sign that there is no need of opening it; and if there be any occasion to open it, it is easily done. As for the rest your Lordship may please to keep account of all the particulars that occur, for the memory of things past is our Tutresse in those that are to come. If occasion shall offer, I intreat you to bow humbly in my name to His Highness the Grand Duke, and the most Serene Prince Leopold; and to attend the service of Their Highnesses, for you serve I rinces of extraordinary merit; And to whom I my self am also exceedingly obliged. In the controversies that arise respect the pious end of speaking the Truth, for then every thing will succeed happily. I kiss the hands of Padre Francesco,of Sig. Bartolotti, and of your Lordship.
Rome, 14. March 1642.
most Obliged Servant
D. BENEDETTO CASTELLI.
Vpon this occasion I will here insert a Discourse that I made upon the Draining and improvement of the Pontine Fens,for that I think that whatsoever may be done well and to purpose in this matter hath absolute dependance on the perfect knowledge of that so important Proposition, by me demonstrated and explained in my Treatise of the Mensuration of Running Waters, namely, That the same water of a River doth continually change Measures, according as it altereth and changeth the velocity of its course; so that the measure of the thicknesse of a River in one Site, to the measure of the same River in another Site, hath the same proportion reciprocally that the velocity in this site hath to the velocity in the first site. And this is a Truth so constant and unchangeable, that it altereth not in the least point on any occurrences of the Waters that change: and being well understood, it openeth the way to the knowledge of sundry advertisements in these matters, which are all resolved by this sole Principle; and from it are derived very considerable benefits; and without these it is impossible to do any thing with absolute perfection
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