Galilei, Galileo, Mechanics, 1665

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            <pb xlink:href="070/01/022.jpg" pagenum="292"/>
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              * Or three parts
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              of one Rope.</s>
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            <p type="head">
              <s>
                <emph type="italics"/>
              Of the
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              SCREW.</s>
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              <s>Amongſt the reſt of Mechanick Inſtruments for ſundry uſes
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              found out by the Wit of Man, the Screw doth, in my opi­
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              nion, both for Invention and for Utility, hold the firſt
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              place, as that which is appoſitely accommodated, and ſo contrived
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              not only to move, but alſo to ſtay and preſs with very great Force,
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              that taking up but little room, it worketh thoſe effects which other
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              Inſtruments cannot, unleſs they were reduced to a great Machine.
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              </s>
              <s>The Screw therefore being of moſt ingenious and commodious
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              contrivance, we ought deſervedly to be at ſome pains in explaining,
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              with all the plainneſs that is poſſible, the Original and Nature of
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              it. </s>
              <s>The which that we may do, we will begin at a Speculation,
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              which, though at firſt bluſh it may appear ſomewhat remote from
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              the conſideration of this Inſtrument, yet is the
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              Baſis
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              and Founda­
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              tion thereof.</s>
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              <s>No doubt, but that Natures operation in the Motions of Grave
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              Bodies is ſuch, that any whatever Body that hath a Gravity in it
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              hath a propenſion of moving, being at liberty, towards the Cen­
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                <arrow.to.target n="marg1116"/>
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              ter, and that not only ^{*} by the Right Line perpendicularly, but al­
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              ſo (when it cannot do otherwiſe) by any other Line, which ha­
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              ving ſome inclination towards the Center goeth more and more
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              abaſing. </s>
              <s>And thus we ſee the Water not only to fall downwards
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              along the Perpendicular from ſome eminent place, but alſo to run
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              about the Surface of the Earth along Lines though very little en­
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              clined; as we ſee in the Courſe of Rivers, the Waters of which, if ſo
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              be that the Bed have any the leaſt declivity, go freely declining
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              downwards. </s>
              <s>Which very effect, like as it is diſcerned in all Fluid
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              Bodies, would appear alſo in hard Bodies, if ſo be, that their Fi­
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              gure and other Accidental and Extern Impediments did not hinder
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              it. </s>
              <s>So that we, having a Superficies very well ſmoothed and poli­
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              ſhed, as for inſtance, that of a Looking-glaſs, and a Ball exactly
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              rotund and ſleek, either of Marble, or of Glaſs, or of any other
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              Matter apt to be poliſhed, this being placed upon that Superficies
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              ſhall trundle along, in caſe that this have any, though very ſmall,
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              inclination; and ſhall lie ſtill only upon that Superficies which is
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              exactly levelled and parallel to the Plane of the Horizon: as is
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              that, for example, of a Lake or ſtanding Water being frozen, up­
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              on which the ſaid Spherical Body would ſtand ſtill, but in a con­
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              dition of being moved by every ſmall Force. </s>
              <s>For we having ſup­
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              poſed that if that Plane did incline but an hairs breadth only, the
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              ſaid Ball would move along it ſpontaneouſly towards the part de­
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              clining, and on the oppoſite would have a Reſiſtance, nay, would
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              not be able without ſome Violence to move towards the part </s>
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          </chap>
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