directly or obliquely upon any dense Body, of a smooth (whether Plane or Arch'd) Superficies, is beat back again and reflected, or does eccho more or less*.
I say (1.) falling directly or obliquely; because, if the Sound be sent out and propagated parallel to the Surface of the Dense Body, or be made so far off and so weak, that it cannot reach it, there will be no Reflection of Sound, no Eccho.
I say (2.) upon a Body of a smooth Superficies; because if the Surface of the Corpus Obstans be uneven, the Air by reverberation will be put out of its regular Motion, and the Sound thereby broken and extinguish'd: So that tho' in this case also the Air be beaten back again, yet Sound is not reflected, nor is there any Eccho.
I say (3.) it does eccho more or less, to shew, that when all things are, as is before describ'd, there is still an Ecchoing, though it be not always heard; either because the direct Sound is too weak to be beaten quite back again to him that made it; or that it does return home to him, but so weak, that without the help of a good Otacoustick, it cannot be discern'd; or that
he stands in a wrong place, to receive the reflected Sound, which passes over his head, under his Feet, or to one side of him; which therefore may be heard by a Man standing in that place, where the reflected Sound will come, provided no interpos'd Body does intercept it; but not by him, that first made it.
I shall further make out the comparison 'twixt Reflex'd Vision and Audition, by these following Propositions.
- As a Plain Speculum reflects the Object in its due Dimensions and Colours; allowing for their difference of appearance, according to their distance: So a Plane Corpus Obstans reflects the Sound back in its due Tone and Loudness; if allowance be likewise made for the