From what has been said, it will be easily conceived, that with respect to echoes, a vast variety of effects may be pro duced, by varying the form, the distance, and the number of reflecting surfaces ; and hence we hear of various surprising echoes being met with at different places.
In Woodstock Park, near Oxford, there is a famous echo, which repeats seventeen syllables in the daytime, and twenty at night, when the air is somewhat more dense. On the north side of Shipley Church, in Sussex, there is another remark able echo, which, in favourable circumstances, repeats twenty one syllables. At Posneath, near Glasgow, in Scotland, is an echo, that repeats three times, completely and distinctly, a tune played with a trumpet.
Whispering-places, are those where a whisper, or other small noise, is conveyed from one part to another, at a great distance. They depend upon this principle, that the voice, being applied to one end of an arch, easily passes by a repe tition of reflections to the other.
Hence sound is conveyed from one side of a whispering gallery to the opposite one, without being perceived by those who stand in the middle. The form of a whispering-gallery is that of a sphere, or the segment of a sphere. The principle of whispering being that of continued reflection. If a person whisper softly against a wall, the rays which proceed from his mouth issue in all directions against the wall ; we shall only take the rays which emanate from the whisperer's mouth (which we shall suppose to be a point) in a horizontal plane, then it is evident, that they will proceed to the right and to the left, and each particle of sound, as we may call it, for want of a inure specific name, will cut off equal segments of the circle which forms the section of the wall ; or, in other words, will pass along equal chords; and there will he an infinite number of such reflections; each particle describing chords different from those described by another, and an in definite number of these will divide the semi-circumference into parts all equal to each other, in the same system of chords ; therefore, all the describing particles of sound passing along the equal chords, will meet upon the other extremity of the diameter opposite the whisperer, and thus form a loud whispering noise. It is evident, that polished surfaces are the
most favourable for this purpose. Accordingly, all the contrivance requisite in whispe•ing-Idaces is, that near the person who whispers, there may be a smooth arched wall, either cylindric or cylindruidie; though a body with circular sections will do, but nut so well.
The most considerable place in England, is the whispering gallery in the dome of St. Paul's Cathedral, London, where the ticking of a watch may be heard from side to side, and a very easy whisper be sent all round the dome. The famous whispering-gallery in Gloucester Cathe dral, is no other than a gallery above the east end of the choir, leading from one side thereof to the other. It consists of live angles, and six sides, the middlemost of which is a naked window ; yet two whisperers hear each other at the distance of twenty-five yards.
ECIIOMETItY, the art of constructing vaults to produce echoes.