curyup to 2 or scarce, Part of an Inch, or about of a Line which is of an Inch.

This being supposed, every one that ever faw a living Creature put under the Receiver of an Air-Pump, near which a Barometer was likewise placed, has been convinced with the utmost certainty, that long before the Mercury fubfided down to of a Line, that Creature would fall into Convulfions, and for the most part expire

too.

So that from hence, and other Experiments made by the Air-Pump (one of which we have quoted above in the XVIIth Contemplation from Mr. Otho Gueric, which had almost cost a Man his Life) it is plain enough, that if the Air in the Chamber at LM, bears no greater Preffure than that which happens to it from the Height of the Cieling ON, all the Creatures that live either upon the Earth, or in the Air, would immediately die in the faid Chamber. And that all Chambers and Houses would therefore be useless, were it not alone that this great Inconvenience is prevented by the aforefaid Law of Preffure, every way according to the Depth of the Fluid, to which all fluid Matters obey, and by which the Preffure in the fame Horizontal Plane P M is equally strong upon the like Parts PQ and LM, whether within or without the Room.

For the fame Reafon it would not be poffible that a Ship could go under a Bridge without occafioning a fudden Death to all that were therein. No Fish could even fwim under a Bridge without being in danger of lofing its Life, because the Air that is under a Bridge, would gravitate much less upon the Water; juft as it happens to fuch Fish that are put into the Air-Pump, when the Air is beginning to be exhaufted from thence, when the Rarefaction of the Air, and the Dimi

nution

nution of its Preffure upon the Water firft puts them into Convulfions, and foon after kills them; that the fame does not likewife happen under every Bridge, is alone owing to the Laws of Hydroftatics.

To this may be added, that the Air at L M in a Chamber undergoing fo fmall a Preffure, that it can hardly keep up the Mercury in the Barometer to of a Line, wou'd become fo thin, as to be uncapable of conveying Sounds to their Ears, of which we have already given Inftances in the XVII Contemplation; fo that tho' one fhould be able to live in fuch an Air, yet no Man could fpeak to another therein: Not to mention that Fire will not burn in fuch a thin Air, nor Smoak afcend; that none of the Particles which are the Object of Smelling, could pass from any Bodies to us, befides many other Things which wou'd be occafion'd by the Thinness of the Air.

If against this it fhould be objected, that altho' the Air in the Chamber undergoes fo little Preffure and Expansion, yet the more compreffed Air would run thither from P QRS; as Water it felf would do, tho' there is little or no Elafticity in it, if it were in the place of the Air. To which we anfwer, that this Objection has no other Foundation but the very Action of Gravity, and the Law of Preffure, &c. which is only peculiar to Fluids, which in this Cafe we do not fuppofe to obtain, fince we only endeavour to fhew what would happen if the Particles of Air operated by their Gravity, not like Fluid, but other folid Bodies.

To Illuftrate this Matter, let Tab. XXIX. Fig. 4. be a high Sand-bank (only confifting of folid Bodies for that Reason) and of the Figure reprefented here by ABCDMHN; 'tis plain then, that the Body G is preffed with the Sand

above it at E F CD, and if you will, with that on the fide of it at QR; but if there be in the fame Horizontal Plane B H, another Body equally as big as K, which is no more than a Hands breadth L S below the Sand, every one knows, that this K bears a fmall Preffure, and much lefs than the Body G, tho' all the Sand were contained in a Veffel equal to the whole Circumference of the aforefaid Bank; and therefore that a Man that were at G under this heavy Sand-Bank, would not be able to ftir from thence, whereas if he were at K, he could raife himself with little Trouble.

But now if inftead of this Sand there were a Veffel of liquid Matter in the very faid Form, the Body K would be preffed with equal Gravity as G, in Confequence of the Law of Preffure. From whence it is manifeft, that if we rightly diftinguish the Action of Solid from that of Fluid Bodies, this Objection will fall itself.

SECT. LXII. Convictions from the foregoing Obfer

vations.

AT least, without infifting upon any farther Particulars, it will be unquestionable to fuch as are verfed in the modern Natural Experiments, that without the Operation of this Law of Preffure, &c. in fluid Matters, Men would be entire ly deprived of the use of their Houses, and greateft Conveniences.

And this being fo, what Reward would not a Man have deferved that had invented a Method to hinder the fame, or that could have prefcribed fuch a Law to fluid Bodies? Would not every one, even an Atheist himself, think he was very much wronged, fuppofing he had only prevented all these Inconveniences after a much more imperfect manner, if instead of returning him the

Thanks

Thanks that were fo juftly due to him, People fhould look upon him as an ignorant or foo.ifh Fellow.

SECT. LXIII. Even Lead it felf will float upon the Water, by the Preffure thereof upwards.

BUT if we now turn our Eyes towards that wonderful Action of the Law of Preffure, &c. namely, the Pressure of Fluids upwards; there will here likewife be vifibly manifefted the Glory, Power, and Goodness of the fupream Director.

Now that all Liquids which have others on the fide of them, do exert a real Force which preffes upwards, has been fhewn before, and may appear likewife from the Experiment of Mr. Boyle, which we shall reprefent here below, after a cheap and eafie manner.

Could any Body who is unexperiencd in Hy droftatics eafily believe that a piece of Lead, which is fo much heavier than a like quantity of Water, fhould only by the Preffure thereof upwards, without being fupported by any other Matter, be kept floating and hinder'd from fubfiding? And yet we fee this happen, when there is no Water above the Lead, which by its Freffure might fink it down, and when the lateral Water is 13 or 14 times deeper than the thickness of the Lead.

They who defire to make an eafie Experiment thereof, may take a Tin Tube dab f, (Tab. XXVI. Fig. 1) the lower Orifice of which, a b, is smooth and even; then taking a round piece of Lead abm n, the thickness of which is about or of an Inch, and its Breadth ab fuch, as being laid upon the Mouth of the faid Tin Tube a b, may Hop the fame; let there be likewise a little Hook

e, fixed

e, fixed as near as poffible in the middle of the Lead, and a String e k tied to it. Then covering the Lead a b with a little piece of Leather, or a wet Bladder with a Hole in the middle of it, to the end that the Hook and String may pafs through, you must draw therewith the faid Lead ABNM pretty tight against the Mouth a b of the Tin Tube a b df; then holding them thus together, let them down fuddenly into the Water to the Depth d m (which is about thirteen or fourteen times as much as the Thicknefs of the Lead am) in a large Veffel N T CO, in which the Water is at the Height of N O; you will then find that the little String e k, and confequently the piece of Lead a b n m being free, will not fink down at that place, but will be fupported by the Force of the Water preffing upwards, and perfectly fwim, according to the Experiments we have frequently made thereof.

Now that this happened only by the Preffure upwards, appears from hence, That if the Lead were not let down lower in the Water than about ten or eleven times its own Thickness, it would presently, upon loofing the little String e k, fubfide; but being placed much deeper under the Water, we found, that both the Tube and the String itfelf being loofened, the Lead did not only not fubfide, but even the whole Apparatus was lifted upwards, rifing and finking like a piece of Wood.

The reason whereof is clear enough, as well from what has been said before, as from the Law of Preffure, &c. for fince the Lead is about twelve times as heavy as the like Quantity of Water when it is brought to be thirteen times deeper than its own Thickness a m below the Water N O, and the Tube being held fast and still by the Hand, as the Lead is by the String, it is plain that the part VOL. III. PPP