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Of Mobility, and the Nature of Motion. 49 A. What do you call relative Motion?

B. It is the Change of Place with refpect to fome other Things at reft: And abfolute and relative Reft is just the contrary to thefe..

A. What do you obferve hence?

B. That a Perfon may relatively be at Reft, who yet is really and truly in abfolute Motion.

A. Please to exemplify this Matter.

B. I will: Suppofe a Perfonfeated in a Ship, he appeareth to all within the Ship to be at Reft; though at the fame Time he is moved with the fame Motion, with the fame Swiftnefs, and according to the fame Course, as the Ship it felf is, with regard to abfolute Space.

A. What are the general Affections of Motion?

B. They are these three, viz.

1. The Celerity or Velocity of Motion.
2. The Quantity of Motion.

3. The Direction of Motion.

A. Pray what do you mean by the Celerity of Motion?

B. It is that whereby a Body paffeth over a given Space, in a given Time: So that if in one Minute a Body A paffeth over the Space ab, and another Body B paffeth over the Space cd, in the fame Time; then the Ε Cele

Celerity or Swiftness of the Body A will be to the Celerity of the Body B, as the Line ab is to the Line cd. See Fig. VI.

A. How do you compute the Quantity of

Motion?

B. By compounding the Proportion of the Quantity of Matter, and the Velocity of Motion: Thus, if the Body A hath two Parts of Matter, and fix Degrees of Velocity, the Motion of that Body will be twelve. See Fig. VII.

A. Then by this Means I perceive you have an eafy Method of comparing the Quantity of Motion in any two or more Bodies.

B. Yes, very truly: For, fuppose the Body A hath two Parts of Matter, and fix Degrees of Velocity; and the Body B hath four Parts of Matter, and ten Degrees of Velocity; then the Quantity of Motion in A, paffing from a to b, will be to the Quantity of Motion in B, paffing the Space cd, as twelve to forty. See Fig. VII.

A. Why then I find, that if the Quantity of Matter in any two Bodies be equal, and their Velocities unequal, and the contrary, their Quantity of Motion will likewife be unequal.

B. It will fo; for the Quantity of Motion in two Bodies will never be equal, if their Matter or Velocity differs, unless in this one Cafe, when the Quantities of

I

Matter

Of Motion, and its Affections.

51 Matter and Velocity are in reciprocal Proportion to each other: As thus, 4: 2::6:3. Wherefore the Motion of B, with three Degrees of Velocity, is equal to the Motion of A, with fix Degrees thereof. See Fig. VII.

A. What do you call the Direction of Motion?

B. It is a right Line, fuppofed to be drawn towards the Place where the moving Body tends As ab is the Direction of the Body A, in Fig. VI.

A. What other Divifions of Motion do you make?

B. Why, according to the Force impreffed on Bodies, their Motion is either simple or compound.

A. Pray, Sir, what is fimple Motion?

B. When only one Force or Power acts on any Body, the Motion produced thereby is fimple, and according to the Direction of the impreffed Force.

A. And, next, pleafe to explain compound Motion.

B. This is produced when two or more Forces act on any Body in different Directions: Thus, fuppofe a Power H act on the Body A, with eight Degrees of Force, in the Direction ab; and another Power I act on it with fix Degrees of Force, in the Direction ac; I fay, the Motion of A, is not fimply in either of the Directions ab or ac, but

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but in a Direction compounded of both, viz. ad, and that with ten Degrees of Force; and arrives to d in the fame Time as it would have arrived to b orc with those Forces fingly. See Fig. VIII.

B.

H

СНА Р. VI.

Of LIGHT.

Aving thus taken a brief Survey of the most general and effential Properties of Matter; let us now proceed to thofe which are more particular, called the fpecific or accidental Qualities of Bodies.

A. By all means, Sir, proceed; for 'tis a Converfation of fo noble a Sort, that I fhall never recompenfe the Time and Trouble you take to inftruct me: But, to continue the Thread, pray, why are they called fpecific or accidental Qualities?

B. They are called Specific, because thereBy the feveral Kinds of Bodies have their Diftinction, and are what they are, and are differenced from one another: Thus, Fire by Heat, Glafs by Transparency, Liquors by Fluidity, Solids by their Fixity, &c. are diftinguished from each other; and are

account

Of the Nature of Light.

53

accounted particular Species of Being or

Matter.

A. But you do not fay why they are termed Accidental.

B. I was just going to tell you; they are thus called, because they are not effential to Matter, but happen to part thereof per Accidens, or by Cafualty; for 'tis purely fo, that one Part of Matter is disposed to be hot, another cold, another dry, another light, another of this Colour, and another of that.

A. Which of thofe Qualities will you please to begin withal?

B. Light; this being the most confiderable and wonderful of all others; and by which, all Things are made visible and known.

A. Pray what do you call Light?

B. That Quality of certain Bodies, whereby they become vifible to us, and render others fo.

A. What is the true Caufe of Light in those Bodies?

B. No Man can certainly tell; however, fome affirm it to confift in the extreme vibrating Motions of the Parts of thofe luminous Bodies.

A. By what Means doth Light come to affect our Eyes?

B. 'Tis fuppofed this is owing to a very fine ætherial Matter, vaftly rarer than Air, which

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