2. As _Perspective-Glasses_ and Telescopes help the Eye to see Objects at a very great distance, which otherwise would not be discernable; in like manner may a sort of _Otacousticks_ be so contriv'd, as that they shall receive in _Sounds_ made at a very great distance also, but with so much advantage, that the Ear shall be able to hear them, which otherwise would have been _inaudible_. And these _Otacousticks_ in some respects would be of greater use than _Perspectives_. For whereas at Land _Perspectives_ are many times render'd almost useless, by the interposition of Woods and Mountains, which hinder the Sight from reaching very far: Our _Otacousticks_ would, notwithstanding these Obstacles, take in a Sound made some Leagues off. Which might be of notable use in the time of War, for discovering the Enemy at a good distance, when he marches or lyes incamp'd behind a Mountain or Wood, or any such place of shelter. Yea, even at Sea also, where _Perspectives_ are of most use, by reason of the plainess of the Surface of the Water; yet sometimes there _Otacousticks_ may be of more benefit, when in dark hazy Weather the Air is too thick, or in Stormy Tempestuous Weather the Waves arise too high for the _Perspective_ to be made use of. But, whether at Sea or Land, _Perspectives_ become altogether insignificant in the Night-time (unless it be for viewing the Stars) which is the chief time for using _Otacousticks_; as it is generally, for Soldiers to take their March, when they would surprise their Enemies. And therefore this sort of _Otacousticks_ have then their chief use, when _Perspectives_ are of no use at all; besides that they may be imploy'd in the Day-time, as well as _Perspectives_, whence they may (not unfitly) be term'd _the most useful Instrument_ of the two. 3. As _Microscopes_ or _Magnifying-Glasses_ help the Eye to see near Objects, that by reason of their smallness were _Invisible_ before; which Objects they Magnify to a strange greatness: So _Microphones_ or _Micracousticks_, that is Magnifying Ear Instruments, may be contriv'd after that manner, that they shall render the most minute Sound in nature distinctly audible, by Magnifying it to an unconceivable loudness. By the help hereof we may hear the different Cries and Tones, as well as by _Microscopes_ see the divers Shapes and Figures of the smallest Animals. 4. As by _Polyscopes_ or _Multiplying-Glasses_, one thing is represented to the Eye as many, whether in the same or different Shapes (for so Multiplying-Glasses may be contriv'd:) So by a _Polyphone_ or _Polyacoustick_ well order'd, one _Sound_ may be heard as many, either of the same or a different Note. Insomuch, that who uses this Instrument, he shall, at the Sound of a single Viol, seem to hear a whole Consort and all true Harmony. By which means this Instrument has much the advantage of the _Polyscope_.