Gravesande, Willem Jacob 's, An essay on perspective

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        <div xml:id="echoid-div327" type="section" level="1" n="177">
          <p>
            <s xml:id="echoid-s2049" xml:space="preserve">
              <pb o="90" file="0158" n="183" rhead="An ESSAY"/>
            until C D be equal to twice the Length of the
              <lb/>
            given Perpendicular. </s>
            <s xml:id="echoid-s2050" xml:space="preserve">This being done, move
              <lb/>
            the Needle along the Horizontal Line, ſuppoſe
              <lb/>
            to R, until the Point of the Thread R S paſſes
              <lb/>
            through the Point T; </s>
            <s xml:id="echoid-s2051" xml:space="preserve">then keeping the Thread
              <lb/>
            tight in this Manner, if the Ruler M N be
              <lb/>
            mov’d, till its Edge alſo paſſes through the
              <lb/>
            Point T, and P is the Point wherein the Edge of
              <lb/>
            the ſaid Ruler croſſes the Part of the Thread
              <lb/>
            R D, the Line T P will be the Repreſentation
              <lb/>
            ſought.</s>
            <s xml:id="echoid-s2052" xml:space="preserve"/>
          </p>
          <p>
            <s xml:id="echoid-s2053" xml:space="preserve">The Demonſtration of this is evident from
              <lb/>
            what is ſaid in n. </s>
            <s xml:id="echoid-s2054" xml:space="preserve">59.</s>
            <s xml:id="echoid-s2055" xml:space="preserve"/>
          </p>
        </div>
        <div xml:id="echoid-div328" type="section" level="1" n="178">
          <head xml:id="echoid-head190" xml:space="preserve">
            <emph style="sc">Method</emph>
          II.</head>
          <p style="it">
            <s xml:id="echoid-s2056" xml:space="preserve">112. </s>
            <s xml:id="echoid-s2057" xml:space="preserve">When all the Perpendiculars have the ſame
              <lb/>
            Length.</s>
            <s xml:id="echoid-s2058" xml:space="preserve"/>
          </p>
          <p>
            <s xml:id="echoid-s2059" xml:space="preserve">Let F G be parallel to the Baſe Line, and F O
              <lb/>
              <note position="left" xlink:label="note-0158-01" xlink:href="note-0158-01a" xml:space="preserve">Fig. 60.</note>
            equal to the Height of the Eye; </s>
            <s xml:id="echoid-s2060" xml:space="preserve">aſſume F f, e-
              <lb/>
            qual to the Length of either of the given Per-
              <lb/>
            pendiculars, and faſten the Thread fixed in F, in
              <lb/>
            the Point f. </s>
            <s xml:id="echoid-s2061" xml:space="preserve">Then raiſe R S perpendicular to
              <lb/>
            the Baſe Line, which make equal to F f, and
              <lb/>
            draw S Q parallel to the Baſe Line. </s>
            <s xml:id="echoid-s2062" xml:space="preserve">This being
              <lb/>
            done, tranſpoſe the Figures in the
              <note symbol="*" position="left" xlink:label="note-0158-02" xlink:href="note-0158-02a" xml:space="preserve">60.</note>
            cal Plane, in ſuch Manner, that the Point R co-
              <lb/>
            incides with S, and R H with S Q. </s>
            <s xml:id="echoid-s2063" xml:space="preserve">Then if S Q
              <lb/>
            be taken for a Baſe Line, and the Appearances
              <lb/>
            of the Feet of the Perpendiculars be
              <note symbol="*" position="left" xlink:label="note-0158-03" xlink:href="note-0158-03a" xml:space="preserve">109.</note>
            the Repreſentations of their Extremities will be
              <lb/>
            had.</s>
            <s xml:id="echoid-s2064" xml:space="preserve"/>
          </p>
        </div>
        <div xml:id="echoid-div330" type="section" level="1" n="179">
          <head xml:id="echoid-head191" xml:space="preserve">
            <emph style="sc">Method</emph>
          III.</head>
          <p style="it">
            <s xml:id="echoid-s2065" xml:space="preserve">113. </s>
            <s xml:id="echoid-s2066" xml:space="preserve">For Perpendiculars of the ſame Length.</s>
            <s xml:id="echoid-s2067" xml:space="preserve"/>
          </p>
          <p>
            <s xml:id="echoid-s2068" xml:space="preserve">The Figures in the Geometrical Plane being
              <lb/>
              <note position="left" xlink:label="note-0158-04" xlink:href="note-0158-04a" xml:space="preserve">Fig. 61.</note>
            tranſpoſed in the Manner aforeſaid , aſſume T
              <note symbol="*" position="left" xlink:label="note-0158-05" xlink:href="note-0158-05a" xml:space="preserve">112.</note>
            </s>
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