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· The Young GENTLEMAN and LADY'S PHILOSOPHY.

PART III.

PNE U M A TI CS:

OR, The Nature and Properties of the Air explained.

The Structure and Use of the AIR-PUMP, and

other Instruments.

- A N D
The Doctrine of Wind, Sound, Meteors, &c.

DIALOGUE I.
On the Nature, Form, and Magnitude of the

ATMOSPHERE, or Body of AIR.

Euphrosyne. VITELL, you are refulved to make a Philofopher

of me, I find; you have brought me into a new Field of Enquiry, and a very airy One too, Cleonicus.

Cleon, 'Indeed it is, Sister, literally lo; you are now to explore the Regions of the Air, and dwell, for some Time, amongst the Clouds.

Euphrof. That is, I suppose, in plain English, I must now inform myself of the Nature of the Air, and its

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Use in the System of the World. To this I shall address myself with great Readiness and Spirit; especially as I know of nothing in Nature that I have heard more about, and am less acquainted with, than this extraordinary unfeen Something, you call Air. — Pray, what Definition do you give to this Subject ?

Cleon. The Air is defined to be a fine, invisible, heavy, elastic, compressible Fluid, of a different Density, environing the Earth on every Side to an indefinite Height.

Euphrof. So many "Characters entering into a bare Definition of it seem to make it an important Subject; pray, what are its general Qualities and Uses ?

Cleon. I will tell you in a few Words.

The principal Property of the Air is its Weight or Gravity ; for though a small Portion of it be light, yet, considering the great Height to which this Body of Air extends, the Weight of any Column of Air upon a given Surface must be very great. For by some Experiments that you will see hereafter, you will be easily convinced, that the Pressure of a Column of Air upon a Square Inch only is equal to fifteen Pounds Weight.

Euphrof. Such an Experiment I shall gladly see; for, when you talk of such a Weight of the Air, I scarce know how to understand. you, as I never yet experienced any Weight or Pressure in the Air at all; nor should I have thought of any such Thing, had it not been for what I have heard you and other Gentlemen speak of, when you have been talking of the Air-Pump.

Cleon.Most People say, and think as you do ; they know little of this Matter for Want of the proper Means of being acquainted with it, and never fail of being wonderfully surprized, when they are told the Pressure of the Air upon the Surface of their Bodies amounts in general to at least 13 Ton Weight.

Euphrof. I don't wonder at their being surprized at such a strange Doctrine as this; for which Way is it possible a Person should sustain so prodigious a Pressure, and yet, at the same Time, be entirely insensible of it?

Cleon. This you will be satisfied of when you consider, that the Air is a fluid Body, and you will be taught hereafter to understand, that all Fluids press with

an equal Force every Way, as well upwards as downwards, Side-ways, and in all Directions you can conceive, and then, allowing that every square Inch upon a Man's Body sustains a Pressure of 15 Pounds, the Pressure upon the whole Surface will amount at least to 13,320 Pounds upon the Surface of a middle Size Man (as it is found by Computation) which is very nearly 14 Ton Weight.

Euphrof. This is a Subject I cannot at present dispute · with you; but if this be the Case, what I most wonder at, at present, is, how it comes to pass, that I am so far from apprehending any such great Weight from the usual Effects in other Cases, that in reality I find nothing at all of it.

Cleon. The Reason of this strange Phänomenon, if I may so call it, is this, that our Bodies, as well as all others, are filled with Air throughout, and the Spring of this internal Air is a Force juit equal to the Pressure of the Air without, and when two Forces equal to each other act in contrary Directions, they intirely destroy each others Effects; and any Body, being pressed with great Force from the ambient Air, is really in the same Case as if it was affected by no Pressure of the Air at all; of this too you will be made thoroughly sensible by Experiment.

Euphrof. The Knowlege of such mysterious Things will be a very agreeable Acquisition; but pray, by the Way, tell me how the Air, a Subitance which I can neither fee nor feel, can be so heavy a Body.

Cleon. The Weight of the Air arises from the same common Cause as the Weight of any other Body does, viz. from that Power, or Force in Nature which is usually called Attraćtion, or Gravitation. This Power, as Sir Ijaac Newton tells us, equally affects all the Parts of Matter, and produces in them a Tendency towards each other; and this Tendency or Force is that which we call the Weight of any Body. Thus, what we commonly call the Weight of a Stone is only its Endeavour to fall, or approach towards the Body of the Earth, and thus every Particle of Air endeavours equally to fall towards the same Surface of the Earth, and therefore the Sum of all those Forces, in all the Particles, make a considerable Sum Total of Weight, or Prefire in the Air.

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