Part 34

I have further taken notice, that not onely the Sun, Moon and Starrs, and high tops of mountains have suffer’d these kinds of refraction, but Trees, and several bright Objects on the ground: I have often taken notice of the twinkling of the reflections of the Sun from a Glass-window at a good distance, and of a Candle in the night, but that is not so conspicuous, and in observing the setting Sun, I have often taken notice of the tremulation of the Trees and Bushes, as well as of the edges of the Sun. Divers of these _Phænomena_ have been taken notice of by several, who have given several reasons of them, but I have not yet met with any altogether satisfactory, though some of their conjectures have been partly true, but partly also false. Setting my self therfore upon the inquiry of these _Phænomena_, I first endeavour’d to be very diligent in taking notice of the several particulars and circumstances observable in them; and next, in making divers particular Experiments, that might cleer some doubts, and serve to determine, confirm, and illustrate the true and adæquate cause of each; and upon the whole, I find much reason to think, that the true cause of all these _Phænomena_ is from the _inflection_, or _multiplicate refraction_ of those Rays of light within the body of the _Atmosphere_, and that it does not proceed from a _refraction_ caus’d by any terminating _superficies_ of the Air above, nor from any such exactly defin’d _superficies_ within the body of the _Atmosphere_.

This Conclusion is grounded upon these two Propositions:

First, that a _medium_, whose parts are unequally _dense_, and mov’d by various motions and transpositions as to one another, will produce all these visible effects upon the Rays of light, without any other _coefficient_ cause.

Secondly, that there is in the Air or _Atmosphere_ such a variety in the constituent parts of it, both as to their _density_ and _rarity_, and as to their divers mutations and positions one to another.

By _Density_ and _Rarity_, I understand a property of a transparent body, that does either more or less refract a Ray of light (coming obliquely upon its superficies out of a third _medium_) toward its perpendicular: As I call Glass a more dense body then Water, and Water a more rare body then Glass, because of the refractions (more or less deflecting towards the perpendicular) that are made in them, of a Ray of light out of the Air that has the same inclination upon either of their superficies.

So as to the business of Refraction, spirit of Wine is a more _dense_ body then Water, it having been found by an accurate Instrument that measures the angles of Refractions to Minutes that for the same refracted angle of 30°.00′. in both those _Mediums_, the angle of incidence in Water was but 41°.35′. but the angle of the incidence in the trial with spirit of Wine was 42°.45′. But as to gravity, Water is a more _dense_ body then spirit of Wine, for the proportion of the same Water, to the same very well rectify’d spirit of Wine was, as 21. to 19.

So as to Refraction, Water is more Dense then Ice; for I have found by a most certain Experiment, which I exhibited before divers illustrious Persons of the _Royal Society_, that the Refraction of Water was greater then that of Ice, though some considerable Authors have affirm’d the contrary, and though the Ice be a very hard, and the Water a very fluid body.

That the former of the two preceding Propositions is true, may be manifested by several Experiments; As first, if you take any two liquors differing from one another in density, but yet such as will readily mix: as Salt Water, or Brine, & Fresh; almost any kind of Salt dissolv’d in Water, and filtrated, so that it be cleer, spirit of Wine and Water; nay, spirit of Wine, and spirit of Wine, one more highly rectify’d then the other, and very many other liquors; if (I say) you take any two of these liquors, and mixing them in a Glass Viol, against one side of which you have fix’d or glued a small round piece of Paper, and shaking them well together (so that the parts of them may be somewhat disturb’d and move up and down) you endeavour to see that round piece of Paper through the body of the liquors, you shall plainly perceive the Figure to wave, and to be indented much after the same manner as the limb of the Sun through a _Telescope_ seems to be, save onely that the mutations here, are much quicker. And if, in steed of this bigger Circle, you take a very small spot, and fasten and view it as the former, you will find it to appear much like the twinkling of the Starrs, though much quicker: which two _Phænomena_, (for I shall take notice of no more at present, though I could instance in multitudes of others) must necessarily be caus’d by an _inflection_ of the Rays within the terminating superficies of the compounded _medium_, since the surfaces of the transparent body through which the Rays pass to the eye, are not at all altered or chang’d.

This _inflection_ (if I may so call it) I imagine to be nothing else, but a _multiplicate refraction_, caused by the unequal _density_ of the constituent parts of the _medium_, whereby the motion, action or progress of the Ray of light is hindred from proceeding in a streight line, and _inflected_ or _deflected_ by a _curve_. Now, that it is a _curve_ line is manifest by this Experiment: I took a Box, such as ADGE, in the first _Figure_ of the 37. _Scheme_, whose sides ABCD, and EFGH, were made of two smooth flat plates of Glass, then filling it half full with a very strong solution of Salt, I filled the other half with very fair fresh water, then exposing the opacous side, DHGC, to the Sun, I observ’d both the _refraction_ and _inflection_ of the Sun beams, ID & KH, and marking as exactly as I could, the points, P, N, O, M, by which the Ray, KH, passed through the compounded _medium_, I found them to be in a _curve_ line; for the parts of the _medium_ being continually more dense the neerer they were to the bottom, the Ray _pf_ was continually more and more deflected downwards from the streight line.

This Inflection may be mechanically explained, either by Monsieur _Des Cartes_ principles by conceiving the Globuls of the third Element to find less and less resistance against that side of them which is downwards, or by a way, which I have further explicated in the Inquisition about Colours, to be from an obliquation of the pulse of light, whence the under part is continually promoted, and consequently refracted towards the perpendicular, which cuts the Orbs at right angles. What the particular Figure of the _Curve line_, describ’d by this way of light, is, I shall not now stand to examine, especially since there may be so many sorts of it as there may be varieties of the Positions of the _intermediate_ degrees of _density_ and _rarity_ between the bottom and the top of the inflecting Medium.

I could produce many more Examples and Experiments, to illustrate and prove this first Proposition, _viz._ that there is such a constitution of some bodies as will cause inflection. As not to mention those I have observ’d in _Horn_, _Tortoise-Shell_, _transparent Gums_, and _resinous Substances_: The _veins_ of Glass, nay, of melted _Crystal_, found, and much complained of by Glass-grinders, and others, might sufficiently demonstrate the truth of it to any diligent Observator.

But that, I presume, I have by this Example given proof sufficient (_viz. ocular demonstration_) to evince, that there is such a modulation, or bending of the rayes of light, as I have call’d _inflection_, differing both from _reflection_, and _refraction_ (since they are both made in the superficies, this only in the middle); and likewise, that this is able or sufficient to produce the effects I have ascribed to it.

It remains therefore to shew, that there is such a property in the Air, and that it is sufficient to produce all the above mentioned _Phænomena_, and therefore may be the principal, if not the only cause of them.

First, That there is such a property, may be proved from this, that the parts of the Air are some of them more condens’d, others more rarified, either by the differing heat, or differing pressure it sustains, or by the somewhat heterogeneous vapours interspers’d through it. For as the Air is more or less rarified, so does it more or less refract a ray of light (that comes out of a denser medium) from the perpendicular. This you may find true, if you make tryal of this Experiment.

Take a small Glass-bubble, made in the form of that in the second Figure of the 37. _Scheme_, and by heating the Glass very hot, and thereby very much rarifying the included Air, or, which is better, by rarifying a small quantity of water, included in it, into vapours, which will expel the most part, if not all the Air, and then sealing up the small neck of it, and letting it cool, you may find, if you place it in a convenient Instrument, that there will be a manifest difference, as to the refraction.

As if in this second Figure you suppose A to represent a small sight or hole, through which the eye looks upon an object, as C, through the Glass-bubble B, and the second sight L; all which remain exactly fixt in their several places, the object C being so cized and placed, that it may just seem to touch the upper and under edge of the hole L: and so all of it be seen through the small Glass-ball of rarified Air; then by breaking off the small seal’d neck of the Bubble (without at all stirring the sights, object, or glass) and admitting the external Air, you will find your self unable to see the utmost ends of the object; but the terminating rayes AE and AD (which were before refracted to G and F by the rarified Air) will proceed almost directly to I and H; which alteration of the rayes (seeing there is no other alteration made in the Organ by which the Experiment is tryed, save only the admission, or exclusion of the condens’d Air) must necessarily be caused by the variation of the _medium_ contain’d in the Glass B; the greatest difficulty in the making of which Experiment, is from the uneven surfaces of the bubble, which will represent an uneven image of the object.

Now, that there is such a difference of the upper and under parts of the Air is clear enough evinc’d from the late improvement of the _Torricellian_ Experiment, which has been tryed at the tops and feet of Mountains; and may be further illustrated, and inquired into, by a means, which some whiles since I thought of, and us’d, for the finding by what degrees the Air passes from such a degree of Density to such a degree of Rarity. And another, for the finding what pressure was requisite to make it pass from such a degree of Rarefaction to a determinate Density: Which Experiments, because they may be useful to illustrate the present Inquiry, I shall briefly describe.

[16] I took then a small Glass-pipe AB, about the bigness of a Swans quill, and about four foot long, which was very equally drawn, so that, as far as I could perceive, no one part was bigger then another: This Tube (being open at both ends) I fitted into another small Tube DE, that had a small bore just big enough to contain the small Pipe, and this was seal’d up at one, and open at the other, end; about which open end I fastned a small wooden box C with cement, so that filling the bigger Tube, and part of the box, with Quicksilver, I could thrust the smaller Tube into it, till it were all covered with the Quicksilver: Having thus done, I fastned my bigger Tube against the side of a wall, that it might stand the steadier, and plunging the small Tube cleer under the _Mercury_ in the box, I stopt the upper end of it very fast with cement, then lifting up the small Tube, I drew it up by a small pully, and a string that I had fastned to the top of the Room, and found the height of the _Mercurial Cylinder_ to be about twenty nine inches.

Then letting down the Tube again, I opened the top, and then thrust down the small Tube, till I perceived the Quicksilver to rise within it to a mark that I had plac’d just an inch from the top; and immediately clapping on a small piece of cement that I had kept warm, I with a hot Iron seal’d up the top very fast, then letting it cool (that both the cement might grow hard, and more especially, that the Air might come to its temper, natural for the Day I try’d the Experiment in) I observ’d diligently, and found the included Air to be exactly an Inch.

Here you are to take notice, that after the Air is seal’d up, the top of the Tube is not to be elevated above the superficies of the Quicksilver in the box, till the surface of that within the Tube be equal to it, for the Quicksilver (as I have elsewhere prov’d) being more heterogeneous to the Glass then the Air, will not naturally rise up so high within the small Pipe, as the superficies of the _Mercury_ in the box, and therefore you are to observe, how much below the outward superficies of the _Mercury_ in the box, that of the same in the Tube does stand, when the top being open, free ingress is admitted to the outward Air.

Having thus done, I permitted the _Cylinder_, or small Pipe, to rise out of the box, till I found the surface of the Quicksilver in the Pipe to be two inches above that in the box, and found the Air to have expanded it self but one sixteenth part of an inch; then drawing up the small pipe, till I found the height of the Quicksilver within to be four inches above that without, I observed the Air to be expanded only ⅐ of an inch more then it was at first, and to take up the room of 1⅐ inch: then I raised the Tube till the Cylinder was six inches high, and found the Air to take up 1²⁄₉ inches of room in the Pipe; then to 8, 10, 12. &c. the expansion of the Air that I found to each of which Cylinders are set down in the following Table; where the first row signifies the height of the _Mercurial Cylinder_; the next, the expansion of the Air; the third, the pressure of the _Atmosphere_, or the highest _Cylinder_ of _Mercury_, which was then neer thirty inches: The last signifies the force of the Air so expanded, which is found by substracting the first row of numbers out of the third; for having found, that the outward Air would then keep up the Quicksilver to thirty inches, look whatever of that height is wanting must be attributed to the Elater of the Air depressing. And therefore having the Expansion in the second row, and the height of the subjacent _Cylinder_ of _Mercury_ in the first, and the greatest height of the _Cylinder_ of _Mercury_, which of it self counterballances the whole pressure of the _Atmosphere_; by substracting the numbers of the first row out of the numbers of the third, you will have the measure of the _Cylinders_ so deprest, and consequently the force of the Air, in the several Expansions, registred.

The height of the The Expansion The height of The strength Cylinder of Mercury, of the Air. the Mercury of the Elater that, together with that counter- of the expanded the Elater of the ballanc’d the Air. included Air, Atmosphere. ballanced the pressure of the Atmosphere. ---------- ---------- ---------- ---------- 00 01 30 30 02 01¹⁄₁₆ 30 28 04 01⅐ 30 26 06 01²⁄₉ 30 24 08 01⅓ 30 22 10 01½ 30 20 12 01⅔ 30 18 14 01⅚ 30 16 16 02²⁄₂₇ 30 14 18 02⁴⁄₉ 30 12 20 03 30 10 22 03⁷⁄₉ 30 8 24 05⁷⁄₁₈ 30 6 25 06⅔ 30 5 26 08½ 30 4 26¼ 09½ 30 3¾ 26½ 10¾ 30 3½ 26¾ 13 30 3¼ 27 15½ 30 3

I had several other Tables of my Observations, and Calculations, which I then made; but it being above a twelve month since I made them; and by that means having forgot many circumstances and particulars, I was resolved to make them over once again, which I did _August_ the second 1661. with the very same Tube which I used the year before, when I first made the Experiment (for it being a very good one, I had carefully preserv’d it:) And after having tryed it over and over again; and being not well satisfied of some particulars, I, at last, having put all things in very good order, and being as attentive, and observant, as possibly I could, of every circumstance requisite to be taken notice of, did register my several Observations in this following Table. In the making of which, I did not exactly follow the method that I had used at first; but, having lately heard of Mr. _Townly_’s _Hypothesis_, I shap’d my course in such sort, as would be most convenient for the examination of that _Hypothesis_; the event of which you have in the latter part of the last Table.

The other Experiment was, to find what degrees of force were requisite to compress, or condense, the Air into such or such a bulk.

The manner of proceeding therein was this: I took a Tube about five foot long, one of whose ends was sealed up, and bended in the form of a _Syphon_, much like that represented in the fourth Figure of the 37. _Scheme_, one side whereof AD, that was open at A, was about fifty inches long, the other side BC, shut at B, was not much above seven inches long, then placing it exactly perpendicular, I pour’d in a little Quicksilver, and found that the Air BC was 6⅞ inches, or very near to seven; then pouring in Quicksilver at the longer Tube, I continued filling of it till the Air in the shorter part of it was contracted into half the former dimensions, and found the height exactly nine and twenty inches; and by making several other tryals, in several other degrees of condensation of the Air, I found them exactly answer the former _Hypothesis_.

But having (by reason it was a good while since I first made) forgotten many particulars, and being much unsatisfied in others, I made the Experiment over again, and, from the several tryals, collected the former part of the following Table: Where in the row next the left hand 24. signifies the dimensions of the Air, sustaining only the pressure of the _Atmosphere_, which at that time was equal to a _Cylinder_ of _Mercury_ of nine and twenty inches: The next Figure above it (20) was the dimensions of the Air induring the first compression, made by a _Cylinder_ of _Mercury_ 5³⁄₁₆ high, to which the pressure of the _Atmosphere_ nine and twenty inches being added, the elastick strength of the Air so comprest will be found 34³⁄₁₆, &c.

_A Table of the Elastick power of the Air, both Experimentally and Hypothetically calculated, according to its various Dimensions._

The dimensions The height The Mercurial The sum or What they of the included of the Cylinder difference ought to Air. Mercurial added, or of these be according Cylinder taken from two to the counter- the former. Cylinders. Hypothesis. pois’d by the Atmosphere. ---------- ---------- ---------- ---------- ---------- 12 29 + 29 = 58 58 13 29 + 24¹¹⁄₁₆ = 53¹¹⁄₁₆ 53⁷⁄₁₃ 14 29 + 20³⁄₁₆ = 49³⁄₁₆ 49⁵⁄₇ 16 29 + 14 = 43 43½ 18 29 + 9⅛ = 38⅛ 38⅔ 20 29 + 5³⁄₁₆ = 34³⁄₁₆ 34⅘ 24 29 0 = 29 29 48 29 − 14⅝ = 14⅜ 14½ 96 29 − 22⅛ = 6⅞ 7²⁄₈ 192 29 − 25⅝ = 3⅜ 3⅝ 384 29 − 27²⁄₈ = 1⁶⁄₈ 1⁷⁄₁₆ 576 29 − 27⅞ = 1⅛ 1⁵⁄₂₄ 768 29 − 28⅛ = 0⅞ 0[7¼]⁄₈ 960 29 − 28⅜ = 0⅝ 0[5⅘]⁄₈ 1152 29 − 28⁷⁄₁₆ = 0⁹⁄₁₆ 0¹⁰⁄₁₆