Salusbury, Thomas
,
Mathematical collections and translations (Tome I)
,
1667
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be perpendicular to the ſame D E, by reaſon that the Colure is
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erect upon the grand Orb; and therefore the ſaid D E,
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ſhall be the Tangent of the grand Orb in the point A.
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<
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>So that in this Poſition the motion of the Centre by the arch
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A E; that is, of one degree every day differeth very little; yea,
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is as if it were made by the Tangent D A E. </
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<
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>And becauſe by
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means of the diurnal motion the point D, carried about by G,
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unto E, encreaſeth the motion of the Centre moved almoſt in the
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ſame line D E, as much as the whole diameter D E amounts
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unto; and on the other ſide diminiſheth as much, moving about
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the other ſemicircle E F D. </
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<
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>The additions and ſubductions
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in this place therefore, that is in the time of the ſolſtice, ſhall be
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meaſured by the whole diameter D E.</
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<
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>Let us in the next place enquire, Whether they be of the ſame
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bigneſs in the times of the
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E
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quinoxes; and tranſporting the
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Centre of the Earth to the point I, diſtant a Quadrant of a
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Circle from the point A. </
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>Let us ſuppoſe the ſaid Equinoctial
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to be G E F D, its common ſection with the grand Orb D E, the
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Axis with the ſame inclination C B; but the Tangent of the grand
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Orb in the point I ſhall be no longer D E, but another which
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ſhall cut that at right Angles; and let it be this marked H I L,
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according to which the motion of the Centre I, ſhall make its
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greſs, proceeding along the circumference of this grand Orb.
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>Now in this ſtate the Additions and Subſtractions are no longer
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meaſured by the diameter D E, as before was done; becauſe that
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diameter not diſtending it ſelf according to the line of the annual
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motion H L, rather cutting it at right angles, thoſe terms D E, do
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neither add nor ſubſtract any thing; but the Additions and
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Subſtractons are to be taken from that diameter that falleth
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in the plane that is errect upon the plane of the grand Orb, and
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that interſects it according to the line H L; which diameter in this
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caſe ſhall be this G F and the Adjective, if I may ſo ſay, ſhall
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be that made by the point G, about the ſemicircle G E F, and the
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Ablative ſhall be the reſt made by the other ſemicircle F D G.
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<
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>Now this diameter, as not being in the ſame line H L of the
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annual motion, but rather cutting it, as we ſee in the point I, the
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term G being elevated above, and E depreſſed below the plane
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of the grand Orb, doth not determine the Additions and
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ſtractions according to its whole length, but the quantity of thoſe
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firſt ought to be taken from the part of the line H L, that is
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tercepted between the perpendiculars drawn upon it from the
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terms G F; namely, theſe two G S, and F V: So that the
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ſure of the additions is the line S V leſſer then G F, or then D E;
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which was the meaſure of the additions in the Solſtice A. </
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<
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>And
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ſo ſucceſſively, according as the centre of the Earth ſhall be </
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