Gravesande, Willem Jacob 's, An essay on perspective

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          <p>
            <s xml:id="echoid-s2352" xml:space="preserve">
              <pb o="107" file="0183" n="212" rhead="on PERSPECTIVE."/>
            a Hole at its End, through which the above-men-
              <lb/>
            tion’d Screw may paſs, and ſo the Mirrour H be
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            fixed to any Inclination before the Aperture N.</s>
            <s xml:id="echoid-s2353" xml:space="preserve"/>
          </p>
          <p>
            <s xml:id="echoid-s2354" xml:space="preserve">14. </s>
            <s xml:id="echoid-s2355" xml:space="preserve">Beſides the Mirrour H, there is another leſſer
              <lb/>
            one, L, faſten’d near its Middle to a Ruler going
              <lb/>
            out through the Middle of the Top of the Box.
              <lb/>
            </s>
            <s xml:id="echoid-s2356" xml:space="preserve">This Ruler may Screw on, and ſerves to raiſe or
              <lb/>
            lower the Mirrour faſten’d to it, ſo that it may be
              <lb/>
            fixed to all Angles of Inclination.</s>
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          </p>
        </div>
        <div xml:id="echoid-div370" type="section" level="1" n="201">
          <head xml:id="echoid-head219" xml:space="preserve">
            <emph style="sc">Remarks</emph>
          .</head>
          <p>
            <s xml:id="echoid-s2358" xml:space="preserve">If the Tubes mention’d in N. </s>
            <s xml:id="echoid-s2359" xml:space="preserve">6. </s>
            <s xml:id="echoid-s2360" xml:space="preserve">be not thought
              <lb/>
            ſufficient for giving Air to the Machine, a ſmall
              <lb/>
            Pair of Bellows may be put under the Seat,
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            which may be blown by one’s Foot. </s>
            <s xml:id="echoid-s2361" xml:space="preserve">And by
              <lb/>
            this means the Air within the Machine may be
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            continually remov’d; </s>
            <s xml:id="echoid-s2362" xml:space="preserve">for the Bellows driving the
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            Air out of the Machine, obliges the external Air
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            to enter through the Tubes therein.</s>
            <s xml:id="echoid-s2363" xml:space="preserve"/>
          </p>
        </div>
        <div xml:id="echoid-div371" type="section" level="1" n="202">
          <head xml:id="echoid-head220" style="it" xml:space="preserve">Uſe of the Machine.</head>
          <head xml:id="echoid-head221" xml:space="preserve">
            <emph style="sc">Problem</emph>
          I.</head>
          <p style="it">
            <s xml:id="echoid-s2364" xml:space="preserve">15. </s>
            <s xml:id="echoid-s2365" xml:space="preserve">To repreſent Objects in their natural Diſpo-
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            ſition.</s>
            <s xml:id="echoid-s2366" xml:space="preserve"/>
          </p>
          <p>
            <s xml:id="echoid-s2367" xml:space="preserve">When Objects are to be repreſented within the
              <lb/>
              <note position="right" xlink:label="note-0183-01" xlink:href="note-0183-01a" xml:space="preserve">Fig. 70.</note>
            Machine, we extend a Sheet of Paper upon the
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            Table A; </s>
            <s xml:id="echoid-s2368" xml:space="preserve">or, which is better, we lay a Sheet of
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            Paper upon another Board, ſo that it ſpreads be-
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            yond the Edges of the Board; </s>
            <s xml:id="echoid-s2369" xml:space="preserve">then we ſqueeze
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            the ſaid Paper and Board into a Frame, ſo that
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            it be fixed therein by means of two Dove-tail’d
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            Rulers. </s>
            <s xml:id="echoid-s2370" xml:space="preserve">This being done, we place a Convex
              <lb/>
            Glaſs in the Cylinder C, which ſcrews into
              <note symbol="*" position="right" xlink:label="note-0183-02" xlink:href="note-0183-02a" xml:space="preserve">9.</note>
            Top of the Machine, having its Focal </s>
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