The Young GENTLEMAN and LADY'S PHILOSOPHY. PART III. PNEUMATICS: OR, The Nature and Properties of the AIR explained. The Structure and Ufe of the AIR-PUMP, and other Inftruments. AND The Doctrine of Wind, Sound, Meteors, &c. DIALOGUE I. On the Nature, Form, and Magnitude of the WE Euphrofyne. ELL, you are refolved 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, Sifter, literally fo; you are now to explore the Regions of the Air, and dwell, for fome Time, amongft the Clouds. Euphrof. That is, I fuppofe, in plain English, I must now inform myfelf of the Nature of the Air, and its Ufe 1 Ufe in the Syftem of the World.To this I fhall addrefs myself with great Readinefs and Spirit; efpecially as I know of nothing in Nature that I have heard more about, and am lefs 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, invifible, heavy, elaftic, compreffible Fluid, of a different Denfity, environing the Earth on every Side to an indefinite Height. Euphrof. So many "Characters entering into a bare Definition of it feem to make it an important Subject; pray, what are its general Qualities and Ufes? 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, confidering 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 fome Experiments that you will fee hereafter, you will be eafily convinced, that the Preffure of a Column of Air upon a Square Inch only is equal to fifteen Pounds Weight. Euphrof. Such an Experiment I fhall gladly fee; for, when you talk of fuch a Weight of the Air, I fcarce know how to underftand. you, as I never yet experienced any Weight or Preffure in the Air at all; nor fhould I have thought of any fuch Thing, had it not been for what I have heard you and other Gentlemen fpeak of, when you have been talking of the Air-Pump. Cleon. Moft People fay, 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 furprized, when they are told the Preffure 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 furprized at fuch a ftrange Doctrine as this; for which Way is it poffible a Perfon fhould fuftain fo prodigious a Preffure, and yet, at the fame Time, be entirely infenfible of it? Cleon. This you will be fatisfied of when you confider, that the Air is a fluid Body, and you will be taught hereafter to understand, that all Fluids prefs with an an equal Force every Way, as well upwards as downwards, Side-ways, and in all Directions you can conceive, and then, allowing that every fquare Inch upon a Man's Body fuftains a Preffure of 15 Pounds, the Preffure 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 prefent difpute with you; but if this be the Cafe, what I moft wonder at, at prefent, is, how it comes to pafs, that I am so far from apprehending any fuch great Weight from the usual Effects in other Cafes, that in reality I find nothing at all of it. Cleon. The Reafon of this ftrange Phænomenon, if I may fo 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 Preffure 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 preffed with great Force from the ambient Air, is really in the fame Cafe as if it was affected by no Preffure of the Air at all; of this too you will be made thoroughly fenfible by Experiment. Euphrof. The Knowlege of fuch myfterious Things will be a very agreeable Acquifition; but pray, by the Way, tell me how the Air, a Subftance which I can neither fee nor feel, can be fo heavy a Body. Cleon. The Weight of the Air arifes from the fame common Cause as the Weight of any other Body does, viz. from that Power, or Force in Nature which is ufually called Attraction, or Gravitation. This Power, as Sir Ifaac 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 fame Surface of the Earth, and therefore the Sum of all thofe Forces, in all the Particles, make a confiderable Sum Total of Weight, or Preffure in the Air. Euphraf. If then the Particles of Air are like so many Stones, why then do they not fall like the Stones, and all abide upon the Surface of the Earth? Whereas, on the contrary, you tell me, that they fill a very great Space in the circumambient Regions. Cleon. In order to answer this Question, which you have very properly propofed, you must be acquainted with one other Power in Nature, by which the Parts of Matter, when they are at a certain very fmall Distance from each other, are made to repel or fly from each other; and this is what the Philofophers ufually call the repulfive Force, or Elafticity in the Parts of Matter. The Force which I before mentioned under the Title of Gravitation, is, with Refpect to the fmall Particles of Matter, called the Attraction of Cohefion, or it is that divine Power in Nature which fo affects the Parts of Matter, that while they are in Contact, or can touch each other, they are by this Means made firmly to cohere or abide together, and according to the different Figures of the Parts of Matter, this Power will produce a greater or leffer Degree of the Force of Cohefion, which is the general Caufe of all Degrees of Hardnefs or Softnefs in Bodies, and what we ufually call Fixity and Fluidity. But more of this Subject hereafter. The Parts of Matter, as I faid before, when they are once beyond the Sphere of this attracting Force, are found to be ftrongly actuated by the repelling Force; and this is the Cafe of Air, the Parts of which do conftantly repel each other, and therefore those which are next to the Surface of the Earth will prevent the other Particles above them from coming fo near to it by their repulfive Force or Elafticity; and this is the second great Property of the Air. Euphrof. By this fecond Property of the Air, which you call Elafticity, I fuppofe, if I understand you right, the Parts of Air are kept at a Distance from each other; but, ftill it is not clear to me how this can be the Cafe; because of the great Weight of the Air, which one would think fhould precipitate thofe Particles at once down to the Surface of the Earth. Pray, why is not this the Cafe? Cleon. Because the Elafticity of the Air is a Force fu perior to that of Gravitation; or in other Words, the repelling Force in the Parts of Air which keeps them afunder afunder is greater than the attracting Force between the Earth and thofe Particles by which they tend toward the Earth, and at a proper Time you will underftand, that this repulfive Force is the ftrongest Power in Nature; that thefe two Principles an attracting and repelling Force are the efficient natural Caufes of all the Appearances in Nature; and that the Properties, Qualities and Effects of all Bodies entirely refult from them. Euphrof. But if the Parts of Air do repel each other, as you. fay, with fo great a Force, how comes it to pass that we have any Air at all? Why does not this elastic Force drive all the Particles of Air quite away? What do you find confines them hovering round this Ball of Earth? Clean. Your Queftion is à propos, Sifter; but ftill you are to confider, that the Power of Gravity is very confiderable, though not entirely equal to the Elafticity of Air, and prevents this latter Force from having its full Effect. Were the Power of Gravity for a Moment to be fufpended from Matter, and the Power of Elafticity to remain, the Atmosphere, or Body of Air, like a Parcel of Gun-powder fet on Fire, would inftantly be diffipated through the infinite Regions of Space; but by Means of Gravity, by far the greatest Part is detained near the Surface of the Earth, and the Distance between the Particles only leffened; and more fo, as the Particles of Air are nearer to the Earth. For at a greater Distance they are lefs affected by the Power of Gravity, and are less heavy, and therefore the Effect of Elafticity will be greater, or keep the Parts of Air at a greater Distance from each other; and thus it will be eafy to understand, that the farther you go from the Surface of the Earth, the greater the Distance will be found between the Particles of Air; and the nearer the Surface of the Earth, the lefs those Distances; and this leffer or greater Distance between the Parts of the Air is ufually called its Denfity, or Rarity; for the less the Diftance is, the greater will be the Number of Particles in the fame Space, and the Air is in such a Cafe, faid to be more denfe: On the contrary, the greater the Distance is between the Particles, the Air, in fuch a Cafe, is faid to be more rare; from whence it will appear, that the Denfity of the Air in any Part will be always in Proportion to the Weight of that above, and confequently, greatest |