Castelli, Benedetto
,
Of the mensuration of running waters
,
1661
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the ſame water in paſſing under the Bridge, in our caſe meeteth
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with eight of the ſame impediments, bearing, and thruſting upon
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two ſides of each Arch (to omit the impediment of the bottom,
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for that it is the ſame in the River, and under the Bridge) from
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which inadvertency ſometimes follow very great diſorders, as
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quotidian practice ſhews us.</
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>APPENDIX IX.</
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>It is alſo worthy to conſider the great and admirable benefit
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that thoſe fields receive, which are wont to drink up the Rain
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water with difficulty, through the height of the water in the
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principal Ditches; in which caſe the careful Husbandman cutteth
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away the reeds and ruſhes in the Ditches, through which the
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waters paſs; whereupon may be preſently ſeen, ſo ſoon as the
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reeds and ruſhes are cut, a notable Ebb in the level of the water
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in the Ditches; inſomuch that ſometimes it is obſerved, that the
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water is abated after the ſaid cutting a third and more, of what it
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was before the cutting. </
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>The which effect ſeemingly might de
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pend on this, That, before thoſe weeds took up room in the
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Ditch, and for that cauſe the water kept a higher level, and the
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ſaid Plants being afterwards cut and removed, the water came to
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abate, poſſeſſing the place that before was occupied by the
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weeds: Which opinion, though probable, and at firſt ſight ſa
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tisfactory, is nevertheleſs inſufficient to give the total reaſon of
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that notable abatement which hath been ſpoken of: But it is ne
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ceſſary to have recourſe to our confideration of the velocity in
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the courſe of the water, the chiefeſt and true cauſe of the vari
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ation of the meaſure of the ſame Running-Water; for, that
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multitudes of reeds, weeds, and plants diſperſed through the cur
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rent of the Ditch, do chance notably to retard the courſe of the
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water, and therefore the meaſure of the water increaſeth; and
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thoſe impediments removed, the ſame water gaineth velocity,
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and therefore decreaſeth in meaſure, and conſequently in
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height.</
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>And perhaps this point well underſtood, may be of great
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profit to the fields adjacent to the
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Pontine
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Fens, and I doubt not
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but if the River
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Ninfa,
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and the other principal Brooks of thoſe
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Territories were kept well cleanſed from weeds, their waters
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would be at a lower level, and conſequently the drains of the
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fields would run into them more readily; it being alwayes to be
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held for undoubted, that the meaſure of the water before the
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cleanſing, hath the ſame proportion to the meaſure after clean
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ſing, that the velocity after the cleanſing hath to the velocity
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before the cleanſing: An dbecauſe thoſe weeds being cleanſed </
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