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is well known to all that have taken any Pains to
enquire into the Modern Philosophy, that the said
Quicksilver that is in the Tube will stop about F,
at the height F I of 28, 29, 30, or 31 Inches
above the uppermost Superficies B D of the
Quicksilver that is in the Glass Vessel. Now
that this happens because the Air does press up-
on that part of the Superficies BD, that is out
of the Tube, as much as the Quicksilver within
does upon the Part CI, which is dire&ly under
the Tube, will appear from the following Rea.
sons.
* I. Because when the Pressure of the Air upon
the Quicksilver B D out of the Tube is greater
or less, that within the Tube does either rise or
fall, asis obvious in all the Barometers or Weather-
Glasses which are only made after this manner. ;
..^ II. This may be likewise deduced from thence,
that in case we'pour Water, Lye, or any other
heavy Liquor to the Height WĶ, upon the Quick-
silver BD, and so augment the Pressure with that
additional Weight, the Quicksilver at F will be
proportionably higher; and again lower, if we
draw the Water off by a Pipe or Crane,
by lessen the Pressure upon B D.

III. The same is very plain, if we cover the
whole with a long Glass Receiver, H GL, on
the Air-Pump, and by exhausting the Air in P,
or in the faid Receiver, from thence into the
empty Pump remove the Pressure, which this Air
made upon the Quicksilver BD; for then we
fhall see that the other in the Tube between
I and F, will descend to CI; or about as low as
that which is in the Glass out of the Tube, and
rise again to the same Height F, when we let
in the Air again to the Receiver, whereby the
Preffure upon the Superficies B D may be in-
çreased.

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Hence then it is plain, that while the Quickfilver stands thus ftill in the Barometer, and in the Glass Vellel in the open Air, every similar part of the Horizontal Superficies of the Quicksilver YX (which may be supposed to pass through the Mercury under the Orifice of the Tube O M) suffers a like Preflure ; because otherwise the QuickGlver would not remain at rest, but the Parts

of it that were more strongly presled, would recede downwards; and the Parts that were least pressed, would be compelled to ascend; which is sufficiently known from the Principles of Hydrostaticks : for which reason then, if one supposes the Part NQ to be equal to OM, both of 'em will undergo an equal Prçffure; for the Parts of the Quicksilver RNOS, and COMI, being of an equal Height, are likewise of equal Weight; and fince they are at rest, they must have the same perpendicular Pressure ; the Part RS, which is in the open Air, will be as much pressed by the perpendicular Column of Air TR SV, as the Part CI, which is in the Tube, by the incumbent Column of Quicksilver ZFC1. And to conclude; each part

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of every thing thaf has Air impending over it, fuffers as great a Pressure as if there were a Column of Quicksilver of 28, 29, 30, or 31 Inches upon it, according to the Heighth in which it is found at that time in the Barometer.

- Now, according to our Experiments, as well as those of others, Quicksilver is about fourteen times as heavy as the like quantity of Water; and so the Air presses as strongly upon every thing pver which it is impending, as if there were fourteen times twenty eight Inches, or reducing the same to Feet, as if there were 32 Feet of Water (taking it at the very lowest) lying upon it..

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5

SECT.

Sect. X. A Barometer of Water and Lye, and

. Some Experiments, Now that we may not be here mistaken in the Dedu&ion of Consequences, which often happens in Physical Enquiries (forasmuch as when we think to have deduced by good Arguments a fecond Phænomenon from a once made Experiment, we do not always find the matter of Fact to agree with our Thoughts ; since in the second Trial, other Causes may likewise intervene and co-operate, which we did not think of in the Deduction, as it happens to those that exercise themselves in such Enquiries more frequently than they could wish;) I therefore took a Tin Tube of 36 Foot in Length, but found, tho' it had been made with great Exactness, that it was not compleatly Wind-tight ; wherefore there was another Tube of Glass of about the same length prepared, in order to make it a Barometer of Water: This was fasten'd to a piece of Wood, and then tied to the Sail of a Wind-mill, and so let down perpendicalarly, its lower end being first stopp'd with a Cork and Bladder; after which, it was filled full of Water from above, stopping at every turn till the Air got above the Water : being full, it was after the same manner carefully stopp'd with a Cork and Bladder ; then the lower Orifice of the Tube that stood in the Water being open'd, the Water in the Tube immediately descended, but stood fțill at the Height of about 33 Feet, as the Quicksilver does in a Barometer, till the upper Orifice being likewife unstopt, and the Pressure of the external Air thereby admitted, the whole Mass of Water that was in the Tube suddenly fubfided into the Cistern. Thus this Experiment fhews the Agreement between the Matter of

Fact,

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Fact, and the Consequences that we have before deduced touching the proportionable Gravity of Water and Quicksilver ; namely, that Air presses upon all Bodies with the same Force as Water would, if it were incumbent on them about 33 Foot. Į

If any one should have a Mind to try the same Experiment, but had not the opportunity of procuring from proper Glass-Blowers such a Tube of 36 Foot in Length, he may, as we do, make use of the broken Necks of Bolt Heads or little Chymical Phiols, which being thrust into one another, may be joined with the Emplaftrum de Minio, or Red Lead, mix'd with Oil of Olives, and boil'd up to the Consistency of a Salve ; and putting a wet Bladder over it, bind it about with a small Packthread: This will make a Tube as perfe&ly Wind-tight for a while, and as good for the Purpose, as if it had been one whole piece. .

Another thing which must not be here past by, is, that the subsiding of the Water with an infinite Number of little Bladders, appeared ascending thro' the Water ; which did not proceed from the external Air, but from that which was in the Water; the Cause of which was, that by the sub{iding of the Water there was an empty space left above in the Tube, and consequently the Pressure upon the Water was remov'd; whereupon the Air that was in the Water, expanding it self, afcended just after the same manner as we see it happen in Warer, under the Bell of the Air-Pump, when the Air, that pressed upon it at first is exhausted.. . • They that desire to be entirely satisfied of what we here mention, may fill the Tube of the Barometer (Tab. XIII. Fig. 2.) A OM, with Water infead of Quicksilver, and place it in the Glass Veffel that is likewise filled with Water up to BD;

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then pumping the Air out of the Receiver HGL, they will see the Water subside from A to F, and lower, but in the mean while, numberless little Bubbles ascending in the Water for the Reasotis before-mencion'd; and that those Bubbles are really Air, and not Water it self, may appear, First, By letting the Air into the Bell again, because that the faid Air remaining above at A F, will hinder the Water from being pressed by the Air P, and rising higher in the Tube than F. Secondly, Because if you exhaust the Air that is in the Receiver at P any farther, the Air at A Fexpanding it self, will press the Water a great way beneath CI, or BD, where descending only by its ownWeight, it would have stoppd by it felf

. . Thirdly, For a farther Proof of the aforesaid Proposition, you may fee by taking away the Receiver HG L, and holding a Coal of Fire near the Air at AF, that the Water being rarified by the heat of the Coal, will be pressed down to ZF, which assoon as the Air at A F becomes cold, will ascend again.

I find these Particulars among my Notes upon this Experiment, to prove that it is not possible to make a lasting Barometer of Water, which would otherwise have a great many Advantages over those of Quicksilver. If inftead of Water one should take

Lye (which tho’it had itood six Years in the open Air, had never admitted any Air into it, at least as far as could be discover'd by the help of an Air-Pump) it might perhaps furnish us a useful Barometer, and in my opinion, even better than one of Water, out of which the Air has been driven by Boiling, because after a while the Air mingles it felf again with the Water.

I hope this Account will not be uncacceptable to such as do not understand the true Properties of the Barometer, tho’ it be now very common the rather, because what we have said above

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