“of the Position of Sonorous Bodies, 634 
two equal and similar solids, constructed upon 
the equal and similar:: planes,, ECS and FE 
CS. . Now sound, though it be formed in the 
ears, is very much increased by the vibrations 
excited in the contiguous parts of the head by 
the pulses which fall :upon them, as I-shall.en- 
deavour to prove hereafter: therefore, as oftenas . 
the two portions of the*head, which are sepa- 
rated by the vertical plane perpendicular to the 
axis of hearing, are equally agitated by the pulses 
of the same sound, the ears are‘also equally affected 
from the same cause; which never happens, as we 
learn from the testimony of the other senses, 
unless the sounding body be placed somewhere 
in the right liné that bisects the axis of hearing 
at right angles, In this manner men are taught 
by experience to draw a general inference from 
a general observation; they therefore conclude 
a body to be situated directly before or behind 
their persons, as often as the sound of it strikes 
both their ears with equal forces. The preced- 
ing demonstration elucidates the case of direct 
hearing, as far as this can be done by the assistance 
of a diagram; but the perception which deter- 
mines the place of a sounding body to be in front 
of the hearer or behind him, requires a separate 
investigation ; and] shall endeavour in the next 
place to ascertain the cause of it, by the follow- 
ing observations, 
