Castelli, Benedetto, Of the mensuration of running waters, 1661

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              paſſing by its extreameſt parts, wherewith it communicateth with
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              the River; in which parts, the Torrent being meaſured, ſhall
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              have ſuch a certain meaſure: but the River ſwelling and riſing,
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              alſo thoſe parts of the Torrent augment in greatneſſe and mea­
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              ſure, though the Torrent, in that inſtant, diſ-imbogue no more
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              water than it did before: ſo that the River being ſwelled, we
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              are to conſider two mouths of the ſame Torrent, one leſſe be­
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              fore the riſing, the other greater after the riſing, which mouths
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              diſcharge equal quantities of water in equal times; therefore the
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              velocity by the leſſer mouth ſhall be greater than the velocity by
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              the greater mouth; and thus the Torrent ſhall be retarded from
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              its ordinary courſe.</s>
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              <s>
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              COROLLARIE
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              V.</s>
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              <s>From which operation of Nature proceedeth another effect
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              worthy of conſideration; and it is, that the courſe of the water
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              retarding, as hath been ſaid in thoſe ultimate parts of the Tor­
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              rent, if it ſhall happen that the Torrent grow torbid and mud­
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              dy, and its ſtreame be retarded in ſuch a degree, that it is not
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              able to carry away thoſe minute grains of Earth, which com­
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              poſe the muddineſſe; in this caſe the Torrent ſhall clear away
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              the mud, and carry away the Sand at the bottome of its own
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              Chanel, in the extream parts of its mouth, which raiſed and
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              voided Sand, ſhall again afterwards be carried away, when the
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              River abating, the Torrent ſhall return to move with its primitive
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              velocity.</s>
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              <s>
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              COROLLARIE
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              VI.</s>
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              <s>Whilſt it is demonſtrated, that the ſame water hath different
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              meaſures in its Chanel or courſe, according as it varieth in
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              velocity; ſo that the meaſure of the water is alwayes greater, where
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              the velocity is leſſer; and on the contrary, the meaſure leſſer,
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              where the velocity is greater: from hence we may moſt ele­
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              gantly render the reaſon of the uſual Proverb,
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              Take heed of the
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              ſtill waters:
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              For that if we conſider the ſelf ſame water of a
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              River in thoſe parts, wherein it is leſs ſwift, and thence called
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              ſtill
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              or
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              ſmooth
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              water, it ſhall be, of neceſſity, of greater meaſure
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              than in thoſe parts, in which it is more ſwift, and therefore ordi­
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              narily ſhall be alſo more deep and dangerous for paſſengers;
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              whence it is well ſaid,
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              Take heed of the ſtill Waters
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              ; and this
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              ſaying hath been ſince applied to things moral.</s>
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