A philosophical essay of musick directed to a friend.

Of SOUND.

MY design being to explain the nature of Musick it will not be improper to enquire first, concerning the cause of Sound; and here though I cannot hope to give full satisfaction, because our Senses want subtilty to discover the motions of the minute parts of the Air whereupon it depends, yet perhaps I may assign a possible cause for the producing of Sound, the knowledge whereof will give some ease to the wondring Philosopher; and if I have the good fortune to shew a probable cause, the Curious will acquiesce from any farther enquiry, which is all the benefit they can have by speculations of this na∣ture.

The Phaenomena of Sound which I think considerable are these, 1. It may be produced in the Torricellian va∣cuity. 2. It causes motion in solid bodies. 3. It is diminish∣ed by interposition of solid bodies, and 4. If the bodies in∣terposed are very thick, its passage is wholly obstructed. 5. It seems to come to the Ear in strait lines when the object is so scituated that it cannot come in a straight line to the Ear. 6. When the air is not in motion its extent is sphaerical. 7. When there is a wind, the sphaere is enlarged on that part to which the wind blows, and diminished on the contrary part. 8. That it arrives not to the Ear in an instant but con∣siderably slower than sight. 9. That it comes as quick against the wind as with it, though not so loud nor so far.

There are many other observable Appearances, and more will daily occurre to the Curious upon Experiments, but the consideration of these qualities of Sound and the visible actions that produce it, hath made me entertain the following Hypothesis.

I suppose the air we breath in to be a mixture of divers minute bodies which are of different sorts and sizes, though all of them are so small as to escape our senses: the grosser of them are elastical, and are resisted by solid bodies, which are in no sort pervious to them; the smaller parts pass through solid bodies, but not with that ease but that upon a sudden and violent start of them, they shock the parts of solid bodies that stand in their way, and also the grosser parts of the air; and there may be another degree of most subtil Ethereal parts with which the interstices of these and all o∣ther bodies are repleat, which find freer passage every where, and are capable of no compression, and consequently be the medium and cause of the immediate communication of sight. The middle sort of these I esteem the medium and cause of Sound, and that at any time when the grosser air is driven off any space and leaves it to be possest by these and other more subtil bodies, and returns by its elastici∣ty to its former place, then are these parts extruded with violence as from the center of that space, and communicate their motion as far as the sound is heard.

Or when any solid body is moved with a sudden and vi∣olent motion, these parts must be affected thereby: for as these parts are so much resisted by solid bodies as to shock them, so on the contrary they must needs be moved by the sudden starting of solid bodies.

So that sound may be caused by the tremble of solid bo∣dies without the presence of gross Air, and also by the re∣stitution of gross Air when it has been divided with any violence.

We see a Bell will sound in the Torricellian space: and when the Air is divided with any sudden force, as by the end of a Whip having all the motion of the Whip contract∣ed in it, and by a sudden turn throwing off the Air; by ac∣cension, as in Thunder and Guns; or by any impression of force carrying it where other Air cannot so forcibly follow, as upon compressing of Air in a bladder till it breaks, or in a Potgun, a sudden crack will be caused.

I shall leave others to apply this Hypothesis to the afore mentioned Phaenomena (which they may easily do) and pro∣ceed to the discourse of Musick, where I am in its due place to shew how this action that causes sound is performed by the several instruments of Musick.