Alberti, Leone Battista, Architecture, 1755

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            <p type="main">
              <s>
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              which are turned upon Centres, when they are
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              cloſed with their Key-ſtones, it will be proper
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              immediately to eaſe the Props a little, that
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              thoſe Centres reſt upon; not only to prevent
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              the Stones freſh laid from floating in the Beds
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              of Mortar they are ſet in, but that the whole
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              Vault may ſink and cloſe by its own Weight
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              epually, into its right Seat: Otherwiſe in drying,
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              the Work would not compact itſelf as it ought,
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              but would be apt to leave Cracks when it came
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              afterwards to ſettle. </s>
              <s>And therefore you muſt
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              not quite take away the Centre immediately,
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              but let it down eaſily Day after Day, by little
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              and little, for Fear, if you ſhould take it away
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              too ſoon, the Building ſhould never duly cement.
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              </s>
              <s>But after a certain Number of Days, according
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              to the Greatneſs of the Work, eaſeit a little, and
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              ſo go on gradually, till the Wedges all compact
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              themſelves in their Places, and are perfectly
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              ſettled. </s>
              <s>The beſt Way of letting down the
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              Frame is this: When you place your Centre
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              upon the Pilaſters, or whatever elſe it is to
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              reſt upon, put under each of its Feet two
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              Wedges of Wood; aud when afterwards you
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              want to let it down, you may with a Ham­
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              mer ſafely drive out theſe Wedges by little
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              and little, as you ſhall judge proper.</s>
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              <s>LASTLY, it is my Opinion, that the Centres
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              ought not to be taken away till after Winter,
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              as well for other Reaſons, as becauſe the
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              Waſhing of the Rains may weaken and de­
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              moliſh the whole Structure; though elſe we
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              cannot do greater Service to a Vault than to
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              give it Water enough, and to let it be
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              thoroughly ſoak'd, that it may never feel Thirſt.
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              </s>
              <s>But of this Subject we have ſaid enough.</s>
            </p>
            <p type="head">
              <s>CHAP. XV.</s>
            </p>
            <p type="head">
              <s>
                <emph type="italics"/>
              Of the Shell of the Covering, and its Uſefulneſs; the different Sorts and
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              Shapes of Tiles, and what to make them of.
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              </s>
            </p>
            <p type="main">
              <s>I now come to cover the Roof. </s>
              <s>And cer­
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              tainly, if we weigh the Matter duly, there
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              is no Convenience in the whole Building
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              greater than the having Shelter from the burn­
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              ing Sun, and the inclement Seaſons; and this
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              is a Benefit which you owe the Continuance
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              of, not to the Wall, nor to Area, nor any of
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              theſe; but principally to the outward Shell of
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              the Roof; which all the Art and Induſtry of
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              Man, though they have tried all Means, has
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              not yet been able to make ſo ſtrong and im­
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              penetrable againſt the Weather as might be
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              wiſh'd: Nor do I think, it will be an eaſy
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              Matter to do it; for where, not only Rains,
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              but Extremes of Heat and Cold, and above
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              all, bluſtering Storms of Wind, are continu­
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              ally aſſaulting the ſame Place; what Mate­
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              rials are ſtrong enough to reſiſt ſuch unwearied
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              and powerful Adverſaries? </s>
              <s>Hence it happens,
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              that ſome Coverings preſently rot, others open,
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              others oppreſs the Wall, ſome crack, or break,
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              others are waſhed away; inſomuch, that even
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              Metals, which are ſo hardy againſt the Wea­
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              ther, in other Places, are not here able to hold
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              out againſt ſuch frequent Aſſaults. </s>
              <s>But Men
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              not deſpiſing ſuch Materials as Nature furniſh­
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              ed them with in their reſpective Countries,
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              have provided againſt theſe Inconveniences as
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              well as they were able; and hence aroſe various
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              Methods of Covering in a Building.
                <emph type="italics"/>
              Vitruvi­
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              us
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              tells us, that the
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              Pyrgenſes
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              covered their
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              Houſes with Reeds, and the People of
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              Mar­
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              ſeilles
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              with Clay kneaded, and mixed with
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              Straw. </s>
              <s>The
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              Chelonophagi,
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              near the
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              Garaman­
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              tes, Pliny
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              tells us, cover'd theirs with the
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              Shells of Tortoiſes. </s>
              <s>The greateſt Part of
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              Ger­
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              many
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              uſe Shingles. </s>
              <s>In
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              Flanders
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              and
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              Picardy,
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              they cut a white Sort of Stone which they
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              have (which Saws eaſier than Wood itſelf) in­
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              to their Scantlings, which they uſe inſtead of
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              Tiles. </s>
              <s>The
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              Genoueze,
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              and
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              Florentines
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              uſe thin
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              Pieces of a ſcaly Sort of Stone. </s>
              <s>Others have
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              tried the Pargets, which we ſhall ſpeak of by
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              and by. </s>
              <s>But after having made Experiment of
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              every Thing, the Wit and Invention of Man
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              has found out nothing yet more convenient
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              than Tiles of baked Clay. </s>
              <s>For all Sorts of
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              Parget grow rugged in Froſts, and ſo crack and
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              break: Lead is melted by the Sun's Heat:
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              Braſs, if laid in thick Plates, is very coſtly;
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              and if it is thin, it is apt to warp, and to be
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              eaten and conſumed with Ruſt.</s>
            </p>
            <p type="main">
              <s>ONE
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              Grinias
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              of
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              Cyprus,
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              the Son of a Pea­
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              ſant, is ſaid to be the Inventer of Tiles, which
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              are of two Sorts, the one broad and flat, one
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              Foot broad, and a Foot and a half long, </s>
            </p>
          </chap>
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