A continuation of new experiments physico-mechanical, touching the spring and weight of the air and their effects. The I. part whereto is annext a short discourse of the atmospheres of consistent bodies / written by way of letter to the right honourable the Lord Clifford and Dungarvan by the honourable Robert Boyle ...

EXPERIMENT XXIV.

Shewing that the Pressure of the Atmosphere may be exercis'd enough to keep up the Mercury in the Torricellian Experiment, though the Air press upon it at a very small Orifice.

BY a very slight variation of the foregoing 22^th Experiment we may both confirm one of the most important and the least likely Truths of the Hydrostaticks, and remove an Objection, which, for want of the knowledg of this Truth, is wont to be urg'd against your Hypothesis even by Learned men. For divers of these, when they see the same Phaenomena happen in the Torricel∣lian Experiment, whether it be made in the open Air, or in a

Chamber, are forward to object, That if it were, as we say tis, the weight of the Air, incumbent on the stagnant Mercury, which keeps that suspended in the Tube from falling down, the Mercury would not be sustain'd at any thing near the same height in the open Air, where the Pillar that is suppos'd to lean upon the stag∣nant Mercury, may reach up to the top of the Atmosphere, as in a close room, where they imagine that no more Air can press upon it, than what reaches directly up to the Roof or Sealing. And when to this tis answer'd, that though if a Room were indeed ex∣actly clos'd, the Sustentation of the Mercury ought to be ascrib'd to some other cause than the weight of the Imprison'd Air, (which other Cause I have elsewhere shewn to be its Spring;) yet in ordinary Rooms there is still a Communication between the in∣ternal and external Air, either by the Chimney, or, if the Room have none, by some Crevice in the Window, or by some Chink between the Wall and the Door, or at least by the Key-hole. And when to this tis objected, that the Orifice of the Keyhole is much narrower than the Superficies of the stagnant Mercury, and consequently, though the Atmosphere were not reduc'd to press obliquely on the Mercury, yet, entring at so small an O∣rifice, it could not press sufficiently upon it; when, I say, in an∣swer to this Objection I have alleadg'd that Hydrostatical Theo∣reme, That the Pressure, in such cases as ours, is to be estimated by the heights of the Liquors and not the breadths, the Asserti∣on has been thought unlikely and precarious.

To confirm therefore this Hydrostatical Truth, one may take the bended Tube, mention'd in the 22^th Experiment; and incli∣ning it till the greatest part of the Mercury pass from the shorter Leg into the longer, the upper end of this shorter Leg may by the flame of a Lamp be drawn out so slender, that the Orifice of it shall not be above an 8^th or 10^th part (not to say a much lesse) as big as 'twas before. For this being done, and the Tube ere∣cted again, if the tall Cylinder of Mercury be of the usual or for∣mer height, as we have found it, 'twill appear congruous to our

Hypothesis, that the weight of the external Air may exercise as much Pression upon the stagnant Mercury through a little hole, as when all the upper Superficies of that Mercury was directly expos'd to it.

And if one have not the conveniency to draw out the shorter Leg as is prescrib'd, one may nevertheless make the Tryal, by carefully stopping up the Orifice with a Cork and Cement, lea∣ving onely (or afterwards making) a very small hole for the Air to pass in and out. If I had not wanted a fit Instrument, I would have tried to exemplifie the Truth of what has been delivered, by adding to the Glasses we imploy'd to make the V^th. Experi∣ment, such a Cover, as might be cemented on to the Edge of the Glass, having onely a very small hole in the midst, at which the Atmosphere would be reduc'd to exercise its Pressure; and the like Cover I would have made use of in the X^th Experiment, about the breaking of Glass-plates in the unexhausted Receiver, by the bare Spring of the Air.