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

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1for that amongſt ſo many tryals as have been made, that alſo
was light upon, on which the improvement and remedy to the
diſorder did depend.
And to us my fore-named Treatiſe ſhall
ſerve for a Rule, which being well underſtood, ſhall make us to
know wherein conſiſteth, and whereon dependeth this miſcarri­
age, and conſequently it will be eaſie to apply thereunto a ſeaſo­
nable remedy.
And firſt I ſay, That there is no doubt but that the waters
continue ſo high on thoſe Plains becauſe they are ſo high in the
principal River, which ought to receive them, and carry them
into the Sea.
Now the Cauſes of the height of the River, may
in my judgement be reduced to one alone; which is that by me
ſo often mentioned for the moſt Potent one, and declared in my
afore-named Tractate; to wit, The tardity of the motion of the
waters, which doth alwayes infallibly, and preciſely cauſe the
ſelf ſame Running Water to change the meaſure of its thickneſs
at ſuch a rate, that the more it encreaſeth in velocity, the more
it decreaſeth in meaſure; and the more it decreaſeth in velocity,
the more it encreaſeth in meaſure: As for example; If a River
run in ſuch a place with the velocity of moving a mile in the
ſpace of an hour, and afterwards the ſame River in another place
doth encreaſe in velocity, ſo as to make three miles an hour;
that ſame River ſhall diminiſh in thickneſs two thirds: And on
the contrary, If it ſhall diminiſh in velocity ſo, as that it runneth
but half a mile in the ſame time, it ſhall encreaſe the double in
thickneſs and meaſure.
And in a word, look what proportion
the velocity in the firſt place, hath to the velocity in the ſecond,
and ſuch hath reciprocally the meaſure of the thickneſs in the
ſecond place, to the meaſure in the firſt; as I have clearly demon­
ſtrated in my Treatiſe: Which I repeat ſo frequently, that I
fear the Profeſſors of Polite Learning will charge me with Tua­
tologie, and vain Repetition.
But I am ſo deſirous in this moſt
important point to be well underſtood, becauſe it will then be
eaſie to comprehend all the reſt; and without this it is impoſſible
(I will not ſay difficult, but abſolutely impoſſible) to underſtand,
or ever to effect any thing to purpoſe.
And the better to ex­
plain the example, let it be ſuppoſed,
17[Figure 17]
That the water of a River A D,
runneth high at the level of A F,
with ſuch a certain velocity; and let
it, by the ſame water, be velocitated
three times more; I ſay, that it will
abate 1/3, and ſhall ſtand at the level
in B E; and if it ſhall more veloci­
tate, it will abate the more at the Sea; But if it ſhould retard

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