The works of the Honourable Robert Boyle, Esq., epitomiz'd by Richard Boulton ... ; illustrated with copper plates.

EXPERIEMINT XVIII. The Variation of the Rise of the Mercury in the Glass-Cylinder, and the Reason of it consider'd.

HAving fill'd a Tube about 3 foot long, with Mercury, and plac'd it in a wooden Frame in the Window, I observ'd, the Mercury was sensibly depress'd in hot Weather, by the Ex∣pansion of the Air, which swam about it, and rais'd again in Cold; the Altitude very often varying without any manifest Cause: So that in five Weeks time it had ascended and descended about two Inches; the utmost descent below the Altitude of it's first Suspension, being 9/16 of an Inch; and it's utmost Descent being 7/16, and it is

not improbable, but that the Variation of the Altitude of the Mercury, would have been more considerable, had the Experiment been try'd in a longer Tube, and in the open Air.

And here it may be seasonable to take notice, that could there be any sensible Variation, ob∣serv'd in the Altitude of the Mereury upon the Ebbing and Flowing of the Sea; it would be of no small Moment in determining whether the Pressure made upon the Air, by the Moon, were any ways concern'd in causing the Ebbing and Flowing of it, and such like Phaenomena.

But to return to what we observ'd further: Having taken the Cylindrical Tube out of the wooden Frame, on a snowy day we observ'd, that the Mercury was rais'd Twenty nine Digits and three Quarters, above the Basis which it lean'd on.

If it should be ask'd, from whence these Va∣riations in the Altitude of the Mercury. pro∣ceeded? I shall offer the following Considera∣tions.

• ...* 1.1 First. That the Air above the Mercury, being very weak, and not able to make any strong Resistance to the Rising Mercury, it may be esteem'd a Cause, why it rises no higher, be∣cause the Asmosphere is able to sustain no more of it; forasmuch as the Mercury and the At∣mosphere are ballanc'd in an Aequilibrium; for the Resistance which the Air above the Mer∣cury can make, is so small, that it would rise but very little higher, were there none at all in the Top of the Tube.

• Secondly, We may consider, that the External Air is subject to many more Alterations and Changes, than the Internal contain'd in the Top of the Tube, the latter being subject to be wrought on only by Heat and Cold: But the former is subject to many and considerable Alterations, besides those observable in this Experiment, the Effect of its fainter Changes being evident by their Effects on bruis'd and feeble Bodies. And that there are considerable Changes in the Air,* 1.2 is further evident from what Kircherus, during his Stay in Malta observes concerning Mount Aetna, which he could see from that place on some days, tho' on others, which seem'd clear, the Air was so condensed, that he could not dis∣cern it; and thet the like Changes of the Air have sensibly alter'd the Prospects of several Pla∣ces, is too commonly known to need further Instances: And we our selves have often taken Notice of plentiful Steams and Exhalations in the Air, by the use of Telescopes, which could not otherwise be taken notice of, which after a Showr of Rain would presently disappear: And that such Steams do rise from the Earth, hath been observ'd by Miners, who are often too sen∣sible of Damps, which except timely prevented, make the Air so thick and muddy, as to put out their very Candles. And that the Thickness of the Air may contribute to the raising of the Mercury in the Pipe, appears from what hath been before observ'd in the Torrecellian Experi∣ment.

And since the External Air is liable to be al∣ter'd so many ways, by the Mixture of insensi∣ble Corpuscles of Matter, its Rarity and Densi∣ty

are sufficient to account for the several Varia∣tions in the Height of the suspended Mercury; since accordingly as the Air is Rarifi'd, its Pres∣sure in Bodying up the Mercury must accordingly be varied.

Notes

• * 1.1

  The Reasons why Mercu∣ry is not al∣ways equal∣ly suspend∣ed.
• * 1.2

  Considera∣ble Changes in the Qua∣lities of the Air.