90 A C O U 
the number of pulfes, by increafing the points of new 
propagation. 3. The reflections of the pulfes from the 
tremulous fides of the tube, which impel the particles of 
air forward, and thus increafe their velocity. 
An echo is a reflection of found ftriking again ft fome 
object, as an image is reflected in a glafs : but it has been 
difputed what are the proper qualities in a body for thus 
reflecting founds. It is in general known, that caverns, 
grottoes, mountains, and ruined buildings, return this 
image of found. We have heard of a very extraordinary 
echo, at a ruined fortrefs near l.ouvain, in Flanders. If 
a perfon fang, he only heard his own voice, without any 
repetition : on tire contrary, thofe who flood at fome dif- 
tance, heard the echo but not the voice; but then they 
heard it with furprifing variations, fometimes louder, 
fometimes foftcr, now more near, then more diflant. 
There is an account in the Memoires of the French Aca¬ 
demy, of a fimilar echo near Rouen. 
As it is plain, that every point againft which the pulfes 
of found flrike becomes the centre of a new feries of pulfes, 
and found defcribes equal diftances in equal times, there¬ 
fore, when any found is propagated from a centre, and its 
pulfes flrike againft a variety of obftacles, if the fum of 
the right lines drawn from that point to each of tire obfta¬ 
cles, and from each obftacle to a fecond point, be equal, 
then will the latter be a point in which an echo will be 
heard. All the points of the obftacles which produce an 
echo, muft he in the furface of the oblong fphasroid, ge¬ 
nerated by the revolution of this ellipfe round its major 
axis ; but, as there maybe feveral fphaeroids of different 
magnitudes, fo there may be feveral different echoes of 
the fame original found. And, as there may happen to be 
a greater number of reflecting points in the furface of an 
exterior fphaeroid than in that of an interior, a fecond ora 
third echo may be much more powerful than the firft, 
provided that the fuperior number of reflecting points, 
that is, the fuperior number of reflected pulfes propagated 
to the ear, be more than fufficient to compenfate for the 
decay of found which arifes from its being propagated 
through a greater fpace. This is finely illuftrated in the 
celebrated echoes at the lake of Killarney in Kerry, where 
the firft return of the found is much inferior in ltrength to 
thofe w hich immediately fucceed it. 
From what has been laid down it appears, that for the 
moft powerful echo, the founding body fhould be in one 
focus of the ellipfe which is the feCtion of the echoing 
fphseroid, and the hearer in the other. However, an 
echo may be heard in other fituations, though not fo fa¬ 
vourably ; as fuch a number of reflected pulfes may arrive 
at the fame time at the ear as may be fufficient to excite a 
diftinCt perception. Thus a perfon often hears the echo 
of his own voice; but for this purpofe he fhould ftand at 
leaft fixty-three or lixty-four feet from the reflecting obfta¬ 
cle, according to what has been faid before. At the 
common rate of fpeaking, we pronounce not above three 
fyllables and an half, that is, feven half-fyllables in a fe¬ 
cond; therefore, that the echo may return juft as foon as 
three fyllables are exprefted, twice the diftance of the 
fpeaker from the reflecting objeCt muft be equal to 1000 
feet; for, as found defcribes 1142 feet in a fecond, £ths of 
that fpace, that is, 1000 feet nearly, will be delcribed 
while fix half or three whole fyllables are pronounced: 
that is, the fpeaker muft ftand near 300 feet from the ob¬ 
ftacle. And in general, the diftance of the fpeaker from 
the echoing furface, for any number of fyllables, muft be 
equal to the feventh part of the product of 1142 feet mul¬ 
tiplied by that number. In churches we never hear a 
diftinCt echo of the voice, but a confuted found when the 
fpeaker utters his words too rapidly; becaufe the greateft 
difference of diftance between the direCt and reflected 
courfes of fuch a number of pulfes as would produce a 
diftinCt found, is never in any church equal to 127 feet, the 
limit of echoes. 
But though the firft reflected pulfes may produce no 
S T I C S. 
echo, both on account of their being too few in number, 
and too rapid in their return to the ear; yet it is evident, 
that the reflecting furface may be fo formed, as that the 
pulfes which come to the ear after two reflections or more 
may, after having defcribed 127 feet or more, arrive at the 
ear in fufficient numbers, and alfo fo nearly at the fame 
inftant, as to produce an echo, though the diftance of the 
reflecting furface from the ear be lefs than the limit of 
echoes. This is confirmed by a Angular echo in a grotto 
on the banks of the little brook called the Dinan, about 
two miles from Caftlecomber, in the county of Kilkenny. 
As you enter the cave, and continue fpeaking loud, no re¬ 
turn of the voice is perceived; but on your arriving at a 
certain point, which is not above fourteen or fifteen feet 
from the reflecting furface, a very diftinCt echo is heard. 
Now this echo cannot arife from the firft courfe of pulfes 
that are reflected to the ear> becaufe the breadth of the 
cave is fo fmall, that they would return too quickly to 
produce a diftinCt fenfation from that of the original found: 
it therefore is produced by thofe pulfes, which, after hav¬ 
ing been reflected feveral times from one fide of the grot¬ 
to to the other, and having run over a greater fpace than 
127 feet, arrive at the ear in conliderable numbers, and 
not more diflant from each other, in point of time, than 
the ninth part of a fecond. 
Entertaining Experiments and Contrivances. 
I. Place a concave mirror of about two feet diameter; 
in a perpendicular direction. The focus of this mirror 
may be at fifteen or eighteen inches diftance from its fur¬ 
face. At the diftance of about five or fix feet let there be 
a partition, in which there is an opening equal to the fize 
of the mirror; againft this opening muft be placed a pic¬ 
ture, painted in water-colours, on a thin cloth, that the 
found may ealily pafs through ft. 
Behind the partition, at the diftance of two or three 
feet, place another mirror, of the fame ftze as the former* 
and let it be diametrically oppolite to it. 
At the central point let there be placed the figure of a 
man feated on a pedeftal, and let his ear be placed exaCfly 
in the focus of the firft mirror: his lower jaw muft be 
made to open by a wire, and ftmt by a fpring; and there 
may be another wire to move the eyes: thefe wires muft: 
pafs through the figure, go under the floor, and come up 
behind the partition. 
Let a perfon, properly inftruded, be placed behind the 
partition near the mirror. You then propofe to any fpCc- 
tator to I'peak foftly to the ftatue, by putting his mouth to 
the ear of it; alluring him that it will anfwer ir.ftantly. 
You then give the fignal to the perfon behind the parti¬ 
tion, who, by placing his ear to the focus of the mirror, 
will hear diftinCtty what the perfon faid; and, moving the 
jaw and eyes of the ftatue by the wires, will return an 
anfwer direCtly, which will in like manner be diftin&ly 
heard by the firft fpeaker. 
This experiment appears to be taken from the Century 
of Inventions of the Marquis of Worcefter; whofe de- 
figns, at the time they were publiffied, were treated with 
ridicule and negleft as being impracticable, but are now 
known to be generally, if not univerfally, practicable. 
The words of the marquis are thefe: “ How to make a. 
brazen or (tone head in the midft of a great field or gar¬ 
den, fo artificial and natural, that though a man fpeak 
ever fo foftly, and even whnper into the ear thereof, it 
will prefently open its mouth, and refolve the queftion in 
French, Latin, Welffi, Irilh, or Engliftr, in good terms, 
uttering it out of its mouth, and then (hut it until the 
next queftion be afked.”—The two following, of a fimilar 
nature, appear to have been the inventions of Kircher, by 
means of which, (as he informs us) he ufed to “ utter 
feigned and ludicrous confultations, with a view to fhew 
the fallacy and impofture of ancient oracles.’! 
II. Let there be two heads of plafter of Paris, placed 
on pedeitals, on the oppofite fides of a room. There muft 
fco 
