Page:Philosophical Transactions - Volume 012.djvu/26

 and other more subtile bodies, and returns by its elasticity to its former place, then, are these parts extruded with violence as from the center of that place, and communicate their motion as far as the sound is heard. Or, when any solid body is moved with a sudden and violent 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 (according to him) Sound may be caused by the tremble of solid bodies without the presence of gross Air; and also by the restitution of gross Air, when it hath been divided with any violence. Thus, (saith he) we see, that 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 contracted in it, and by a sudden turn throwing off the Air; or by accension, 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 Pot-gun; a sudden crack will be caused.

Having laid down this Hypothesis, and left his Reader to apply it to the afore-mentioned phænomena, he proceeds to the Discourse of Musick it self, and maketh it a considerable part of his business to shew, How this Action that causes Sound, is performed by the several Instruments of Musick; having taught his Reader, first, What a Tone is, and that the Tones useful in Musick are those within the Scale, in which they are placed as they have relation to one another. Secondly, wherein consists that Relation of Tones & the union of mixt Sounds. Which done, he explains, how Tones are produced, and what assistances are given to the Sound by Instruments. Where he teaches, that wherever a Body stands upon a Spring that vibrates in equal Terms, such a Body, put into motion, will produce a Tone, which will be more grave or acute, according to the velocity of the returns: Wherefore Strings vibrating have a Tone according to the Bigness or Tension of them; and Bells that vibrate by cross Ovals, produce Notes according to the bigness of them, or the thickness of their sides; and so do all other bodies, whose superficies, being displaced by force, results or comes back by a spring which carries it beyond its first station. And here, to make it to be understood, how every pulse upon such vibrations causes Sound, our Author gives us to consider, that the gross Air is thrown off by the violence of the motion, which continues some moments of