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43

DISCOURS II
Of ye Equilibrium of Fluid Bodies by ye Spring

The Air and flame act by their spring to make an equilibrium with other bodys ye spring of ye air is manifest by many experiments as in ye Barometers where it is mch dilated, and in ye wind guns where it is extreamly condensed;  but it is very difficult to explain well those dilations and condensations.  For to give some idea we may consider ye whole extent of ye air from ye top to ye bottom as a great heap of sponges or balls of cotton whereof ye heighest have their naturall extent, but ye lower being pressed by ye weight of ye upper are reduced into a very small thickness and that they regain their first dilatation, when they are discharged of ye weight of ye other.  According to ye hypothesis we may say that ye air at ye bottom makes an equilibrium by its spring with ye weight of all ye rest of ye air wherewith it is charged;  so that is that superior air becomes heavier or more of it, ye inferior air is a little more condensed and if ye upper becomes less heavy or if it is of a less extent, ye inferior is more dliated.  We may compare also ye spring of ye air, so ye spring (dacier steel) of wooll wch is pressed and is more contracted when it is charged with a greater weight, and wch is elevated and extended when one part of ye weight is taken away;  as we may say that ye spring of ye (steel) wooll being pressed and reduced to a certain figure by a weight, makes an equilibrium with that weight [[strikethrough]] makes [[/strikethrough]] we may say likewise that ye lowest air condensed as it is makes an equilibrium by its spring with all ye weight of ye atmosphere.

  We see by many experiments that ye condensation of ye air is made according to ye proportion of ye weights wherewith it is charged.  See one easy enough.  Take a crooked pipe of glass A B C stopped at ye [[strikethrough]] glass [[/strikethrough]] end C and open at ye other.  Pour into it a little mercury to ye horizontal height D E to ye end that ye inclosed air C E be neither more or less dilated than that wch is in ye other [[?branch]] [[strikethrough]] ye air should [[/strikethrough]] for if ye mercury was a little higher in one of ye branches than in ye other, ye air should be there less pressed.  It is necessary that E C be of a moderate height as of 12 inches as it is supposed in that figure and ye other D A as great as you please.  Ye mercury being therefore on both sides at ye same height towards D and E and ye air E C having no communication with that of B A pour by ye end A with a small Funnell of glass new mercury; takeing head that no air enter into ye space C E you will observe that ye mercury will mount by little and little towards C, and will condence ye air wch was in C E, and that if E F is six inches F G being an horizontal line, ye mercury shall be mounted in ye other branch to H if that point is distant 28 inches from ye point G, and that ye Barometers would be than at ye height of 28 inches in ye place of ye observation;  for if they were but at 27 1/2 then G H would be but 27 1/2:  Now in that estate ye air in F C is pressed by ye weight of ye atmosphere wch is supposed equal to that of 28 inches wch are in ye space G H; and by consequence is charged with a weight double to that where with ye air is charged wch is in ye place where ye experiment is made, and wch is like to that wch was in E C before it was condensed with ye weight of ye mercury G H:  It is therefore manifestly seen in that experiment that ye air E C will follow ye proportion of ye weights we may find ye same proposition in ye other experiments by making ye calculation in this sort;  we must take for ye first term ye summ of ye weights of ye atmosphere and of ye mercury wch shall be mounted heigher than ye bottom of ye air in ye branch E C.  For ye second term, ye weight of ye atmosphere that is to say 28 inches of mercury;  for ye third, ye distance E C and ye forth proportionable shall be ye space or height where ye inclosed air shall be reduced in ye pipe E C:  as if ye air was reduced only to ye space I C of 8 inches, we shall find that ye mercury shall be in ye other pipe only fourteen inches heigher than ye horizontall line I L.  Now these fourteen inches with ye 28 of ye atmosphere are 42, we may therefore say according to that rule as [[strikethrough]] 22 [[/strikethrough]] 42 inches is to 28 inches so ye extent 

Transcription Notes:
mandc: "D'acier" is French for steel. Changed a few typos, J to I, B to D, etc.