Salusbury, Thomas
,
Mathematical collections and translations (Tome I)
,
1667
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ly. </
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<
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>The like ſhall alſo hold true in the
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P
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ortion of the Sphære
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leſs than an Hemiſphere that lieth with its whole Baſe above the
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Liquid.</
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* Or according
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to the Perpendi
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cular.</
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<
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>COMMANDINE.</
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The Demonſtration of this Propoſition is defaced by the Injury of Time, which we have re
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ſtored, ſo far as by the Figures that remain, one may collect the Meaning of
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Archimedes,
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for we thought it not good to alter them: and what was wanting to their declaration and ex
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planation we have ſupplyed in our Commentaries, as we have alſo determined to do in the ſe
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cond Propoſition of the ſecond Book.
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<
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>If any Solid Magnitude lighter than the Liquid.]
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Theſe words, light-
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er than the Liquid, are added by us, and are not to be found in the Tranſiation; for of theſe
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kind of Magnitudes doth
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Archimedes
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ſpeak in this Propoſition.
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A</
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<
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>Shall be demitted into the Liquid in ſuch a manner as that the
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Baſe of the Portion touch not the Liquid.]
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That is, ſhall be ſo demitted into
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the Liquid as that the Baſe ſhall be upwards, and the
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Vertex
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downwards, which he oppoſeth
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to that which he ſaith in the Propoſition following
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; Be demitted into the Liquid, ſo, as
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that its Baſe be wholly within the Liquid;
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For theſe words ſignifie the Portion demit
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ted the contrary way, as namely, with the
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Vertex
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upwards and the Baſe downwards. </
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>The
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ſame manner of ſpeech is frequently uſed in the ſecond Book; which treateth of the Portions
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of Rectangle Conoids.
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B</
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>Now becauſe every Portion of a Sphære hath its Axis in the Line
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that from the Center of the Sphære is drawn perpendicular to its
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Baſe.]
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For draw a Line from B to C, and let K L cut the Circumference A B C D in the
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Point G, and the Right Line B C in M
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:
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and becauſe the two Circles A B C D, and
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E F H do cut one another in the Points
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B and C, the Right Line that conjoyneth
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their Centers, namely, K L, doth cut the
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Line B C in two equall parts, and at
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Right Angles; as in our Commentaries
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upon
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Prolomeys
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Planiſphære we do
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prove: But of the Portion of the Circle
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B N C the Diameter is M N; and of the
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Portion B G C the Diameter is M G;
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for the
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(a)
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Right Lines which are drawn
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on both ſides parallel to B C do make
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Right Angles with N G; and
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(b)
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for
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that cauſe are thereby cut in two equall
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parts: Therefore the Axis of the Portion
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of the Sphære B N C is N M; and the
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Axis of the Portion B G C is M G:
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from whence it followeth that the Axis of
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the Portion demerged in the Liquid is
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in the Line K L, namely N G. </
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>And ſince the Center of Gravity of any Portion of a Sphære is
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in the Axis, as we have demonstrated in our Book
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De Centro Gravitatis Solidorum,
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the
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Centre of Gravity of the Magnitude compounded of both the Portions B N C & B G C, that is,
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of the Portion demerged in the Water, is in the Line N G that doth conjoyn the Centers of Gra
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vity of thoſe Portions of Sphæres. </
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<
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>For ſuppoſe, if poſſible, that it be out of the Line N G, as
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in Q, and let the Center of the Gravity of the Portion B N C, be V, and draw V
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Becauſe
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therefore from the Portion demerged in the Liquid the Portion of the Sphære B N C, not ha
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ving the ſame Center of Gravity, is cut off, the Center of Gravity of the Remainder of the
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Portion B G C ſhall, by the 8 of the firſt Book of
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Archimedes, De Centro Gravitatis </
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