uttered to the one will seem to proceed from the other by the reflection of sound.”
Mr. Seymour now removed a shell from a group of corallines which decorated a part of the temple, and desired Tom to place it to his ear.
“I hear a noise which appears to me to resemble that of the sea,” cried Tom.
“Ay,” said the vicar, “and there is a popular belief that it is the murmur of the sea, which the shell actually sends forth, betraying, as it were, its marine origin.”
“And what produces the sound?” inquired Louisa.
“The interior of the shell merely concentrates, and thus magnifies the sounds around us, so as to render them audible: a goblet applied to the ear will be found to produce the same effect,” replied her father.
“I suppose it is upon the same principle that the speaking-trumpet is made to convey sound to a distance,” remarked Louisa.