Humber, and too rapid in their return to the 
ear; yet it is evident, that the reflecting 
surface may be so formed, as that the 
pulses which come to the ear after two re- 
flections or more, may, after having de- 
scribed 127 feet or more, arrive at the ear 
in sufficient numbers, and also so nearly at 
the same instant, as to produce an echo, 
though the distance of the reflecting sur- 
face from the ear be less than the limit of 
echoes. This is confirmed by a singular 
echo in a grotto on the banks of the little 
brook called the Dinan, about two miles from 
Castleeomber, in the county of Kilkenny. 
As you enter the cave, and continue speak- 
ing loud, no return of the voice is per- 
ceived ; but on your arriving at a certain 
point, which is not above 14 or 15 feet 
from the reflecting surface, a very distinct 
echo is heard. Now this echo cannot arise 
from the fifst course of pulses that are re- 
flected to the ear, because the breadth of 
the cave is so small, that they would re- 
tail too quickly to produce a distinct sen- 
sation from that of the original sound: it 
therefore is produced by those pulses, which, 
alter having been reflected several times 
from one side of the grotto to the other, and 
having run over a greater space than 127 
feet, arrived at the ear in considerable num- 
bers, and not more distant from each other 
in point of time than the ninth part of a se- 
cond. M. De la Grange demonstrated that 
all impressions are reflected by an -obstacle 
terminating an elastic fluid with the same 
velocity with which they arrived at that ob- 
stacle. When the walls of a passage, or of an 
unfurnished room, are smooth and perfectly 
parallel, any explosion, or a stamping with 
the foot, communicates an impression to 
the air, which is reflected from one wall to 
the other, and from the second again to- 
wards the ear, nearly in the same direction, 
with the primitive impulse : this takes place 
as frequently in a second, as double the 
breadth of the passage is contained in 1130 
feet ; and the ear receives a perception of a 
musical sound, thus determined its pitch 
by the breadth of the passage. On mak- 
ing the experiment, the result will be found 
accurately to agree with this explanation. 
If the sound is predetermined, and the fre- 
quency of vibrations such as that each pulse, 
when doubly reflected, may coincide with 
the subsequent impulse, proceeding directly 
from the sounding body, the intensity of 
the sound will be much increased by the 
reflection; and also, in a less degree, if the 
reflected pulse coincides with the next but 
one, the next but two, or more of the direct 
pulses. The appropriate notes of a room 
may readily be discovered by singing the 
scale in it; and they will be found to de- 
pend on the proportion of its length or 
breath to 1130 feet. 
By altering our situation in a room, and 
expressing a sound, or hearing the sound of 
another person, \ in different situations, or 
when different objects are alternately placed 
in the room, that sound may be heard 
louder or weaker, and more or less dis- 
tinct. Hence it is, that blind persons, who 
are under the necessily of paying great at- 
tention to the perceptions of their sense of 
hearing, acquire the habit of distinguishing 
from the sound even of their own voices, 
whether a room is empty or furnished ; whe- 
ther the windows are open or shut, and 
sometimes they can even distinguish whe- 
ther any person be in the room or not. A 
great deal of furniture in a room checks, in 
a great measure, the sounds that are pro- 
duced in it, for they hinder the free com- 
munication of the vibrations of the air from 
one part of the room to the other. The 
fittest rooms for declamation, or for music, 
are such as contain few ornaments that ob- 
struct the sound, and at the same time have 
the least echo possible. 
A strong and continued sound fatigues the 
ear. 1 he strokes of heavy hammers, of ar- 
tillery, &c. are apt to make people deaf 
for a time : and it has been known that 
persons who have been long exposed to the 
continued and confused noise of certain 
manufactories, or of water-falls, or other 
noisy places, can hoar what is spoken to 
them much better in the midst of that noise 
than elsewhere. 
We shall conclude this article with an 
experiment or two for the amusement of 
the younger part of our readers. 
Experiment 1. Place a concave mirror, 
AB, fig. 6, of two feet in diameter, in a 
perpendicular direction, and at the dis- 
tance of about five or six feet from a par- 
tition EF, in which there is an opening 
equal in size to the mirror; against this 
opening must be placed a picture painted in 
water-colours, on a thin cloth, that the 
sound may easily pass through it. Behind 
the partition, at the distance of a few feet, 
place another mirror GH, of the same size 
as the former, and directly opposite to it. 
At the point C is to be placed the figure of 
a man seated on a pedestal, with his ear ex- 
actly in the focus of the first mirror ; Ids 
lower jaw must be made to open by a 
C 2 
