Archimedes
,
Natation of bodies
,
1662
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For if, the Line B R being prolonged unto G, G R hath the ſame proportion to R B as the Por
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tion of the Conoid I P O S hath to the remaining Figure that lyeth above the Surface of the
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Liquid, the Toine G ſhall be its Centre of Gravity; by the 8 of the ſecond of
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Archimedes
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de Centro Gravitatis Planorum, vel de
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Æ
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quiponderantibus.
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D</
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E</
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<
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>R O ſhall be leſs than
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quæ uſque ad Axem
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(or than the Semi
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parameter.]
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By the 10 Propofit. </
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Euclids
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fifth Book of Elements. </
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>The Line
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quæ
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uſque ad Axem,
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(or the Semi-parameter) according to
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Archimedes,
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is the half of that
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juxta quam poſſunt, quæ á Sectione ducuntur, (
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or of the Parameter;) as appeareth
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by the 4 Propoſit of his Book
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De Conoidibus & Shpæroidibus:
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and for what reaſon it is
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ſo called, we have declared in the Commentaries upon him by us publiſhed.
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F</
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<
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>Whereupon the Angle R P
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ſhall be acute.]
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Let the Line N O be
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continued out to H, that ſo RH may be equall to
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the Semi-parameter. </
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>If now from the Point H
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a Line be drawn at Right Angles to N H, it ſhall
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meet with FP without the Section; for being
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drawn thorow O parallel to A L, it ſhall fall
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without the Section, by the 17 of our ſirst Book of
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Conicks;
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Therefore let it meet in V: and
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becauſe F P is parallel to the Diameter, and H
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V perpendicular to the ſame Diameter, and R H
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equall to the Semi-parameter, the Line drawn
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from the Point R to V ſhall make Right Angles
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with that Line which the Section toucheth in the Point P: that is with K
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as ſhall anon be
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demonstrated: Wherefore the Perpendidulat R T falleth betwixt A and
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and the Argle R
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P
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ſhall be an Acute Angle.
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<
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>Let A B C be the Section of a Rightangled Cone, or a Parabola,
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and its Diameter B D; and let the Line E F touch the
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ſame in the Point G: and in the Diameter B D take the Line
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H K equall to the Semi-parameter: and thorow G, G L be
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ing drawn parallel to the Diameter, draw KM from the
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P
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oint K at Right Angles to B D cutting G L in M: I ſay
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that the Line prolonged thorow Hand Mis perpendicular to
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E F, which it cutteth in N.</
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For from the Point G draw the Line G O at Right Angles to E F cutting the Diameter in
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O: and again from the ſame Point draw G P perpendicular to the Diameter: and let the
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ſaid Diameter prolonged cut the Line E F in
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P B ſhall be equall to B Q, by the 35 of
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<
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our firſt Book of
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Conick
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Sections,
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(a)
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and G
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P a Mean-proportion all betmixt Q P and PO
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;
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(b)
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and therefore the Square of G P ſhall be e
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quall to the Rectangle of O P Q: But it is alſo
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equall to the Rectangle comprehended under P B
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and the Line
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juxta quam poſſunt,
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or the Par
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ameter, by the 11 of our firſt Book of
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Conicks:
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(c)
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Therefore, look what proportion Q P hath to
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P B, and the ſame hath the Parameter unto P O:
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But Q P is double unto
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P B,
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for that
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P B
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and B
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Q are equall, as hath been ſaid: And therefore
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the Parameter ſhall be double to the ſaid P O:
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and by the ſame Reaſon P O is equall to that which we call the Semi-parameter, that is, to K H
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:
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But
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(d)
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P G is equall to K M, and
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(e)
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the Angle O P G to the Angle H K M; for they are both
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Right Angles: And therefore O G alſo is equall to H M, and the Angle P O G unto the
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