Salusbury, Thomas
,
Mathematical collections and translations (Tome I)
,
1667
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<
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>NIC. </
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<
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>You ſay truth.</
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<
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>RIC. </
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<
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>I have another queſtion to aske you, which is this, Why the Author
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uſeth the word
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L
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iquid, or Humid, inſtead of Water.</
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<
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>NIC. </
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<
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>It may be for two of theſe two Cauſes; the one is, that Water being the
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principal of all
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L
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iquids, therefore ſaying
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Humidum
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he is to be underſtood to mean
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the chief Liquid, that is Water: The other, becauſe that all the Propoſitions of
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this Book of his, do not only hold true in Water, but alſo in every other
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L
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iquid,
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as in Wine, Oyl, and the like: and therefore the Author might have uſed the word
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Humidum,
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as being a word more general than
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Aqua.
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<
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>RIC. </
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<
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>This I underſtand, therefore let us come to the firſt
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Propoſition,
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which, as
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you know, in the Original ſpeaks in this manner.</
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>PROP. I. THEOR. I.</
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If any Superficies ſhall be cut by a Plane thorough any
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Point, and the Section be alwaies the Circumference
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of a Circle, whoſe Center is the ſaid Point: that Su
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perficies ſhall be Spherical.
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<
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>Let any Superficies be cut at pleaſure by a Plane thorow the
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Point K; and let the Section alwaies deſcribe the Circumfe
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rence of a Circle that hath for its Center the Point K: I ſay,
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that that ſame Superficies is Sphærical. </
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<
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>For were it poſſible that the
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ſaid Superficies were not Sphærical, then all the Lines drawn
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through the ſaid Point K unto that Superficies would not be equal,
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Let therefore A and B be two
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Points in the ſaid Superficies, ſo that
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drawing the two Lines K A and
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K B, let them, if poſſible, be une
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qual: Then by theſe two Lines let
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a Plane be drawn cutting the ſaid
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Superficies, and let the Section in
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the Superficies make the Line
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D A B G: Now this Line D A B G
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is, by our pre-ſuppoſal, a Circle, and
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the Center thereof is the Point K, for ſuch the ſaid Superficies was
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ſuppoſed to be. </
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<
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>Therefore the two Lines K A and K B are equal:
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But they were alſo ſuppoſed to be unequal; which is impoſſible:
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It followeth therefore, of neceſſity, that the ſaid Superficies be
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Sphærical, that is, the Superficies of a Sphære.</
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<
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>RIC. </
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<
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>I underſtand you very well; now let us proceed to the ſecond
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Propoſition,
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which, you know, runs thus.</
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