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108.

thrô 6 lines if a pipe of 13 foot heigh give 1 inch thrô 3 lines: therefore thrô 6 lines it will give 4 inches; and by consequence if ye conduit remains of ye same largeness ye water will go 4 times as swift, and have 4 times as much rubbing; it is necessary therefore to make it go as swift as ye square of ye diameter of ye conduit be 4 lines as great, and then ye root of that square shall be to ye root of ye others as 6 to 30.

  There happens an effect surprising enough in ye conduit of some pipes at Chantilly these pipes wch are of wood through one into another pass thrô a small pond, and afterwards thrô a long channel; whence it happens that if we stop of a suddain ye entry of ye reservatory, and that ye water runs no more into ye pipe of ye conduit, ye spout of 14 foot will not wholly cease, but it will continue to spout to near 2 foot without discontinuation, supposing that ye entry of ye reservatory was well sloped, we may attribute this effect to this that ye waters running with a great swiftness, that of ye pond and ye channel doth a little open ye bodys of ye pipes of ye conduit being void we let ye water of ye reservatory enter of a sudden.  for then ye air bring pressed forces ye pipes, and makes a passage wch is made by little of ye water of ye pond and of ye channel, is great enough to furnish this spout of 2 foot.

  There happens also to ye same spout another extraordinary effect wch is, that if we put ones hand upon ye passage and keep it there begins to be elevated by little and little to 3 inches, then to 1 foot and after to two successively in a considerable time.  I have seen ye same effect in a water wch run horizontally in a pipe of copper: for having stopped it with ye hand thinking that that water being retained a considerable time, would make a greater effort and spout further, I was surprised that there came presently almost no water out:  but at last by little and little gained its ordinary force, see how I explain this effect. 

 In ye channel of Chantilly wch hath a very small descent to 8 toises from ye spout.  Ye water would run there but slowly if it was not pressed by ye upper water whose descent is steeper.  now if we suppose A B C D to be ye steep descent, and that ye channel is but half 

[[image:  drawing of a pipe A B C D gently sloping down to the right, showing a less than full level of liquid in the pipe F G H, with an upturned nozzle I L at E.]]

full as from C D to F G, ye water will then run swift enough and will push with ye same impression that wch is in G H D E, and by ye motion it will have acquired in ye way, it will be carried swift enough to ye entrance of ye passage I L wch it will entirely fill, and being shoced  by that wch succeeds it will be elevated to 2 foot, but when retaining it, it is [[?stoped]] in its motion it will doth flow back towards B G D being elevated forwards ye top of ye pipe near to C, wch causeth that this water being in  motion and its height in B being less than ye height of ye point L cannot make an effort to run or spout till its motion begins to be made according to its first running wch is very slow.

  We must avoid making ye pipes of ye conduit cut one another 

Transcription Notes:
mandc: Added a missing line. Revised image description. I think I have discovered why there is so much confusion over I's and J's in the ms script text and the illustrations. J was not a Latin letter. There was no letter J in some medieval English alphabets. So I cannot tell you if the scribe translating the Mariotte document was writing an I or a J, but the Desaguliers translation used all I's and no J's, so that is the convention I have used in transcribing, reviewing and amending image descriptions. https://en.wikipedia.org/wiki/J Desaguliers' diagram: http://echo.mpiwg-berlin.mpg.de/ECHOdocuView?url=/permanent/library/QERNH1MN/pageimg&viewMode=images&mode=imagepath&pn=283&ww=0.1558&wh=0.095&wx=0.7788&wy=0.432