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15 C H 
murs in the air, that are occasioned fay the 
discharge of great guns, &c. are a kind of in- 
definite echoes, and are produced from the 
vaporous particles suspended in the atmos- 
phere, which resist the undulations of sound, 
and reverberate them to the ear. 
There can be no echo, unless the direct 
and reflex sounds follow one another at a suf- 
ficient distance of time ; for if the reflex sound 
arrives at the ear before the impression of 
the direct sound ceases, the sound will not 
be doubled, but only rendered more intense. 
Now if we allow that nine or ten syllables 
can be pronounced in a second, in order to 
preserve the sounds articulate and distinct, 
there should be about the ninth part of a se- 
< ond between the times of their appulse to 
the ear; or, as sound flies about 1142 feet in 
a second, the said difference should be ^ of 
1142, or 127 feet ; and therefore every syllable 
will be reflected to the ear at the distance of 
about 70 feet from the reflecting body ; but 
as in the ordinary way of speaking, three or 
lour syllables only are uttered in a second, 
the speaker, that he may have the echo re- 
turned as soon as they are expressed, should 
stand about 500 feet from the reflecting body, 
and so in proportion for any other number 
of syllables. Mersenne allows for a mono- 
syllable the distance of 69 feet, Morton 90 
feet ; for a dissyllable 1 05 feet, a trisyllable 
160 feet, a tetrasyllable 182 feet, and a pen- 
tasyllable 204 feet. 
From what has been said, it follows, that 
echoes may be applied for measuring inac- 
cessible distances. Thus, Mr. Durham, 
standing upon the banks of the Thames, op- 
posite to Woolwich, observed that the echo 
of a single sound was reflected back from the 
houses in three seconds; consequently the 
sum of the direct and reflex rays must have 
been 1 142 x 3 = 3426 feet, and the half of it 
1713 feet, the breadth of the river in that 
place. 
It also follows that the echoing body beinp- 
removed farther off, it relleots more of the 
sound than when nearer, which is the reason 
why some echoes repeat but one syllable, or 
one word, and some many. Of these some 
are tonical, which only return a voice when 
modulated into some particular musical tone, 
and others polysyllabicah. That fine echo in 
Woods tock-p ark, Dr. Plot assures us, in the 
day-time will return very distinctly 17 syl- 
lables, and in the night 20. 
Echoing bodies may be so contrived and 
placed, as that reflecting the sound from one 
to the other, a multiple echo, or many echoes, 
shall arise. At Rosneath, near Glasgow, in 
Scotland, there is an echo that repeats a tune 
played with a trumpet three times completely 
and distinctly. At the sepulchre of Metclia, 
wife ol Grasses, there was an echo, which re- 
peated what a man said five times. Authors 
mentipn a tower at Cyzicus, where the echo 
repeated seven timse. There is an echo at 
Brussels that answers 15 times. 
One of the finest echoes we read of is that 
mentioned by Barthius, in his notes on Sta- 
tius’s Thebais, lib. 6, ver. 30, which repeated 
the words a man uttered 17 times. This was 
on the banks of the Naha, between Goblentz 
and Bingen. And whereas, in common echoes, 
the repetition is not heard till some time 
after hearing the words spoken, or the notes 
sung ; in this, the person who speaks or sings, 
is seal cely heard at all, but the repetition 
very clearly, and always in surprising varie- 
ties; the echo seeming sometimes to ap- 
proach nearer, and sometimes farther off- 
sometimes the voice is heard very distinctly’ 
and sometimes scarcely at all: one person 
bears only one voice, and another several ; 
one hears the echo on the right, and the other 
on the left, &c. 
Addison, and other travellers in Italv, men- 
tion an echo at Simonetta palace, near Milan, 
stfil more extraordinary, returning the sound 
06 times, 'i he echo is heard behind the 
house, which lias two wings ; the pistol is dis- 
charged from a window in one of these wings 
the sound is returned from a dead wall in the 
other wing, and heard from a window in the 
back-front. 
1’ ai ther, a multiple echo may be made, by 
so placing the echoing bodies, at unequal 
distances, as that they may reflect ali one 
way, and not one on the other; by which 
means, a manifold successive sound* will be 
heaid ; one clap of the hands like many; one 
ha! like a laughter; one single word like 
many ot the same tone and accent; and so 
one musical instrument like many of the 
same kind, imitating each other. 
Lastly, echoing bodies may be so ordered, 
that from any one sound given, they shall 
produce many echoes, different both as to 
tone and intention: by which means a mu- 
sical room may be so contrived, that not only 
one instrument playing in it shall seem many 
°r r C T same sortan fl size, but even a concert 
or different ones; this may be contrived by 
placing certain echoing bodies so, as that any 
11( ^ e played, shall be returned by them in 
3ds, 5 ths, and Sths. See Pneumatics. 
Echo is also used for the place where the 
repetition of the sound is produced or heard 
1 Ins is either natural or artificial. 
In echoes, the place where the speaker 
stands is called the centrum phonicum, and 
the object or. place that returns the voice, the 
centrum phonocampticum. 
Echo, in architecture, a term applied to 
certain kinds of vaults and arches, most com- 
monly of elliptical and parabolical figures 
used to redouble sounds, and produce artifi- 
cml echoes. We learn from Vitruvius, that 
m several parts of Greece and Italy, there 
weie brazen vessels artfully ranged under the 
seats of the theatres, to render the sound of 
the actors voices more clear, and make a kind 
ot echo. A single arch or concavity can 
scarcely ever stop and reflect the sound.; but 
it there is a convenient disposition between 
it, part of the sound that is propagated thi- 
ther, being collected and reflected as before 
will present another echo; or if there is an- 
other concavity opposed at a due distance to 
the former, the sound reflected from the one 
upon the other will be tossed back ao-ain 
upon this latter, &c. ° 
Echo, in poetry, a kind of composition 
wherein the last words or syllables of each 
verse contain some meaning, which being 
repeated apart, answers to some question or 
other matter contained in the verse, as in 
this beautiful one. from Virgil : 
ECL 
and by that means affect it with surprise and 
admiration. 
ECHOMETER, among musicians, a kind 
of scale or rule, with several lines, serving to 
measure the duration and length of sounds, 
and to find their intervals and ratios. 
ECLECTICS, antient philosophers, who, 
without attaching themselves to any parti- 
cular sect, selected whatever appeared to. 
them the best and most rational from each. 
1 otamon of Alexandria was the first of the 
CC i rr^ S; lle lly ed in the reigns of Augustus 
and 1 i bertus ; and being tired with the scep- 
ticism of the pyrrlionians, he resolved upon 
a scheme that would allow him to belies e 
something, but without being so implicit as 
to swallow any entire hypothesis. 
f^ L.L’^E, in astronomy, the deprivation 
ot the light of the sun, or of some heavenly 
body, by the interposition of another hea- 
venly body between our sight and it. bee 
Astronomy. 
Eclipse, partial, is when only a part of 
the luminary is eclipsed. And " 
Eclipse, total, is that in which the whole 
luminary is darkened. 
r w CL r P f E °f themo °n, is a privation of the 
light ot the moon, occasioned by an inter- 
position of the body of the earth directly be- 
tween the sun and moon, and so intercepting 
be sun s rays that they cannot arrive at the 
moon to illuminate her. Or, the obscura- 
tion of the moon may be considered as a sec- 
11011 °1 the earth's comical shadow, by the 
moon passing through some part of it. 
. . . le .manner of this eclipse is represented 
in this figure, where S is the sun, E the earth, 
and M or M the moon. 
Crudehs mater magis, an piier improbus ille> 
Improbus file puer, crudelis tu quoque mater. 
I he elegance of an echo consists in givim- 
a new sense to the last words, which rever- 
berate, as it were, the motions of the mind. 
Lunar eclipses only happen at the time of 
ml moon, because it is only then the earth is 
between the sun and moon; nor do they 
happen every full moon, because of the ob- 
liquity of the moon’s path with respect to the 
sun s, but only in such full moons as happen 
either at the intersection of those two paths, 
called the moon’s nodes, or very near them 
viz when the moon’s latitude, or distance 
between the centres of the earth and moon, 
is less than the sum of the apparent semidia- 
mete is ot the moon and the earth’s shadow 
1 he chief circumstances in lunar eclipses 
are the following: 1. Ail lunar eclipses are 
universal, or visible in a!] parts of the earth 
winch have the moon above their horizon; 
and are every where of the same magnitude, 
with the same beginning and end. 2. In fill 
lunar eclipses, the eastern side is what first 
lmmerges and emerges again, i. e. the left- 
hand side of the moon as we look towards 
her, from the north; for the proper motion 
of the moon being swifter than that of the 
eartn s snadow, the moon approaches it from 
the west, overtakes and passes through it 
with the moon’s east side foremost, leaving 
the shadow behind, or to the westward. 
3 lotal eclipses, and those of the longest 
duration, happen in the very nodes of "the 
eclipse ; because the section of the earth’s 
shadow, then falling on the moon, is con- 
