1. 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 proportionable decrease of the Sound, according to its distance.
2. As a Convex Speculum reflects the Object less, but somewhat brighter or clearer: So a Convex Corpus Obstans repels the Sound (insensibly) smaller; but somewhat quicker (though weaker) than otherwise it would be.
3. As a Concave Speculum reflects the Object bigger, more obscure and Inverted: So a Concave Corpus Obstans ecchoes back the Sound (insensibly) bigger, slower (though stronger) and also inverted; but never according to the order of Words. Nor do I think it possible for the Art of Man to contrive a Single Eccho, that shall invert the Sound, and repeat backwards; because then the Words last spoken, that is, which do last occur to the Corpus Obstans, must first be repell'd; which cannot be: For where, in the mean time, should the first Words hang, and be conceal'd, or lie dormant? Or how, after such a pause, be reviv'd and animated again into Motion? Yet in complicated or Compound Ecchoes, where many receive from one another, I know not whether something that way may not be done.
From the determinate Concavity or Archedness of these reflecting Bodies, it comes to pass, that some of them, from a certain distance or posture, will eccho back but one determinate Note, and from no other place will they reverberate any; because of the undue Position of the sounding Body. Such an one (as I remember) is the Vault in Merton College in Oxford.
4. As a Speculum takes in and reflects more of its Object, when plac'd at a great distance from it, than when nearer; because it reflects according to the apparent Magnitude of the Body at such a distance, which is less: So also the Ecchoing Body, being remov'd farther off, reflects more of the Sound, than when nearer. And this is the reason, why some Ecchoes repeat but one Syllable, some one Word, and some many.
5. As Specula's may be so plac'd, that reflecting one upon or into the other, either directly or obliquely, one Object shall appear many; as in Sir Samuel Moreland's Glass-room: After the same manner Ecchoing Bodies may be so contriv'd and plac'd, as that reflecting the Sound from one to the other, either directly and mutually, or obliquely and by Succession, out of one Sound shall many Ecchoes be begotten; which in the first case will be all together, and somewhat involv'd or swallow'd up of each other, and thereby confus'd (as a Face in Looking-glasses obverted) in the other they will be distinct, separate, and succeeding one another; as most multiple Ecchoes do.
Moreover a Multiple-Eccho may be made, by so placing the Ecchoing Bodies, at unequal distances, that they reflect all one way, and not one on the other; by which means a manifold successive Sound will be heard (not without astonishment) one Clap of the hands like many, one Ha, like a laughter, one single Word like many of the same Tone and Accent, and so one Viol like many of the same kind imitating each other.
Furthermore, as Specula's may be so order'd, that by Reflection they shall make one single thing appear many different things; as one single Man to seem many Men, differing as to Shape and Complexion (or a company of Men) which I think Sir Samuel Moreland's Contrivance does not: So may Ecchoing Bodies also be order'd, that from any one Sound given, they shall produce many Ecchoes, different both as to their Tone and Intension. (The ground whereof has elsewhere been laid down in a Treatise concerning the Sympathy of Lute-strings.)
By this means a Musical Room may be so contriv'd, that not only one Instrument, play'd on in it shall seem many of the same sort and size; but even a Consort of (somewhat) different ones; only by placing certain Ecchoing Bodies so, as that any Note (play'd) shall be return'd by them in 3ds, 5ths, and 8ths, which is possible to be done otherwise than was mention'd before in Refracted Audition.
I have now done with my Comparison of the two Noblest Senses, and Sciences, as to their Improvements; wherein I have been thus large, that I might give you a little prospect into the Excellency and Usefulness of Acousticks; and that thereby I might excite all that hear me, to bend their Thoughts towards the making of Experiments for the compleating this (yet very imperfect, tho' noble) Science; a Specimen whereof I will give you in three Problems, and then present you with the Semiplane of an Acoustick or Phonical Sphere, as an Attempt to explicate the great Principle in this Science, which is The Progression of Sounds.