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

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1paſſing by its extreameſt parts, wherewith it communicateth with
the River; in which parts, the Torrent being meaſured, ſhall
have ſuch a certain meaſure: but the River ſwelling and riſing,
alſo thoſe parts of the Torrent augment in greatneſſe and mea­
ſure, though the Torrent, in that inſtant, diſ-imbogue no more
water than it did before: ſo that the River being ſwelled, we
are to conſider two mouths of the ſame Torrent, one leſſe be­
fore the riſing, the other greater after the riſing, which mouths
diſcharge equal quantities of water in equal times; therefore the
velocity by the leſſer mouth ſhall be greater than the velocity by
the greater mouth; and thus the Torrent ſhall be retarded from
its ordinary courſe.
COROLLARIE V.
From which operation of Nature proceedeth another effect
worthy of conſideration; and it is, that the courſe of the water
retarding, as hath been ſaid in thoſe ultimate parts of the Tor­
rent, if it ſhall happen that the Torrent grow torbid and mud­
dy, and its ſtreame be retarded in ſuch a degree, that it is not
able to carry away thoſe minute grains of Earth, which com­
poſe the muddineſſe; in this caſe the Torrent ſhall clear away
the mud, and carry away the Sand at the bottome of its own
Chanel, in the extream parts of its mouth, which raiſed and
voided Sand, ſhall again afterwards be carried away, when the
River abating, the Torrent ſhall return to move with its primitive
velocity.
COROLLARIE VI.
Whilſt it is demonſtrated, that the ſame water hath different
meaſures in its Chanel or courſe, according as it varieth in
velocity; ſo that the meaſure of the water is alwayes greater, where
the velocity is leſſer; and on the contrary, the meaſure leſſer,
where the velocity is greater: from hence we may moſt ele­
gantly render the reaſon of the uſual Proverb, Take heed of the
ſtill waters: For that if we conſider the ſelf ſame water of a
River in thoſe parts, wherein it is leſs ſwift, and thence called ſtill
or ſmooth water, it ſhall be, of neceſſity, of greater meaſure
than in thoſe parts, in which it is more ſwift, and therefore ordi­
narily ſhall be alſo more deep and dangerous for paſſengers;
whence it is well ſaid, Take heed of the ſtill Waters; and this
ſaying hath been ſince applied to things moral.

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