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orld ons, nnuorel abo
our Strength and Understanding, let us take
SECT. II. First of the Air.
Sect. III. The Gravity and Elasticity of the Air.
The Diligence, or rather the good Fortune, of the Philosophers of the last Age, has brought to light two remarkable Discoveries, and which were entirely a Secret to all the Ancients, touching the Constitution of the Air ; namely its Gravity or Weight, and its Spring, called in Latin by the Modern Naturalists, Vis Elastica.
SECT. IV. An Experiment concerning the Gravity
of the Air.
For some thousand Years the Air was esteem'd to be a Body so light; that it would never descend like other Bodies, till the Invention of Barometers gave the first hint to Mankind; that the Air might likewise be a heavy Body.
And how greatly the Experiment of these Weather-Glasles has contributed to the chief Proofs of the Gravity of the Air, may be seen by the Suspension of the Quicksilver in those Tubes in many Cases, which is to be ascrib’d, first to its Elastic Faculty, and afterwards to its Gravity, which causes the said Faculty to exert itself; as will appear by what follows. · Wherefore, in order to prove dire&ly the Gravity and weight of the Air, this Method seems to afford the strongest Proof, or at least the cleareft and simplest : Take a Glass full of Air, and weigh it in a nice and exact Pair of Scales; then drawing out the Air as far as possible with an Air-Pump, and weigh it again, you will find that it was sensibly heavier before the Air was exhausted than it is afterwards. The hollow. Glass Balls which are commonly sold with the great kind of Air-Pumps, are very proper for such an Experiment, and bigger Glasses are yet more so.
I find in my Notes, that such a Ball or Bubble had lost with its Air, fixty two Grains of its Weight, which is more than sufficient to convince us of the Gravity of the Air. According as we make use of bigger or smaller Bubbles, this Difference will appear greater or less.
SECT. V. and VI. The Air's Elastick Faculty,
The Second Property, for the Knowledge of which we are beholden to the Discoveries of later Years, is the Elastick Power or Springiness of the Air; whereby its Parts, like Steel Springs chát are bent with Force, do continually endeavour to expand themselves; and so by their Separation from each other, to take up a larger Space, driving away and pressing on every Side, all that makes any Resistance to them. ..." • To prove this, many Experiments have been made by the Famous Boyle and others. The common Method of shewing it is by a little Bladder E (Tab. XIII. Fig. 1.) which is about as big as a large Goose Egg, when full blown. Squeeze the Bladder so as to leave but a very small quantity of Air in it: Then having tied the Neck close, hang it up by its String to the little Hook D, of the Glass Receiver A B C, which being laid on the Plate of the Air-Pump B A, if you exhaust the Air from the Receiver at F, which pass'd on the outside of the Bladder, the Spring of the Air in the Bladder will exere it self so, that the Bladder will swell as if it was strongly blown up with a Pipe. ..:.
And for a further Proof of this Elastick Power of the Air, several other Experiments; hereafter · quoted in the proper Places, may be serviceable.
SECT. VII. The Pressure of the Air. * Now that.Operation or Effe& which the Air has upon other Bodies, by this its Weight joined to the Expanding or Elastick Force of its Parts, is what the Maderns call the Pressure of the Air:
The surprising Strength of which is incredible to
Sec T. VIII. The Mistakes of some Atheists.
Now before we proceed any farther, let us answer these Men, who to defend their unhappy Notions, viz. That there is not much Wisdom requifite in the Direction of many Things about them, alledge, That most of those Things are either entirely at reft, or at least mov'd but very slowly, and think this a strong Argument for their Affertions, because when things are suppos'd to be without Motion, there does not seem much Wifdom not Power necessary to continue them in the State in which they are ; because a slow and languid Motion is known not to want so much Force and Dire&ion to prevent its doing Mischief, as that Motion which has more Velocity and Strength in it: And if this last be allow'd, the first carries a great deal of Probability with it, at least in the Minds of ignorant Persons : For several people sitting in a Chamber, for instance, are not sensible of any Force upon them from Powers operating externally; the Glass of the Windows, that is known to be so brittle, remains in the same Condition ; the Tapistry or Hangings of the Room immoveable ; not a Hair of their Head stirs; in short, every thing seems to them plainly enough to be in perfea Rest. Let 'em go abroad, and unless the Air be put into Motion by Winds or Storms, they meet with no violent Opposition, but every thing feems still and calm to them, excepting perhaps some uncommon Revolution or Changes, which, because they cannot easily trace the Causes, seem to be merely fortuitous; from whence they conclude, that at
such times they are Safe and Secure enough, and stand in need of no greater Power than they themselves are able to furnish for their own Defence.
This Mistake does oftentimes render the unhappy Atheists very easie for a while, and makes them Hatter themselves, that there is nothing about them which they need to fear. But in order to excite different Thoughts in them, and to make them apprehend Matters as they really are ; let them go on and Contemplate with us those great and terrible Powers, which, even at the very time that they think themselves to be in the surest Calm and Stillness, move continually round about them, and they continually live in the midst of 'em ; which Powers, if they were not most wonderfully restrained by an Equilibrium or Balance, (and so hinder'd from hurting us, and thereby only render'd insensible) would be able, as soon as ever that Equilibrium ceased to operate, in an instant of Time to crush us into Atoms.
SECT. IX. A Description of the Barometers; and
an Experiment of the Presure, and of the Weight
of the Air thereby : Now to the end that this may not appear to any one more marvellous than true; take a Glass Tube AO (Tab. XIII. Fig. 2.) of about three Foot in length, and of the bigness of a Goose or Swan's Quill, closed at A and open at 0; let it be filled with Quickfilver; then stopping the Orifice O with your Finger, turn it down into another Veffel of Quicksilver, as described here in the Glass BOD; then drawing your Finger away, the Quicksilver that is in the Tube will have an opportunity of finking down, some of it running to the other that is in the Glass. But it