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TAke a Glass-Tube of above 29 inches in length, as AB, closed at the end B, and open at A: fill it full of Quicksilver, and so close the end A, exactly with the thumb (as with a stoppel;) then reverse it, and putting it and your finger together into the wooden vessel D, fill'd about two inches deep with Quicksilver, erect it per∣pendicularly therein; then drawing away your finger from the orifice, your shall see a great part of the
Quicksilver in the Tube to make a quick and smart de∣scent into the external Quicksilver in the vessel; and after it hath, by several vibrations up and down, found out a certain point or degree, there to stand still and im∣moveable: so that all the upper part of the Tube (which the Mercury has deserted) viz. from E to B, will seem to be a vacuity.
The first Inventor of this noble Experiment, was Torricellius the eminent Mathematician, and deserved Successour to the famous Gallilaeo, to whom all the Common-wealth of Learning are exceedingly oblieg'd, because thereby he has excited the greatest modern Wits to higher and nobler Experiments.
In this Torricellian Experiment (for so we shall al∣wayes hereafter call it) let me give you notice of these rare Observables:
1. If the Tube be not longer then 29. inches, the Quicksilver will not at all descend: this we have tryed in several Tubes of 18, 21, and 26 ½ inches long.
2. In Tubes of a greater length then 29. inches, the Quicksilver will descend.
3. The Quicksilver will not descend lower then 29. inches, or thereabouts; that is, the Cylinder of Mercury in the Tube will alwayes be 29. inches in height above the superficies of the restagnant Mercury in the vessel.
4. The Quicksilver descends neither more nor less in Tubes of a greater or lesser Bore, provided they ex∣ceed the length of 29. inches.
5. How long soever the Tube be, the Quicksilver will fall down to its wonted pitch and stint of 29. inches or thereabouts; as we have tryed in Tubes of 32, 35 ½, 37, 45, and 50 inches in Longitude, and all of dif∣ferent Diameters and Bores.
6. If you add any more Quicksilver to that in the ves∣sel, then, that in the Tube rises proportionally the higher: and contrariwise, if you take any Quicksilver out of the vessel, that in the Tube descends lower; and so consequently, the internal Quicksilver in the Tube keeps alwayes the same height of that in the vessel.
7. That you may with great facility move the Tube to and fro in the vessel'd Quicksilver, but not draw it up towards the superficies of the external Quicksilver in the vessel without some reluctancy.
8. That if you tilt or incline the Glass-Tube, you shall see the Quicksilver gradually to ascend till it al∣most totally fill the Tube, at which Angle of Inclination the atletus or perpendicular will be equal to 29. inches, let the Tube be of what length soever.
9. That upon removal of your finger from the ori∣fice, you shall see the Quicksilver to make a very Quick and Smart descent six inches at least below the stan∣dard of its Altitude in the Glass of 45. inches long, and in others more or less; and after a few vibrations up and down, to settle at its wonted pitch and altitude of 29. inches, or thereabouts.
10. That if any thing, considerably hot or cold, be ap∣plyed to the Superiour part of the Tube, the Quicksilver therein will more or less ascend or descend, as the water in a Weather-glass, though with farr feebler and more insensible effects: So that any time of the year it will not much desert nor surmount the determinate height and pitch aforesaid of 29. inches.
11. That this seeming vacuity in the Tube would be judged by any one that came in at an adventure, to be nothing but such like illuminated ayr as this we breathe in.
12. If you dip your thumb into the vessel'd Mercury, and close the orifice of the Tube therin, and so gently re∣verse it, you shal see the Quicksilver in the Tube to move more swiftly (though not without resistance, and ebulli∣tions) through that seeming vacuity; and the Mercury will pass with such shoggs towards the depressed ex∣treme of the Tube, as will make you apprehend that the Tube will be either beaten out of your hand or broken: none of which Phaenomena will appear, if you let in the outward ayr into the cavity unpossessed by the Mercury. In which Interim of Motion, your thumb will be drawn and suck'd into the orifice of the Tube, not without some considerable pain.
13. If before the removal of your thumb you reim∣merge it again into the vessel'd Quicksilver as before, & then draw the Tube perpendicularly quite out of the vessell'd Quicksilver, the Quicksilver in the Tube will rise to the top of the Glass with such a violence as will indanger the knocking out of the head of the Glass, and then the ayr will pass by a speedy ebullition through the Quicksilver, and it will totally descend into the vessel. I once brake a Glass-tube of near forty inches long, by plucking it suddenly out of the vessel'd Mercury.
14. That you cannot so cautiously perform this Experiment in any Glass Tube whatsoever, but some little Air will be seen in the top of the Tube, when re∣versed, and before the removal of your Thumb, like the little Cap of Air in the obtuse end of an Egge; so that if you incline the Tube to what Angle soever (as in the eighth Observable aforesaid) the re-ascending Quicksilver will never totally and exactly fill the Tube, but a little Cap of Air will still stand in the top thereof.
15. That, use all the Artifice and Industry you can,
you cannot so cautiously fill the Tube, but that the Cy∣linder of Quicksilver will seem cragged and itched, and never purely smooth and polished, (though your Glass be never so smooth and dry, and your Quicksilver never so well purged) which interstices are filled up with Parti∣cles of Air that lurk 'twixt the Contiguities of the Glass and Quicksilver: and which after the descent of the Quicksilver do bubble up, and shoot themselves little by little into that seeming vacuity (as you may ocularly behold them) and doubtless are the occasion and hin∣drance why upon inclination of the Tube (as in 8. Ob∣serv.) the Quicksilver cannot totally replenish and fill the Tube again.
16. We filled a Tube of 27. inches with Quicksilver, and after inversion of it into a Vessel of Quicksilver, as in the Torricellian-Experiment we perceived, just upon retracti∣on of the finger, the little Particles of Air which re∣mained lurking between the sides of the Tube and the Quicksilver, on the suddain to become more visible, by a violent and rapid dilatation, flying out like so many little Springs wound up, and then all at once set at Li∣berty.
17. If you immerge the Tube into Vessels of Quick∣silver of several Capacities and larger Surfaces, the des∣cent of it will not alter.
18. Observe that the height of the Mercurial Cylin∣der, which here with us is found to be 29. inches at the least (if you order the Tube handsomely in filling of it) may seem greatly different from the French Observati∣ons, and those of Forrain Experimenters, as Parricellius himself, Doctor Pascal, Roberual, Doctor Pettit, and Pecquet, who all assign its Altitude to be but about 27. inches. To this I shall onely at present answer, that this
difference of the Mercurial Cylinder, may partly arise from the variations of the Climates, the Air being more thin and hot then ours, partly from the difference and altitude of the Atmosphere here and there, (as shall hereafter be made more intelligible) and partly from the diversity of our measures and theirs, or from the club and combination of all these causes joyned toge∣ther. To which I may well super-add, the negligence or inconsideration of those that try this Experiment; for you may alter the height of the Mercurial Cylinder, as you do rudely or cautiously tunnel in the Quick∣silver into the Tube; for I have some time with exact caution, made it to rise to 30. inches in altitude from the Surface of the restagnant Quicksilver in the Vessel. I set down 29. inches as its determinate height, to which it will for the most mount, though you use but a careless kind of carefulness in the management of the Experi∣ment.