is well known to all that have taken any Pains to enquire into the Modern Philofophy, that the faid Quickfilver that is in the Tube will ftop about F, at the height FI of 28, 29, 30, or 31 Inches above the uppermoft Superficies B D of the Quickfilver that is in the Glafs Veffel. Now that this happens because the Air does prefs upon that Part of the Superficies B D, that is out of the Tube, as much as the Quickfilver within does upon the Part CI, which is directly under the Tube, will appear from the following Reafons.

I. Because when the Preffure of the Air upon the Quickfilver B D out of the Tube is greater or lefs, that within the Tube does either rife or fall, as is obvious in all the Barometers or WeatherGlaffes which are only made after this manner.

II. This may be likewife deduced from thence, that in cafe we pour Water, Lye, or any other heavy Liquor to the Height WK, upon the Quickfilver B D, and fo augment the Preffure with that additional Weight, the Quickfilver at F will be proportionably higher; and again lower, if we draw the Water off by a Pipe or Crane, and thereby leffen the Preffure upon B D.

III. The fame is very plain, if we cover the whole with a long Glafs Receiver, H GL, on the Air-Pump, and by exhaufting the Air in P, or in the faid Receiver, from thence into the empty Pump remove the Preffure which this Air made upon the Quickfilver B D; for then we fhall fee that the other in the Tube between I and F, will defcend to C I, or about as low as that which is in the Glafs out of the Tube, and rife again to the fame Height F, when we let in the Air again to the Receiver, whereby the Preffure upon the Superficies BD may be increafed.

Hence

Hence then it is plain, that while the Quickfilver ftands thus ftill in the Barometer, and in the Glafs Vellel in the open Air, every fimilar part of the Horizontal Superficies of the Quickfilver YX (which may be supposed to pass through the Mercury under the Orifice of the Tube O M) fuffers a like Preffure; because other wife the Quickfilver would not remain at reft, but the Parts of it that were more, ftrongly preffed, would recede downwards; and the Parts that were leaft preffed, would be compelled to afcend; which is fufficiently known from the Principles of Hydroftaticks: for which reafon then, if one fuppofes the Part NQ to be equal to OM, both of 'em will undergo an equal Preffure; for the Parts of the Quickfilver RNQS, and COMI, being of an equal Height, are likewife of equal Weight; and fince they are at reft, they must have the fame perpendicular Preffure; the Part R S, which is in the open Air, will be as much preffed by the perpendicular Column of Air T R SV, as the Part C I, which is in the Tube, by the incumbent Column of Quickfilver ZFCI. And to conclude; each part of every thing that has Air impending over it, fuffers as great a Preffure as if there were a Column of Quickfilver 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 thofe of others, Quickfilver is about fourteen times as heavy as the like quantity of Water; and fo the Air preffes as ftrongly upon every thing over which it is impending, as if there were fourteen times twenty eight Inches, or reducing the fame to Feet, as if there were 32 Feet of Water (taking it at the very loweft) lying upon it.

SECT.

SECT. X. A Barometer of Water and Lye, and fome Experiments.

Now that we may not be here miftaken in the Deduction of Confequences, which often happens in Phyfical Enquiries (forafmuch 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; fince in the fecond Trial, other Causes may likewife intervene and co-operate, which we did not think of in the Deduction, as it happens to thofe that exercise themselves in fuch Enquiries more frequently than they could wifh;) I therefore took a Tin Tube of 36 Foot in Length, but found, tho' it had been made with great Exactnefs, that it was not compleatly Wind-tight; wherefore there was another Tube of Glafs of about the fame length prepared, in order to make it a Barometer of Water: This was faften'd to a piece of Wood, and then tied to the Sail of a Wind-mill, and fo let down perpendicu larly, its lower end being first stopp'd with a Cork and Bladder; after which, it was filled full of Water from above, ftopping at every turn till the Air got above the Water: being full, it was after the fame manner carefully ftopp'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 defcended, but ftood ftill at the Height of about 33 Feet, as the Quickfilver does in a Barometer, till the upper Orifice being likewife unftopt, and the Preffure of the external Air thereby admitted, the whole Mafs of Water that was in the Tube fuddenly fubfided into the Ciftern. Thus this Experiment fhews the Agreement between the Matter of

Fact,

Fact, and the Confequences that we have before deduced touching the proportionable Gravity of Water and Quickfilver; namely, that Air preffes upon all Bodies with the fame Force as Water would, if it were incumbent on them about 33

Foot.

If any one fhould have a Mind to try the fame Experiment, but had not the opportunity of procuring from proper Glafs-Blowers fuch 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 Confiftency of a Salve; and putting a wet Bladder over it, bind it about with a fmall Packthread: This will make a Tube as perfectly Wind-tight for a while, and as good for the Purpofe, as if it had been one whole piece.

Another thing which must not be here paft by, is, that the fubfiding of the Water with an infinite Number of little Bladders, appeared afcending 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 fubfiding of the Water there was an empty space left above in the Tube, and confequently the Preffure upon the Water was remov'd; whereupon the Air that was in the Water, expanding it felf, afcended juft after the fame manner as we fee it happen in Water, under the Bell of the Air-Pump, when the Air that preffed upon it at first is exhausted.

They that defire to be entirely fatisfied of what we here mention, may fill the Tube of the Barometer (Tab. XIII. Fig. 2.) A OM, with Water inftead of Quickfilver, and place it in the Glass Veffel that is likewife filled with Water up to BD;

then

then pumping the Air out of the Receiver HGL, they will fee the Water fubfide from A to F, and lower, but in the mean while, numberlefs little Bubbles afcending in the Water for the Reafotis before-mention'd; and that thofe Bubbles are really Air, and not Water it felf, 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 preffed by the Air P, and rifing higher in the Tube than F. Secondly, Because if you exhauft the Air that is in the Receiver at P any farther, the Air at A Fexpanding it felf, will prefs the Water a great way beneath CI, or BD, where defcending only by its ownWeight, it would have ftopp'd by it felf. Thirdly, For a farther Proof of the aforefaid Propofition, you may fee by taking away the Receiver HGL, and holding a Coal of Fire near the Air at A F, that the Water being rarified by the heat of the Coal, will be preffed down to Z F; which affoon as the Air at A F becomes cold, will afcend again.

I find thefe Particulars among my Notes upon this Experiment, to prove that it is not poffible to make a lafting Barometer of Water, which would otherwife have a great many Advantages over thofe of Quickfilver. If inftead of Water one fhould take Lye (which tho' it had stood fix Years in the open Air, had never admitted any Air into it, at leaft 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 fuch as do not understand the true Properties of the Barometer, tho' it be now very common; the rather, becaufe what we have faid above (namely,