These Undulations are caused by the Vibration of some elastic Body, which is put into Motion by a Stroke of another Body against it. It must be an elastic
Body (take notice) for upon that Word depends the Truth of what is going to be alledged. To convince the Reader of the Truth of this, he has nothing to do but to take a Rolling Pin, and strike it against a Pound of Butter, and he will find very little or no Sound at all, because Butter has very little Elasticity or Spring in it; but if he strikes the aforesaid Implement against the Table, he will find Sound enough, because most Tables are made of Wood, which is a very elastic Body. If there is no Butter in the House, Wax will do as well or better, for it will prove that a Body may be hard without being elastic, and which will be very much to the Purpose. It will be necessary, before we can get any further, to explain what Vibration is, a Word very commonly made Use of among Musical Men, tho' but little understood.
To be as short as possible; a Piece of Lead hung upon the End of a String, which moves backwards and forwards of itself after being first put into Motion, is
called a Pendulum, and that Motion backwards and forwards is called its Vibration; it is upon this Principle that elastic Bodies are the Cause of Sound. It will be best illustrated in a Musical Instrument, besides that is the Point in Hand; and to be more to the Point still, we will suppose it a Violin, though any other stringed Instrument would answer the same End.
Here we have four Strings stretched out upon a Bridge, or thin Piece of Wood, which communicates to the Belly of the Instrument, from which Belly the greatest Part of the Tone proceeds. Now a String drawn tight at both Ends, when it is struck, will have a Vibration or tremulous Motion, which Vibration, or tremulous Motion, acts upon the same Principle as a Pendulum does in a Cycloid, or, to speak as plain as possible, as a Pendulum does when it is put into it's proper Motion.
It is upon this Principle of Vibration then, or tremulous Motion, that the String
of a Violin, being moved by the Bow, is to act: The String immediately communicates it's tremulous Motion to the Bridge, and the Bridge to the Belly of the Instrument, which Belly being made of a very elastic Wood, by it's Vibration and free Motion, acts upon the Air in the Manner abovementioned.
As it is the great Elasticity of the Wood which is to cause the Tone, it ought to have as little Confinement in it's Vibrations as is possible; the Weight of the Strings must indeed press against it, otherwise they could not communicate their Motion to it. We should therefore be careful not to over-string the Instrument, since it so plainly contradicts the Principles of Pneumatics.
It is easy to hear when an Instrument is over-strung; and sometimes an Octave in a Harpsichord, by it's additional Number of Strings, shall render the Tone of the Instrument so dead, that, though it gives
a Sprightliness peculiar to an Octave, yet it sometimes hardly compensates for the Loss of Tone which it causes in the Unisons, by it's too great Pressure upon the Belly of the Instrument.