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-div525" type="section" level="1" n="249">
          <pb o="224" file="239" n="239" rhead="CONSTRUCTION ET USAGES DU QUART, & c."/>
          <p>
            <s xml:id="echoid-s7013" xml:space="preserve">Ayant donctrouvé l'erreur de l'inſtrument, c'eſt-à-dire, la diffe-
              <lb/>
            rence entre le premier point de la diviſion marqué ſur le bord & </s>
            <s xml:id="echoid-s7014" xml:space="preserve">le
              <lb/>
            point qui répond au Zénith, on tachera de remettre les fils de ſoie
              <lb/>
            en leur vraie poſition, ſi cela ſe peut faire commodément, ſinon il
              <lb/>
            faudra avoir égard à l'erreur que l'on a reconnu en toutes les obſer-
              <lb/>
            vations, ſoit d'élevation, ſoit d'abaiſſement.</s>
            <s xml:id="echoid-s7015" xml:space="preserve"/>
          </p>
          <p>
            <s xml:id="echoid-s7016" xml:space="preserve">Mais il faut remarquer que ſi l'objet eſt proche & </s>
            <s xml:id="echoid-s7017" xml:space="preserve">élevé ſur l'ho-
              <lb/>
            riſon de pluſieurs minutes, il faudra trouver la veritable erreur de
              <lb/>
            l'inſtrument, par le calcul en la maniere ſuivante.</s>
            <s xml:id="echoid-s7018" xml:space="preserve"/>
          </p>
          <p>
            <s xml:id="echoid-s7019" xml:space="preserve">Dans un triangle dont un côté ſoit la diſtance connuë entre le
              <lb/>
            lieu de l'obſervation & </s>
            <s xml:id="echoid-s7020" xml:space="preserve">l'objet, l'autre côté ſoit la diſtance entre le
              <lb/>
            point milieu de la longueur de la lunete, & </s>
            <s xml:id="echoid-s7021" xml:space="preserve">le point de la ſurface de
              <lb/>
            l'eau où elle eſt rencontrée par le raïon refféchi avec l'angle com-
              <lb/>
            pris de ces deux côtez, à ſçavoir l'angle ou l'arc compris entre les
              <lb/>
            obſervations de l'élevation & </s>
            <s xml:id="echoid-s7022" xml:space="preserve">de l'abaiſſement de l'objet, on trou-
              <lb/>
            vera par le calcul l'angle oppoſé au plus petit côté; </s>
            <s xml:id="echoid-s7023" xml:space="preserve">de la quantité
              <lb/>
            de cet angle, il faut diminuer l'arc entre les obſervations du côté
              <lb/>
            du quart de cercle prolongé, & </s>
            <s xml:id="echoid-s7024" xml:space="preserve">le point milieu de l'arc reſtant ſe-
              <lb/>
            ra le vrai commencement de la diviſion.</s>
            <s xml:id="echoid-s7025" xml:space="preserve"/>
          </p>
          <p>
            <s xml:id="echoid-s7026" xml:space="preserve">Ainſi on peut trouver la diſtance entre le point milieu du tuïau
              <lb/>
            de la lunete & </s>
            <s xml:id="echoid-s7027" xml:space="preserve">le point de la ſurface de l'eau rencontrée par le raïon
              <lb/>
            refléchi, par le moyen d'une regle ou d'un fil tendu & </s>
            <s xml:id="echoid-s7028" xml:space="preserve">prolongé de-
              <lb/>
            puis ledit tuïau juſqu'à la ſurface de l'eau.</s>
            <s xml:id="echoid-s7029" xml:space="preserve"/>
          </p>
        </div>
        <div xml:id="echoid-div526" type="section" level="1" n="250">
          <head xml:id="echoid-head371" style="it" xml:space="preserve">Troiſiéme Méthode.</head>
          <p>
            <s xml:id="echoid-s7030" xml:space="preserve">CEtte operation eſt ſimple, mais les obſervations ne ſont pas fa-
              <lb/>
            ciles à faire. </s>
            <s xml:id="echoid-s7031" xml:space="preserve">Nous ſuppoſons en cette méthode, comme en la
              <lb/>
            precedente, qu'il y a ſur le bord du quart de cercle quelques degrez
              <lb/>
            marquez & </s>
            <s xml:id="echoid-s7032" xml:space="preserve">diviſez au-delà du point de 90 de hauteur, qui répond
              <lb/>
            au Zénith. </s>
            <s xml:id="echoid-s7033" xml:space="preserve">Nous obſervons pendant une belle nuit, le tems étant
              <lb/>
            ſerein & </s>
            <s xml:id="echoid-s7034" xml:space="preserve">ttanquille, la hauteur méridiene de quelque étoile qui ap-
              <lb/>
            proche de nôtre Zénith, ayant tourné vers l'Orient la face diviſéé
              <lb/>
            du bord de l'inſtrument. </s>
            <s xml:id="echoid-s7035" xml:space="preserve">La nuit ſuivante, ou peu de tems après,
              <lb/>
            nous obſervons ſem blablement la hauteur méridiene de la même
              <lb/>
            étoile, mais la face diviſée du bord étant tournée vers l'Occident:
              <lb/>
            </s>
            <s xml:id="echoid-s7036" xml:space="preserve">je dis que le point milieu de l'arc entre les obſervations eſt le point
              <lb/>
            de 90 deg. </s>
            <s xml:id="echoid-s7037" xml:space="preserve">c'eſt à dire, par où paſſe le raïon parallele de la ligne
              <lb/>
            de foi de la lunete.</s>
            <s xml:id="echoid-s7038" xml:space="preserve"/>
          </p>
          <p>
            <s xml:id="echoid-s7039" xml:space="preserve">Cette troiſiéme méthode eſt fort utile pour prouver la poſition
              <lb/>
            des lunetes, que l'on ajuſte non ſeulement aux quarts de cercle,
              <lb/>
            </s>
          </p>
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