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principle. By experience it is known that about nine syllables can be articulately and distinctly pronounced in a second of time. But sound travels with the velocity of 1142 feet in a second, therefore in the ninth part of a second it passes over 12, or 127 feet nearly, and consequently the reflected sound, which is the echo, to be distinct, must travel over at least 127 feet more than the direct.
Emma. If cd in the figure represent the garden wall, how far must I be from it to hear distinctly any word I utter? will 63 or 64 feet be sufficient, so that the whole space which the sound has to travel be equal in this case also to 127 feet? Father. It must be something more than this, because the first sound
rests a certain time on the ear, which should vanish before the echo returns, or it will appear a continuation of the former, and not a distinct sound: it is generally supposed that the distance must not be less then 70 or 72 feet; and this will give the distinct echo of one syllable only.
Charles. Must the distance be increased in proportion to the number of syllables that are to be repeated?
Father. Certainly; and at the distance of about 1000 or 1200 feet, 8 or 10 syllables, properly pronounced, will be distinctly repeated by the echo.
I will finish this subject to
Of the Echo.
FATHER. The following are among the most celebrated echoes. At Rosneath, near Glasgow, there is an echo that repeats a tune played with a trumpet three times completely and distinctly. Near Rome there was one that repeated what a person said five times. At Brussels there is an echo that answers 15 times. Thornbury Castle, Gloucestershire, an echo repeats 10 or 11 times very distinctly. Between Coblentz and Bingen an echo is celebrated as different
from most others. In common echoes the repetition is not heard till some time after hearing the words spoken or notes sung; in this the person who speaks or sings is scarcely heard, but the repetition is perceived very clearly, and in surprising varieties: the echo in some cases appears to be approaching, in others receding: sometimes it is heard distinctly, at others scarcely at all: one person hears only one voice, while another hears several. And, to mention but one more instance, in Italy, near Milan, the sound of a pistol is returned 56 times. Emma. This is indeed
To fetch shrill echoes from their hollow earth.
Father. The ingenious Mr. Derham applied the echo to meausring inaccessible distances,
Charles. How did he do this?
Father. Standing on the banks of the Thames, opposite Woolwich, he observed that the echo of a single sound was reflected from the houses in three seconds, consequently in that time it had travelled 3426 feet, the half of which, or 1713 feet, was the breadth of the river in that particular place.
Did you ever hear of the Whispering-Gallery in the dome of St. Paul's Church?
Emma: Yes: and you promised to take us to see it some time.
Father. And I will perform my promise. In the mean time it may be proper to inform you, that the circumstance that attracts every person's attention is, that the smallest whisper made against the wall on