Bion, Nicolas, Traité de la construction et principaux usages des instruments de mathématique, 1723

Table of Notes

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        <div xml:id="echoid-div515" type="section" level="1" n="248">
          <p>
            <s xml:id="echoid-s6990" xml:space="preserve">
              <pb o="223" file="238" n="238" rhead="ASTRONOMIQUE. Liv. VI. Chap. I."/>
            ction. </s>
            <s xml:id="echoid-s6991" xml:space="preserve">Mais comme dans les voïages il faut ſouvent faire la preuve
              <lb/>
            de cet inſtrument, & </s>
            <s xml:id="echoid-s6992" xml:space="preserve">que la ſuſdite méthode eſt ſujete à beaucoup
              <lb/>
            d'incommoditez, tant parce qu'il eſt difficile de renverſer l'inſtru-
              <lb/>
            ment de maniere que le tuïau de la lunete reſte à même hauteur,
              <lb/>
            qu'à cauſe des differentes refractions de l'Atmoſphere autour de
              <lb/>
            l'horiſon à differentes heures du jour comme auſſi pour l'agi-
              <lb/>
            tation & </s>
            <s xml:id="echoid-s6993" xml:space="preserve">l'ondulation de l'air, & </s>
            <s xml:id="echoid-s6994" xml:space="preserve">autres choſes ſemblables, nous
              <lb/>
            ajoûtons ici deux autres méthodes propres à rectifier ces ſortes
              <lb/>
            d'inſtrumens, afin que chacun puiſſe choiſir celle qui lui paroîtra
              <lb/>
            plus commode ſelon les tems & </s>
            <s xml:id="echoid-s6995" xml:space="preserve">les lieux.</s>
            <s xml:id="echoid-s6996" xml:space="preserve"/>
          </p>
        </div>
        <div xml:id="echoid-div525" type="section" level="1" n="249">
          <head xml:id="echoid-head370" style="it" xml:space="preserve">Seconde méthode pour éprouver la poſition des Lunetes
            <lb/>
          de longue vûë.</head>
          <p>
            <s xml:id="echoid-s6997" xml:space="preserve">POur cette operation nous choiſiſſons un lieu d'où l'on puiſſe
              <lb/>
            voir diſtinctement un objet éloigné du moins de mille toiſes,
              <lb/>
            & </s>
            <s xml:id="echoid-s6998" xml:space="preserve">dont l'élevation ſur l'horiſon ne ſurpaſſe point le nombre des de-
              <lb/>
            grez que l'on marque ſur le bord au-delà du commencement de la
              <lb/>
            diviſion. </s>
            <s xml:id="echoid-s6999" xml:space="preserve">Ayant donc obſervé la hauteur de cet objet telle qu'elle
              <lb/>
            paroît par les degrez du bord, on mettra devant le quart de cercle, & </s>
            <s xml:id="echoid-s7000" xml:space="preserve">
              <lb/>
            le plus près qu'il ſe pourra, un vaſe de large embouchure, qu'on
              <lb/>
            remplira entierement d'eau juſqu'au ſommet des bords, le plus à
              <lb/>
            comble que faire ſe pourra. </s>
            <s xml:id="echoid-s7001" xml:space="preserve">Enſuite il faut hauſſer ou baiſſer le vaſe
              <lb/>
            juſqu'à ce que par la lunete on voïe ledit objet ſur la ſurface de l'eau
              <lb/>
            comme dans un miroir, ce qui ne ſera pas difficile, pourvû que la
              <lb/>
            ſurface de l'cau ne ſoit pas troublée par l'agitation de l'air; </s>
            <s xml:id="echoid-s7002" xml:space="preserve">nous au-
              <lb/>
            rons donc l'abaiſſement de cet objet par reflexion, & </s>
            <s xml:id="echoid-s7003" xml:space="preserve">on le verra
              <lb/>
            dans la ſituation droite, car nous nous ſervons de lunetes compo-
              <lb/>
            ſées de deux lentilles de verre convexe qui repreſentent les objets
              <lb/>
            renverſez; </s>
            <s xml:id="echoid-s7004" xml:space="preserve">mais par la reflexion, un objet renverſé ſe renverſe en-
              <lb/>
            core une fois; </s>
            <s xml:id="echoid-s7005" xml:space="preserve">c'eſt pourquoi il paroît droit.</s>
            <s xml:id="echoid-s7006" xml:space="preserve"/>
          </p>
          <p>
            <s xml:id="echoid-s7007" xml:space="preserve">Il faut remarquer cependant que l'abaiſſement de l'objet peut
              <lb/>
            quelquefois ſe voir ſur les degrez du bord comme la hauteur, ce qui
              <lb/>
            arrivera ſi l'angle fait de la ligne de foi & </s>
            <s xml:id="echoid-s7008" xml:space="preserve">du raïon qui paſſe par le
              <lb/>
            premier point de la diviſion eſt plus grand qu'un droit, & </s>
            <s xml:id="echoid-s7009" xml:space="preserve">au con-
              <lb/>
            traire, en d'autres cas la hauteur paroîtra comme l'abaiſſement, ſi
              <lb/>
            l'angle de la ligne de foi avec le raîon qui paſſe par le premier point
              <lb/>
            de la diviſion eſt plus petit qu'un droit. </s>
            <s xml:id="echoid-s7010" xml:space="preserve">Mais en tous les cas, ſans
              <lb/>
            avoir égard à l'élevation ou l'abaiſſement marqué ſur le bord, le
              <lb/>
            point milieu marqué exactement entre les deux obſervations, ſera
              <lb/>
            vertical & </s>
            <s xml:id="echoid-s7011" xml:space="preserve">répondra au Zénith, par rapport à la ligne de foi des lu-
              <lb/>
            netes.</s>
            <s xml:id="echoid-s7012" xml:space="preserve"/>
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