Caverni, Raffaello, Storia del metodo sperimentale in Italia, 1891-1900

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            <p type="main">
              <s>
                <pb xlink:href="020/01/404.jpg" pagenum="385"/>
              stra? </s>
              <s>Ma quando siano uguali, e che la mano del lavorante farà moti irre­
                <lb/>
              golari e stravaganti, cioè spire, ghirigori, circoli, e sopratutto diametri molti
                <lb/>
              e per tutti i versi; allora sì che neanche un angelo potrà dare al vetro figura
                <lb/>
              più perfettamente sferica. </s>
              <s>” </s>
            </p>
            <p type="main">
              <s>“ Il segreto che più m'importa, e che non si sà da altri che da Dio e
                <lb/>
              da me, è questo: Non attaccare i vetri da lavorarsi con pece, nè con altro,
                <lb/>
              per via di fuoco. </s>
              <s>Perchè quelle materie, nel freddarsi, si ritirano più da
                <lb/>
              una parte che dall'altra, ed inarcano il vetro, il quale, finchè sta attaccato
                <lb/>
              al macinello, ha la figura colma, ma quando lo stacchiamo, per metter nel­
                <lb/>
              l'occhiale, egli si spiana come prima e la figura si guasta. </s>
              <s>Questo segreto,
                <lb/>
              che dico adesso a V. S., è stato da me osservato evidentemente, tanto che
                <lb/>
              l'ho toccato con mano, e direi anco a V. S. il come, ma lo lascio per
                <lb/>
              brevità. </s>
              <s>” </s>
            </p>
            <p type="main">
              <s>“ Ora, io attacco i vetri così: piglio un macinello di piombo di que­
                <lb/>
              sta proporzione: (fig. </s>
              <s>28). Alla faccia A spianata metto una rotella di ra­
                <lb/>
                <figure id="id.020.01.404.1.jpg" xlink:href="020/01/404/1.jpg" number="36"/>
              </s>
            </p>
            <p type="caption">
              <s>Figura 28.
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              scia o altro panno fino, cedente, acciò il vetro toc­
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              chi sul morbido, e dopo cingo sopra detto panno
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              il macinetto con una pelle di guanto tiratissima,
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              e la lego con lo spago CD stretta assai. </s>
              <s>Dopo,
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              impiastro la faccia di detta pelle A con cera rossa,
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              calda e distesa sottilmente. </s>
              <s>Così il vetro, purchè
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              non sia bagnato, si attaccherà sempre, sebben
                <lb/>
              freddo, e quando occorresse, si dà una strofina­
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              tina a detta pelle, con una palla della medesima cera rossa, che attaccherà
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              assai forte. </s>
              <s>” </s>
            </p>
            <p type="main">
              <s>“ Così ne seguita che il vetro non sarà sforzato, ma quella figura che
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              riceverà dalla centina, l'istessa riterrà, quando sia staccato dal macinello. </s>
              <s>
                <lb/>
              Oltre di ciò, V. S. averà comodità di cominciare a provare il vetro, se fa
                <lb/>
              bene o male, subito che si comincia a pulire, e potrà staccarlo e attaccare
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              cento volte, senza danno alcuno, e piuttosto con giovamento. </s>
              <s>Chè, quando si
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              adopra la pece, la regola è non lo staccar mai, se non quando egli è finito. </s>
              <s>” </s>
            </p>
            <p type="main">
              <s>“ Quanto all'invenzìone del macinello di piombo, non è mia, ma è bo­
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              nissima, perchè nel dare la pelle, non occorre aggravare quasi niente la
                <lb/>
              mano, ma il piombo medesimo fa quasi da per lui. </s>
              <s>Anco nel pulire aiuta
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              assai, ed acciò faccia meglio il servigio, abbiamo i macinelli, che son quasi
                <lb/>
              due dita più di diametro, che il vetro stesso, acciò gravitino quel di più,
                <lb/>
              ed osservi che il fare il macinello alto assai è male, perchè fa lieva e fa
                <lb/>
              traballare il vetro. </s>
              <s>Quando V. S. proverà queste invenzioni, che non son se
                <lb/>
              non due: centina piccola e non adoprar fuoco, l'assicuro che farà i vetri
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              buoni, anco quando la materia fosse cattiva, e non glie ne riuscirà mai nes­
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              suno cattivo affatto, ma sempre più che mediocri, e bisogna accordar molte
                <lb/>
              cose, la figura, la materia, e il pulimento. </s>
              <s>L'osservazione m'ha insegnato
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              che nei vetri, la figura importa assaissimo, e il pulimento pochissimo. </s>
              <s>La
                <lb/>
              ragione è questa: io ho provato molti de'miei vetri che appena comincia-</s>
            </p>
          </chap>
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