Of these small Eels, which are to be found in divers sorts of Vinegar, I
have little to add besides their Picture, which you may find drawn in the
third Figure of the 25. _Scheme_: That is, they were shaped much like an
Eel, save only that their nose A, (which was a little more opacous then the
rest of their body) was a little sharper, and longer, in proportion to
their body, and the wrigling motion of their body seem’d to be onely
upwards and downwards, whereas that of Eels is onely side-wayes: They
seem’d to have a more opacous part about B, which might, perhaps, be their
Gills; it seeming always the same proportionate distant from their nose,
from which, to the tip of their tail, C, their body seem’d to taper.
Taking several of these out of their Pond of Vinegar, by the net of a small
piece of filtring Paper, and laying them on a black smooth Glass plate, I
found that they could wriggle and winde their body, as much almost as a
Snake, which made me doubt, whether they were a kind of Eal or Leech.
I shall add no other observations made on this minute Animal, being
prevented herein by many excellent ones already publish’d by the ingenious,
Doctor _Power_, among his _Microscopical_ Observations, save onely that a
quantity of Vinegar repleat with them being included in a small Viol, and
stop’d very close from the ambient air, all the included Worms in a very
short time died, as if they had been stifled.
And that their motion seems (contrary to what we may observe in the motion
of all other Infects) exceeding slow. But the reason of it seems plain, for
being to move to and fro after that manner which they do, by waving onely,
or wrigling their body; the tenacity, or glutinousness, and the density or
resistance of the fluid _medium_ becomes so exceeding sensible to their
extremely minute bodies, that it is to me indeed a greater wonder that they
move them so fast as they do, then that they move them no faster. For what
a vastly greater proportion have they of their superficies to their bulk,
then Eels or other larger Fishes, and next, the tenacity and density of the
liquor being much the same to be moved, both by the one and the other, the
resistance or impediment thence arising to the motions made through it,
must be almost infinitely greater to the small one then to the great. This
we find experimentally verify’d in the Air, which though a _medium_ a
thousand times more rarify’d then the water, the resistance of it to
motions made through it, is yet so sensible to very minute bodies, that a
Down-feather (the least of whose parts seem yet bigger then these Eels, and
many of them almost incomparably bigger, such as the quill and stalk) is
suspended by it, and carried to and fro as if it had no weight.
* * * * *
Observ. LVIII. _Of a new Property in the _Air_, and several other
transparent _Mediums_ nam’d _Inflection_, whereby very many considerable
_Phænomena_ are attempted to be solv’d, and divers other uses are hinted._
Since the Invention (and perfecting in some measure) of _Telescopes_, it
has been observ’d by several, that the Sun and Moon neer the Horizon, are
disfigur’d (losing that exactly-smooth terminating circular limb, which
they are observ’d to have when situated neerer the Zenith) and are bounded
with an edge every way (especially upon the right and left sides) ragged
and indented like a Saw: which inequality of their limbs, I have further
observ’d, not to remain always the same, but to be continually chang’d by a
kind of fluctuating motion, not unlike that of the waves of the Sea, so as
that part of the limb, which was but even now nick’d or indented in, is now
protuberant, and will presently be sinking again; neither is this all but
the whole body of the Luminaries, do in the _Telescope_, seem to be
depress’d and flatted, the upper, and more especially the under side
appearing neerer to the middle then really they are, and the right and left
appearing more remote: whence the whole _Area_ seems to be terminated by a
kind of Oval. It is further observ’d, that the body, for the most part,
appears red, or of some colour approaching neer unto it, as some kind of
yellow; and this I have always mark’d, that the more the limb is flatted or
ovalled, the more red does the body appear, though not always the contrary.
It is further observable, that both fix’d Stars and Planets, the neerer
they appear to the Horizon, the more red and dull they look, and the more
they are observ’d to twinkle; in so much, that I have seen the Dog-starr to
vibrate so strong and bright a radiation of light, as almost to dazle my
eyes, and presently, almost to disappear. It is also observable, that those
bright scintillations neer the Horizon, are not by much so quick and sudden
in their consecutions of one another, as the nimbler twinklings of Stars
neerer the Zenith. This is also notable, that the Starrs neer the Horizon,
are twinkled with several colours; so as sometimes to appear red, sometimes
more yellow, and sometimes blue, and this when the Starr is a pretty way
elevated above the Horizon. I have further, very often seen some of the
small Starrs of the fifth or sixth magnitude, at certain times to disappear
for a small moment of time, and again appear more conspicuous, and with a
greater luster. I have several times, with my naked eye, seen many smaller
Starrs, such as may be call’d of the seventh or eighth magnitude to appear
for a short space, and then vanish, which, by directing a small _Telescope_
towards that part they appear’d and disappear’d in; I could presently find
to be indeed small Starrs so situate, as I had seen them with my naked eye,
and to appear twinkling like the ordinary visible Stars; nay, in examining
some very notable parts of the Heaven, with a three foot Tube, me thought I
now and then, in several parts of the constellation, could perceive little
twinklings of Starrs, making a very short kind of apparition, and presently
vanishing, but noting diligently the places where they thus seem’d to play
at boe-peep, I made use of a very good twelve foot Tube, and with that it
was not uneasie to see those, and several other degrees of smaller Starrs,
and some smaller yet, that seem’d again to appear and disappear, and these
also by giving the same Object-glass a much bigger aperture, I could
plainly and constantly see appear in their former places; so that I have
observ’d some twelve several magnitudes of Starrs less then those of the
six magnitudes commonly recounted in the Globes.
It has been observ’d and confirm’d by the accuratest Observations of the
best of our modern Astronomers, that all the Luminous bodies appear above
the Horizon, when they really are below it. So that the Sun and Moon have
both been seen above the Horizon, whil’st the Moon has been in an Eclipse.
I shall not here instance in the great refractions, that the tops of high
mountains, seen at a distance, have been found to have; all which seem to
argue the Horizontal refraction, much greater then it is hitherto generally
believ’d.
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
From which Experiments, I think, we may safely conclude, that the Elater of
the Air is reciprocal to its extension, or at least very neer. So that to
apply it to our present purpose (which was indeed the chief cause of
inventing these wayes of tryal) we will suppose a _Cylinder_ indefinitely
extended upwards, [I say a _Cylinder_, not a piece of a _Cone_, because, as
I may elsewhere shew in the Explication of Gravity, that _triplicate_
proportion of the shels of a Sphere, to their respective diameters, I
suppose to be removed in this case by the decrease of the power of Gravity]
and the pressure of the Air at the bottom of this _Cylinder_ to be strong
enough to keep up a _Cylinder_ of _Mercury_ of thirty inches: Now because
by the most accurate tryals of the most illustrious and incomparable Mr.
_Boyle_, published in his deservedly famous Pneumatick Book, the weight of
Quicksilver, to that of the Air here below, is found neer about as fourteen
thousand to one: If we suppose the parts of the _Cylinder_ of the
_Atmosphere_ to be every where of an equal density, we shall (as he there
deduces) find it extended to the height of thirty five thousand feet, or
seven miles: But because by these Experiments we have somewhat confirm’d
the hypothesis of the reciprocal proportion of the Elaters to the
Extensions we shall find, that by supposing this _Cylinder_ of the
_Atmosphere_ divided into a thousand parts, each of which being equivalent
to thirty five feet, or seven geometrical paces, that is, each of these
divisions containing as much Air as is suppos’d in a _Cylinder_ neer the
earth of equal diameter, and thirty five foot high, we shall find the
lowermost to press against the surface of the Earth with the whole weight
of the above mentioned thousand parts; the pressure of the bottom of the
second against the top of the first to be 1000 − 1 = 999. of the third
against the second to be 1000 − 2 = 998. of the fourth against the third to
be 1000 − 3 = 997. of the uppermost against the 999. or that next below it,
to be 1000 − 999 = 1. so that the extension of the lowermost next the
Earth, will be to the extension of the next below the uppermost, as 1. to
999. for as the pressure sustained by the 999. is to the pressure sustain’d
by the first, so is the extension of the first to the extension of the 999.
so that, from this hypothetical calculation, we shall find the Air to be
indefinitely extended: For if we suppose the whole thickness of the Air to
be divided, as I just now instanced, into a thousand parts, and each of
those under differing Dimensions, or Altitudes, to contain an equall
quantity of Air, we shall find, that the first _Cylinder_, whose Base is
supposed to lean on the Earth, will be found to be extended 35³⁵⁄₉₉₉ foot;
the second equal Division, or _Cylinder_, whose _basis_ is supposed to lean
on the top of the first, shall have its top extended higher by 35⁷⁰⁄₉₉₈
the third 35¹⁰⁵⁄₉₉₇ the fourth 35¹⁴⁰⁄₉₉₆ and so onward, each equal
quantity of Air having its dimensions measured by 35. and some additional
number exprest alwayes in the manner of a fraction, whose numerator is
alway the number of the place multipli’d by 35. and whose denominator is
alwayes the pressure of the _Atmosphere_ sustain’d by that part, so that by
this means we may easily calculate the height of 999. divisions of those
1000. divisions, I suppos’d; whereas the uppermost may extend it self more
then as high again, nay, perhaps indefinitely, or beyond the Moon; for the
Elaters and Expansions being in reciprocal proportions, since we cannot yet
find the _plus ultra_, beyond which the Air will not expand it self, we
cannot determine the height of the Air: for since, as we have shewn, the
proportion will be alway as the pressure sustain’d by any part is to 35. so
1000. to the expansion of that part; the multiplication or product
therefore of the pressure, and expansion, that is, of the two extream
proportionals, being alwayes equal to the product of the means, or 35000.
it follows, since that Rectangle or Product may be made up of the
multiplication of infinite diversities of numbers, that the height of the
Air is also indefinite; for since (as far as I have yet been able to try)
the Air seems capable of an indefinite Expansion, the pressure may be
decreased in _infinitum_, and consequently its expansion upwards indefinite
also.
There being therefore such a difference of density, and no Experiment yet
known to prove a _Saltus_, or skipping from one degree of rarity to another
much differing from it, that is, that an upper part of the Air should so
much differ from that immediately _subjacent_ to it, as to make a distinct
superficies, such as we observe between the Air and Water, &c. But it being
more likely, that there is a continual increase of rarity in the parts of
the Air, the further they are removed from the surface of the Earth: It
will hence necessarily follow, that (as in the Experiment of the salt and
fresh Water) the ray of Light passing obliquely through the Air also, which
is of very different density, will be continually, and infinitely
inflected, or bended, from a streight, or direct motion.
This granted, the reason of all the above recited _Phænomena_, concerning
the appearance of the Celestial Bodies, will very easily be deduced. As,
First, The redness of the Sun, Moon, and Stars, will be found to be caused
by the inflection of the rays within the _Atmosphere_. That it is not
really in or near the luminous bodies, will, I suppose, be very easily
granted, seeing that this redness is observable in several places differing
in Longitude, to be at the same time different, the setting and rising Sun
of all parts being for the most part red:
And secondly, That it is not meerly the colour of the Air interpos’d, will,
I suppose, without much more difficulty be yielded, seeing that we may
observe a very great _interstitium_ of Air betwixt the Object, and the Eye,
makes it appear of a dead blew, far enough differing from a red, or yellow.
But thirdly, That it proceeds from the refraction, or inflection, of the
rays by the _Atmosphere_, this following Experiment will, I suppose,
sufficiently manifest.
Take a sphærical Crystalline Viol, such as is describ’d in the fifth Figure
ABCD, and, having fill’d it with pure clear Water, expose it to the Sun
beams; then taking a piece of very fine _Venice_ Paper, apply it against
that side of the Globe that is opposite to the Sun, as against the side BC,
and you shall perceive a bright red Ring to appear, caus’d by the
refraction of the Rays, AAAA, which is made by the Globe; in which
Experiment, if the Glass and Water be very cleer, so that there be no Sands
nor bubbles in the Glass, nor dirt in the Water, you shall not perceive any
appearance of any other colour. To apply which Experiment, we may imagine
the _Atmosphere_ to be a great transparent Globe, which being of a
substance more dense then the other, or (which comes to the same) that has
its parts more dense towards the middle, the Sun beams that are tangents,
or next within the tangents of this Globe, will be refracted or inflected
from their direct passage towards the center of the Globe, whence,
according to the laws of refractions made in a triangular _Prism_, and the
generation of colour set down in the description of Muscovi-glass there
must necessarily appear a red colour in the _transitus_ or passage of those
tangent Rays. To make this more plain, we will suppose (in the sixth
_Figure_) ABCD, to represent the Globe of the _Atmosphere_, EFGH to
represent the opacous Globe of the Earth, lying in the midst of it, neer to
which, the parts of the Air, sustaining a very great pressure, are thereby
very much condens’d, from whence those Rays that are by inflection made
tangents to the Globe of the Earth, and those without them, that pass
through the more condens’d part of the _Atmosphere_, as suppose between A
and E, are by reason of the inequality of the _medium_, inflected towards
the center, whereby there must necessarily be generated a red colour, as is
more plainly shewn in the former cited place; hence whatsoever opacous
bodies (as vapours, or the like) shall chance to be elevated into those
parts, will reflect a red towards the eye; and therefore those evenings and
mornings appear reddest, that have the most store of vapours and halituous
substances exhaled to a convenient distance from the Earth; for thereby the
inflection is made the greater, and thereby the colour also the more
intense; and several of those exhalations being opacous, reflect several of
those Rays, which, through an _Homogeneous_ transparent _medium_ would pass
unseen; and therefore we see, that when there chances to be any clouds
situated in those Regions they reflect a strong and vivid red. Now, though
one great cause of the redness may be this inflection, yet I cannot wholly
exclude the colour of the vapours themselves, which may have something of
redness in them, they being partly nitrous; and partly fuliginous; both
which steams tinge the Rays that pass through them, as is made evident by
looking at bodies through the fumes of _Aqua fortis_ or spirit of _Nitre_
[as the newly mentioned Illustrious Person has demonstrated] and also
through the smoak of a Fire or Chimney.
Having therefore made it probable at least, that the morning and evening
redness may partly proceed from this inflection or refraction of the Rays,
we shall next shew how the Oval Figure will be likewise easily deduced.
Suppose we therefore, EFGH in the sixth _Figure_ of the 37. _Scheme_, to
represent the Earth; ABCD, the _Atmosphere_; EI, and EL, two Rays coming
from the Sun, the one from the upper, the other from the neather Limb,
these Rays, being by the _Atmosphere_ inflected, appear to the eye at E, as
if they had come from the points, N and O; and because the Ray L has a
greater inclination upon the inequality of the _Atmosphere_ then I,
therefore must it suffer a greater inflection, and consequently be further
elevated above its true place, then the Ray I, which has a less
inclination, will be elevated above its true place; whence it will follow,
that the lower side appearing neerer the upper then really it is, and the
two _lateral_ sides, _viz._ the right and left side, suffering no sensible
alteration from the inflection, at least what it does suffer, does rather
increase the visible Diameter then diminish it, as I shall shew by and by,
the Figure of the luminous body must necessarily appear somewhat
_Elliptical_.
This will be more plain, if in the seventh _Figure_ of the 37. _Scheme_ we
suppose AB to represent the sensible Horizon; CDEF, the body of the Sun
really below it; GHIK, the same appearing above it, elevated by the
inflection of the _Atmosphere_: For if, according to the best observation,
we make the visible Diameter of the Sun to be about three or four and
thirty minutes, and the Horizontal refraction according to _Ticho_ be
thereabout, or somewhat more, the lower limb of the Sun E, will be elevated
to I; but because, by his account, the point C will be elevated but 29.
minutes, as having not so great an inclination upon the inequality of the
Air, therefore IG, which will be the apparent refracted perpendicular
Diameter of the Sun, will be less then CG, which is but 29. minutes, and
consequently six or seven minutes shorter then the unrefracted apparent
Diameter. The parts, D and F, will be likewise elevated to H and K, whose
refraction, by reason of its inclination, will be bigger then that of the
point C, though less then that of E; therefore will the semidiameter IL, be
shorter then LG, and consequently the under side of the appearing Sun more
flat then the upper.
Now, because the Rays from the right and left sides of the Sun, &c. have
been observ’d by _Ricciolo_ and _Grimaldus_, to appear more distant one
from another then really they are, though (by very many Observations that I
have made for that purpose, with a very good _Telescope_, fitted with a
divided Ruler) I could never perceive any great alteration, yet there being
really some, it will not be amiss, to shew that this also proceeds from the
refraction or inflection of the _Atmosphere_; and this will be manifest, if
we consider the _Atmosphere_ as a transparent Globe, or at least a
transparent shell, encompassing an opacous Globe, which, being more dense
then the _medium_ encompassing it, refracts or inflects all the entring
parallel Rays into a point or focus, so that wheresoever the Observator is
plac’d within the _Atmosphere_, between the focus and the luminous body,
the _lateral_ Rays must necessarily be more converg’d towards his eye by
the refraction or inflection, then they would have been without it; and
therefore the Horizontal Diameter of the luminous body must necessarily be
augmented.
This might be more plainly manifest to the eye by the sixth _Figure_; but
because it would be somwhat tedious, and the thing being obvious enough to
be imagin’d by any one that attentively considers it, I shall rather omit
it, and proceed to shew, that the mass of Air neer the surface of the
Earth, consists, or is made up, of parcels, which do very much differ from
one another in point of density and rarity; and consequently the Rays of
light that pass through them will be variously inflected, here one way, and
there another, according as they pass so or so through those differing
parts; and those parts being always in motion, either upwards or downwards,
or to the right or left, or in some way compounded of these, they do by
this their motion inflect the Rays, now this way, and presently that way.
This irregular, unequal and unconstant inflection of the Rays of light, is
the reason why the limb of the _Sun_, _Moon_, _Jupiter_, _Saturn_, _Mars_,
and _Venus_, appear to wave or dance; and why the body of the Starrs appear
to tremulate or twinkle, their bodies, by this means, being sometimes
magnify’d, and sometimes diminished; sometimes elevated, otherwhiles
depress’d; now thrown to the right hand, and then to the left.
And that there is such a property or unequal distribution of parts, is
manifest from the various degrees of heat and cold that are found in the
Air; from whence will follow a differing density and rarity, both as to
quantity and refraction; and likewise from the vapours that are interpos’d,
(which, by the way, I imagine, as to refraction or inflection, to do the
same thing, as if they were rarify’d Air; and that those vapours that
ascend, are both lighter, and less dense, then the ambient Air which boys
them up; and that those which descend, are heavier and more dense) The
first of these may be found true, if you take a good thick piece of Glass,
and heating it pretty hot in the fire, lay it upon such another piece of
Glass, or hang it in the open Air by a piece of Wire, then looking upon
some far distant Object (such as a Steeple or Tree) so as the Rays from
that Object pass directly over the Glass before they enter your eye, you
shall find such a tremulation and wavering of the remote Object, as will
very much offend your eye: The like tremulous motion you may observe to be
caus’d by the ascending steams of Water, and the like. Now, from the first
of these it is manifest, that from the rarifaction of the parts of the Air,
by heat, there is caus’d a differing refraction, and from the ascension of
the more rarify’d parts of the Air, which are thrust up by the colder, and
therefore more condens’d and heavie, is caus’d an undulation or wavering of
the Object; for I think, that there are very few will grant, that Glass, by
as gentle a heat as may be endur’d by ones hand, should send forth any of
its parts in steams or vapours, which does not seem to be much wasted by
that violent fire of the green Glass-house; but, if yet it be doubted, let
Experiment be further made with that body that is accounted, by Chymists
and others, the most ponderous and fix’d in the world; for by heating of a
piece of Gold, and proceeding in the same manner, you may find the same
effects.
This trembling and shaking of the Rays, is more sensibly caus’d by an
actual flame, or quick fire, or anything else heated glowing hot; as by a
Candle, live Coal, red-hot Iron, or a piece of Silver, and the like: the
same also appears very conspicuous, if you look at an Object betwixt which
and your eye, the rising smoak of some Chimney is interpos’d; which brings
into my mind what I had once the opportunity to observe, which was, the Sun
rising to my eye just over a Chimney that sent forth a copious steam of
smoak; and taking a short _Telescope_, which I had then by me, I observ’d
the body of the Sun, though it was but just peep’d above the Horizon, to
have its underside, not onely flatted, and press’d inward, as it usually is
when neer the Earth; but to appear more protuberant downwards then if it
had suffered no refraction at all; and besides all this, the whole body of
the Sun appear’d to tremble or dance, and the edges or limb to be very
ragged or indented, undulating or waving, much in the manner of a flag in
the Wind.
This I have likewise often observ’d in a hot Sunshiny Summer’s day, that
looking on an Object over a hot stone, or dry hot earth, I have found the
Object to be undulated or shaken, much after the same manner. And if you
look upon any remote Object through a _Telescope_ (in a hot Summer’s day
especially) you shall find it likewise to appear tremulous. And further, if
there chance to blow any wind, or that the air between you and the Object
be in a motion or current, whereby the parts of it, both rarify’d and
condens’d, are swiftly remov’d towards the right or left, if then you
observe the Horizontal ridge of a Hill far distant, through a very good
_Telescope_, you shall find it to wave much like the Sea, and those waves
will appear to pass the same way with the wind.
From which, and many other Experiments, ’tis cleer that the lower Region of
the Air, especially that part of it which lieth neerest to the Earth, has,
for the most part, its constituent parcels variously agitated, either by
heat or winds, by the first of which, some of them are made more rare, and
so suffer a less refraction; others are interwoven, either with ascending
or descending vapours; the former of which being more light, and so more
rarify’d, have likewise a less refraction; the latter being more heavie,
and consequently more dense, have a greater.
Now, because that heat and cold are equally diffus’d every way; and that
the further it is spread, the weaker it grows; hence it will follow, that
the most part of the under Region of the Air will be made up of several
kinds of _lentes_, some whereof will have the properties of _Convex_,
others of _Concave_ _glasses_, which, that I may the more intelligibly make
out, we will suppose in the eighth _Figure_ of the 37. _Scheme_, that A
represents an ascending vapour, which, by reason of its being somewhat
_Heterogeneous_ to the ambient Air, is thereby thrust into a kind of
Globular form, not any where terminated, but gradually finished, that is,
it is most rarify’d in the middle about A. somewhat more condens’d about
BB, more then that about CC; yet further, about DD, almost of the same
density with the ambient Air about EE;, and lastly, inclosed with the more
dense Air FF, so that from A, to FF, there is a continual increase of
density. The reason of which will be manifest, if we consider the rising
vapour to be much warmer then the ambient heavie Air; for by the coldness
of the ambient Air, the shell EE will be more refrigerated then DD, and
that then CC, which will be yet more then BB, and that more then A; so that
from F to A, there is a continual increase of heat, and consequently of
rarity; from whence it will necessarily follow, that the Rays of light will
be inflected or refracted in it, in the same manner as they would be in a
_Concave-glase_; for the Rays _GKI_, _GKI_ will be inflected by _GKH_,
_GKH_, which will easily follow from what I before explained concerning the
inflection of the _Atmosphere_.
On the other side, a descending vapour, or any part of the air included by
an ascending vapour, will exhibit the same effects with a _Convex lens_;
for, if we suppose, in the former Figure, the quite contrary constitution
to that last describ’d; that is, the ambient Air FF being hotter then any
part of that matter within any circle, therefore the coldest part must
necessarily be A, as being farthest remov’d from the heat, all the
intermediate spaces will be gradually discriminated by the continuall
mixture of heat and cold, so that it will be hotter at EE, then DD, in DD
then CC, in CC then BB, and in BB then A. From which, a like refraction and
condensation will follow, and consequently a lesser or greater refraction,
so that every included part will refract more then the including, by which
means the Rays, GKI, GKI, coming from a Starr, or some remote Object, are
so inflected, that they will again concurr and meet, in the point M. By the
interposition therefore of this desending vapour the visible body of the
Star, or other Object, is very much augmented, as by the former it was
diminished.
From the quick consecutions of these two, one after another, between the
Object and your eye, caused by their motion upwards or downwards,
proceeding from their levity or gravity, or to the right or left,
proceeding from the wind, a Starr may appear, now bigger, now less, then
really it would otherwise without them; and this is that property of a
Starr, which is commonly call’d twinkling, or scintillation.
The reason why a Star will now appear of one colour, now of another, which
for the most part happens when ’tis neer the Horizon, may very easily be
deduc’d from its appearing now in the middle of the vapour, other whiles
neer the edge; for if you look against the body of a Starr with a
_Telescope_ that has a pretty deep _Convex_ Eye-glass, and so order it,
that the Star may appear sometimes in one place, and sometimes in another
of it; you may perceive this or that particular colour to be predominant in
the apparent Figure of the Starr, according as it is more or less remote
from the middle of the _Lens_. This I had here further explain’d, but that
it does more properly belong to another place.
I shall therefore onely add some few Quæries, which the consideration of
these particulars hinted, and so finish this Section.
And the first I shall propound is, Whether there may not be made an
artificial transparent body of an exact Globular Figure that shall so
inflect or refract all the Rays, that, coming from one point, fall upon any
_Hemisphere_ of it; that every one of them may meet on the opposite side,
and cross one another exactly in a point; and that it may do the like also
with all the Rays that, coming from a _lateral_ point, fall upon any other
_Hemisphere_; for if so, there were to be hoped a perfection of
_Dioptricks_, and a transmigration into heaven, even whil’st we remain here
upon earth in the flesh, and a descending or penetrating into the center
and innermost recesses of the earth, and all earthly bodies; nay, it would
open not onely a cranney, but a large window (as I may so speak) into the
Shop of Nature, whereby we might be enabled to see both the tools and
operators, and the very manner of the operation it self of Nature; this,
could it be effected, would as farr surpass all other kind of perspectives
as the vast extent of Heaven does the small point of the Earth, which
distance it would immediately remove, and unite them, as ’twere, into one,
at least, that there should appear no more distance between them then the
length of the Tube, into the ends of which these Glasses should be
inserted: Now, whether this may not be effected with parcels of Glass of
several densities, I have sometimes proceeded so farr as to doubt (though
in truth, as to the general, I have wholly despair’d of it) for I have
often observ’d in Optical Glasses a very great variety of the parts, which
are commonly called Veins; nay, some of them round enough (for they are for
the most part, drawn out into strings) to constitute a kind of _lens_.
This I should further proceed to hope, had any one been so inquisitive as
to have found out the way of making any transparent body, either more dense
or more rare, for then it might be possible to compose a Globule that
should be more dense in the middle of it, then in any other part, and to
compose the whole bulk, so as that there should be a continual gradual
transition from one degree of density to another; such as should be found
requisite for the desired inflection of the _transmigrating_ Rays; but of
this enough at present, because I may say more of it when I set down my own
Trials concerning the melioration of _Dioptricks_, where I shall enumerate
with how many several substances I have made both _Microscopes_, and
_Telescopes_, and by what and how many, ways: Let such as have leisure and
opportunity farther consider it.
The next Quæry shall be, whether by the same collection of a more dense
body then the other, or at least, of the denser part of the other, there
might not be imagin’d a reason of the apparition of some new fix’d Stars,
as those in the Swan, _Cassiope’s Charr_, _Serpentarius_, _Piscis_,
_Cetus_, &c.
Thirdly, Whether it be possible to define the height of the _Atmosphere_
from this inflection of the Rays, or from the Quicksilver Experiment of the
rarifaction or extension of the Air.
Fourthly, Whether the disparity between the upper and under Air be not
sometimes so great, as to make a reflecting superficies; I have had several
Observations which seem to have proceeded from some such cause, but it
would be too long to relate and examine them. An Experiment, also somewhat
analogous to this, I have made with Salt-water and Fresh, which two
liquors, in most Positions, seem’d the same, and not to be separated by any
determinate superficies, which separating surface yet in some other
Positions did plainly appear.
And if so, Whether the reason of the equal bounding or _terminus_ of the
under parts of the clouds may not proceed from this cause; whether,
secondly, the Reason of the apparition of many Suns may not be found out,
by considering how the Rays of the Sun may so be reflected, as to describe
a pretty true Image of the body, as we find them from any regular
Superficies. Whether also this may not be found to cause the apparition of
some of those _Parelii_, of counterfeit Suns, which appear coloured, by
refracting the Rays so, as to make the body of the Sun appear in quite
another place then really it is. But of this more elsewhere.
5. Whether the _Phænomena_ of the Clouds may not be made out by this
diversity of density in the upper and under parts of the Air, by supposing
the Air above them to be much lighter then they themselves are, and they
themselves to be yet lighter then that which is subjacent to them, many of
them seeming to be the same substance with the Cobwebs that fly in the Air
after a Fog.
Now that such a constitution of the Air and Clouds, if such there be, may
be sufficient to perform this effect, may be confirm’d by this Experiment.
Make as strong a Solution of Salt as you are able, then filling a Glass of
some depth half full with it, fill the other half with fresh Water, and
poyse a little Glass-bubble, so as that it may sink pretty quick in fresh
Water, which take and put into the aforesaid Glass, and you shall find it
to sink till it comes towards the middle, where it will remain fixt,
without moving either upwards or downwards. And by a second Experiment, of
poising such a bubble in water, whose upper part is warmer, and
consequently lighter, then the under, which is colder and heavier; the
manner of which follows in this next Quæry, which is,
6. Whether the rarifaction and condensation of Water be not made after the
same manner, as those effects are produc’d in the Air by heat; for I once
pois’d a seal’d up Glass-bubble so exactly, that never so small an addition
would make it sink, and as small a detraction make it swim, which suffering
to rest in that Vessel of Water for some time, I alwayes found it about
noon to be at the bottom of the Water, and at night, and in the morning, at
the top: Imagining this to proceed from the Rarifaction of the Water,
caus’d by the heat, I made tryal, and found most true; for I was able at
any time, either to depress, or raise it, by heat and cold; for if I let
the Pipe stand for some time in cold water, I could easily raise the Bubble
from the bottom, whither I had a little afore detruded it, by putting the
same Pipe into warm Water. And this way I have been able, for a very
considerable time, to keep a Bubble so poys’d in the Water, as that it
should remain in the middle, and neither sink, nor swim: For gently heating
the upper part of the Pipe with a Candle, Coal, or hot Iron, till I
perceived the Bubble begin to descend, then forbearing, I have observed it
to descend to such or such a station, and there to remain suspended for
some hours, till the heat by degrees were quite vanished, when it would
again ascend to its former place. This I have also often observed naturally
performed by the heat of the Air, which being able to rarifie the upper
parts of the Water sooner then the lower, by reason of its immediate
contact, the heat of the Air has sometimes so slowly increased, that I have
observed the Bubble to be some hours in passing between the top and bottom.
7. Whether the appearance of the _Pike_ of _Tenerif_, and several other
high Mountains, at so much greater a distance then seems to agree with
their respective heights, be not to be attributed to the _Curvature_ of the
visual Ray, that is made by its passing obliquely through so differingly
_Dense_ a Medium from the top to the eye very far distant in the Horizon:
For since we have already, I hope, made it very probable, that there is
such an _inflection_ of the Rays by the differing density of the parts of
the Air; and since I have found, by several Experiments made on places
comparatively not very high, and have yet found the pressure sustain’d by
those parts of the Air at the top and bottom, and also their differing
Expansions very considerable: Insomuch that I have found the pressure of
the _Atmosphere_ lighter at the top of St. _Paul_’s Steeple in _London_
(which is about two hundred foot high) then at the bottom by a sixtieth or
fiftieth part, and the expansion at the top greater then that at the bottom
by neer about so much also; for the _Mercurial Cylinder_ at the bottom was
about 39. inches, and at the top half an inch lower; the Air also included
in the Weather-glass, that at the bottom fill’d only 155. spaces, at the
top fill’d 158. though the heat at the top and bottom was found exactly the
same with a scal’d _Thermometer_: I think it very rational to suppose, that
the greatest Curvature of the Rays is made nearest the Earth, and that the
inflection of the Rays, above 3. or 4. miles upwards, is very
inconsiderable, and therefore that by this means such calculations of the
height of Mountains, as are made from the distance they are visible in the
Horizon, from the supposal that that Ray is a straight Line (that from the
top of the Mountain is, as ’twere, a Tangent to the Horizon whence it is
seen) which really is a _Curve_, is very erroneous. Whence, I suppose,
proceeds the reason of the exceedingly differing Opinions and Assertions of
several Authors, about the height of several very high Hills.
8. Whether this Inflection of the Air will not very much alter the supposed
distances of the Planets, which seem to have a very great dependence upon
the Hypothetical refraction or inflection of the Air, and that refraction
upon the hypothetical height and density of the Air: For since (as I hope)
I have here shewn the Air to be quite otherwise then has been hitherto
suppos’d, by manifesting it to be, both of a vast, at least an uncertain,
height, and of an unconstant and irregular density; It must necessarily
follow, that its inflection must be varied accordingly: And therefore we
may hence learn, upon what sure grounds all the Astronomers hitherto have
built, who have calculated the distance of the Planets from their
Horizontal _Parallax_; for since the Refraction and _Parallax_ are so
nearly ally’d, that the one cannot be known without the other, especially
by any wayes that have been yet attempted, how uncertain must the
_Parallax_ be, when the Refraction is unknown? And how easie is it for
Astronomers to assign what distance they please to the Planets, and defend
them, when they have such a curious _subterfuge_ as that of Refraction,
wherein a very little variation will allow them liberty enough to place the
Celestial Bodies at what distance they please.
If therefore we would come to any certainty in this point, we must go other
wayes to work; and as I have here examined the height and refractive
property of the Air by other wayes then are usual, so must we find the
Parallax of the Planets by wayes not yet practiced; and to this end, I
cannot imagine any better way, then the Observations of them by two persons
at very far distant parts of the Earth, that lye as neer as may be under
the same Meridian, or Degree of longitude, but differing as much in
latitude, as there can be places conveniently found: These two persons, at
certain appointed times, should (as near as could be) both at the same
time, observe the way of the _Moon_, _Mars_, _Venus_, _Jupiter_, and
_Saturn_, amongst the fixt Stars, with a good large _Telescope_, and making
little Iconismes, or pictures, of the small fixed Stars, that appear to
each of them to lye in or near the way of the Center of the Planet, and the
exact measure of the apparent Diameter; from the comparing of such
Observations together, we might certainly know the true distance, or
Parallax, of the Planet. And having any one true Parallax of these Planets,
we might very easily have the other by their apparent Diameters, which the
_Telescope_ likewise affords us very accurately. And thence their motions
might be much better known, and their Theories more exactly regulated. And
for this purpose I know not any one place more convenient for such an
Observation to be made in, then in the Island of St. _Helena_, upon the
Coast of _Africk_, which lyes about sixteen degrees to the Southwards of
the Line, and is very near, according to the latest Geographical Maps, in
the same Meridian with _London_; for though they may not perhaps lye
exactly in the same, yet their Observations, being ordered according to
what I shall anon shew, it will not be difficult to find the true distance
of the Planet. But were they both under the same Meridian, it would be much
better.
And because Observations may be much easier, and more accurately made with
good _Telescopes_, then with any other Instruments, it will not, I suppose,
seem impertinent to explain a little what wayes I judge most fit and
convenient for that particular. Such therefore as shall be the Observators
for this purpose, should be furnished with the best _Telescopes_ that can
be had, the longer the better and more exact will their Observations be,
though they are somewhat the more difficultly manag’d. These should be
fitted with a _Rete_, or divided Scale, plac’d at such a distance within
the Eye-glass, that they may be distinctly seen, which should be the
measures of minutes and seconds; by this Instrument each Observator should,
at certain prefixt times, observe the Moon, or other Planet, in, or very
near, the Meridian; and because it may be very difficult to find two
convenient stations that will happen to be just under the same Meridian,
they shall, each of them, observe the way of the Planet, both for an hour
before, and an hour after, it arrive at the Meridian; and by a line, or
stroke, amongst the small fixed Stars, they shall denote out the way that
each of them observ’d the Center of the Planet to be mov’d in for those two
hours: These Observations each of them shall repeat for many dayes
together, that both it may happen, that both of them may sometimes make
their Observations together, and that from divers Experiments we may be the
better assured of what certainty and exactness such kind of Observations
are like to prove. And because many of the Stars which may happen to come
within the compass of such an _Iconism_, or Map, may be such as are only
visible through a good _Telescope_, whose Positions perhaps have not been
noted, nor their longitudes, or latitudes, any where remarked; therefore
each Observator should indeavour to insert some fixt Star, whose longitude,
and latitude, is known; or with his _Telescope_ he shall find the Position
of some notable _telescopical_ Star, inserted in his Map, to some known
fixt Star, whose place in the _Zodiack_ is well defin’d.
Having by this means found the true distance of the Moon, and having
observed well the _apparent Diameter_ of it at that time with a good
_Telescope_, it is easie enough, by one single Observation of the apparent
Diameter of the Moon with a good Glass, to determine her distances in any
other part of her _Orbit_, or _Dragon_, and consequently, some few
Observations will tell us, whether she be mov’d in an _Ellipsis_, (which,
by the way, may also be found, even now, though I think we are yet ignorant
of her true distance) and next (which without such Observations, I think,
we shall not be sure of) we may know exactly the bigness of that
_Ellipsis_, or Circle, and her true velocity in each part, and thereby be
much the better inabled to find out the true cause of all her Motions. And
though, even now also, we may, by such Observations in one station, as here
at _London_, observe the _apparent Diameter_ and motion of the Moon in her
_Dragon_, and consequently be inabled to make a better ghess at the
_Species_ or kind of Curve, in which she is mov’d, that is, whether it be
sphærical, or _elliptical_, or neither, and with what proportional
velocities she is carried in that Curve; yet till her true _Parallax_ be
known, we cannot determine either.
Next, for the true distance of the Sun, the best way will be, by accurate
Observations, made in both these forementioned stations, of some convenient
Eclipse of the Sun, many of which may so happen, as to be seen by both; for
the _Penumbra_ of the Moon may, if she be sixty Semidiameters distant from
the Earth, and the Sun above seven thousand, extend to about seventy
degrees on the Earth, and consequently be seen by Observators as far
distant as _London_, and St. _Helena_, which are not full sixty nine
degrees distant. And this would much more accurately, then any way that has
been yet used, determine the Parallax, and distance, of the Sun; for as for
the Horizontal Parallax I have already shewn it sufficiently uncertain; nor
is the way of finding it by the Eclipse of the Moon any other then
hypothetical; and that by the difference of the true and apparent
quadrature of the Moon is not less uncertain, witness their Deductions from
it, who have made use of it; for _Vendeline_ puts that difference to be but
4′.30″. whence he deduces a vast distance of the Sun, as I have before
shewn. _Ricciolo_ makes it full 30′.00. but _Reinoldus_, and _Kircher_, no
less then three degrees. And no wonder, for if we examine the _Theory_, we
shall find it so complicated with uncertainties.
First, From the irregular surface of the Moon, and from several Parallaxes,
that unless the _Dichotomy_ happen in the _Nonagesimus_ of the _Ecliptick_,
and that in the Meridian, &c. all which happen so very seldom, that it is
almost impossible to make them otherwise then uncertainly. Besides, we are
not yet certain, but that there may be somewhat about the Moon _analogus_
to the Air about the Earth, which may cause a refraction of the light of
the Sun, and consequently make a great difference in the apparent
_dichotomy_ of the Moon. Their way indeed is very rational and ingenious;
and such as is much to be preferred before the way by the Horizontal
Parallax, could all the uncertainties be remov’d, and were the true
distance of the Moon known.
But because we find by the Experiments of _Vendiline_, _Reinoldus_, &c.
that Observations of this kind are very uncertain also: It were to be
wisht, that such kind of Observations, made at two very distant stations,
were promoted. And it is so much the more desirable, because, from what I
have now shewn of the nature of the Air, it is evident, that the refraction
may be very much greater then all the Astronomers hitherto have imagined
it: And consequently, that the distance of the Moon, and other Planets, may
be much lesse then what they have hitherto made it.
For first, this Inflection, I have here propounded, will allow the shadow
of the Earth to be much shorter then it can be made by the other
_Hypothesis_ of refraction, and consequently, the Moon will not suffer an
Eclipse, unless it comes very much nearer the Earth then the Astronomers
hitherto have supposed it.
Secondly, There will not in this _Hypothesis_ be any other shadow of the
Earth, such as _Kepler_ supposes, and calls the _Penumbra_, which is the
shadow of the refracting _Atmosphere_; for the bending of the Rays being
altogether caus’d by _Inflection_, as I have already shewn, all that part
which is ascribed by _Kepler_, and others after him, to the _Penumbra_, or
dark part, which is without the _umbra terræ_, does clear vanish; for in
this _Hypothesis_ there is no refracting surface of the Air, and
consequently there can be no shadows, such as appear in the ninth Figure of
the 37. _Scheme_, where let ABCD represent the Earth, and EFGH the
_Atmosphere_, which according to _Keplers_ supposition, is like a Sphære of
Water terminated with an exact surface EFGH, let the lines MF, LB, ID, KH,
represent the Rays of the Sun; ’tis manifest, that all the Rayes between
LB, and ID, will be reflected by the surface of the Earth BAD, and
consequently, the conical space BOD would be dark and obscure; but, say the
followers of _Kepler_, the Rays between MF, and LB, and between ID, and KH,
falling on the _Atmosphere_, are refracted, both at their ingress and
egress out of the _Atmosphere_, nearer towards the Axis of the sphærical
shadow CO, and consequently, inlighten a great part of that former dark
Cone, and shorten, and contract, its top to N. And because of this
Reflection of these Rays, say they, there is superinduc’d another shell of
a dark Cone FPH, whose Apex P is yet further distant from the Earth: By
this _Penumbra_, say they, the Moon is Eclipsed, for it alwayes passes
between the lines 12, and 34.
To which I say, That if the Air be such, as I have newly shewn it to be,
and consequently cause such an inflection of the Rays that fall into it,
those dark _Penumbra’s_ FYZQ, HXVT, and ORPS, will all vanish. For if we
suppose the Air indefinitely extended, and to be no where bounded with a
determinate refracting surface, as I have shewn it uncapable of having,
from the nature of it; it will follow, that the Moon will no where be
totally obscured, but when it is below the Apex N, of the dark blunt Cone
of the Earth’s shadow: Now, from the supposition, that the Sun is distant
about seven thousand Diameters, the point N, according to calculation,
being not above twenty five terrestrial Semidiameters from the Center of
the Earth: It follows, that whensoever the Moon eclipsed is totally
darkned, without affording any kind of light, it must be within twenty five
Semidiameters of the Earth, and consequently much lower then any
Astronomers have hitherto put it.
This will seem much more consonant to the rest of the secundary Planets;
for the highest of _Jupiter’s_ Moons is between twenty and thirty _Jovial
Semidiameters_ distant from the Center of _Jupiter_; and the Moons of
_Saturn_ much about the same number of _Saturnial Semidiameters_ from the
Center of that Planet.
But these are but conjectures also, and must be determin’d by such kind of
Observations as I have newly mention’d.
Nor will it be difficult, by this _Hypothesis_, to salve all the
appearances of Eclipses of the Moon, for in this _Hypothesis_ also, there
will be on each side of the shadow of the Earth, a _Penumbra_, not caus’d
by the Refraction of the Air, as in the _Hypothesis_ of _Kepler_; but by
the faint inlightning of it by the Sun: For if, in the sixth Figure, we
suppose ESQ, and GSR, to be the Rays that terminate the shadow from either
side of the Earth; ESQ coming from the upper limb of the Sun, and GSR from
the under; it will follow, that the shadow of the Earth, within those Rays,
that is, the Cone GSE, will be totally dark. But the Sun being not a point,
but a large _area_ of light, there will be a secondary dark Cone of shadow
EPG, which will be caus’d by the earth’s hindring part of the Rays of the
Sun from falling on the parts GPR, and EPQ, of which halved shadow, or
_Penumbra_, that part will appear brightest which lyes nearest the
terminating Rayes GP, and EP, and those darker that lye nearest to GS, and
ES: when therefore the Moon appears quite dark in the middle of the
Eclipse, she must be below S, that is, between S and F; when she appears
lighter near the middle of the Eclipse, she must pass some where between RQ
and S; and when she is alike light through the whole Eclipse, she must pass
between RQ, and P.
* * * * *
Observ. LIX. _Of multitudes of small _Stars_ discoverable by the
_Telescope_._
Having, in the last Observation, premis’d some particulars observable in
the _medium_, through which we must look upon _Cœlestial_ Objects, I shall
here add one Observation of the Bodies themselves; and for a _specimen_ I
have made choice of the _Pleiades_, or seven Stars, commonly so called
(though in our time and Climate there appear no more then six to the naked
eye) and this I did the rather, because the deservedly famous _Galileo_,
having publisht a Picture of this _Asterisme_, was able, it seems, with his
Glass to discover no more then thirty six, whereas with a pretty good
twelve foot _Telescope_, by which I drew this 38 _Iconism_, I could very
plainly discover seventy eight, placed in the order they are ranged in the
Figure, and of as many differing Magnitudes as the _Asterisks_, wherewith
they are Marked, do specifie; there being no less then fourteen several
Magnitudes of those Stars, which are compris’d within the draught, the
biggest whereof is not accounted greater then one of the third Magnitude;
and indeed that account is much too big, if it be compared with other Stars
of the third Magnitude, especially by the help of a _Telescope_; for then
by it may be perceiv’d, that its splendor, to the naked eye, may be
somewhat augmented by the three little Stars immediately above it, which
are near adjoyning to it. The _Telescope_ also discovers a great variety,
even in the bigness of those, commonly reckon’d, of the first, second,
third, fourth, fifth, and sixth Magnitude; so that should they be
distinguish’d thereby, those six Magnitudes would, at least, afford no less
then thrice that number of Magnitudes, plainly enough distinguishable by
their Magnitude, and brightness; so that a good twelve foot Glass would
afford us no less then twenty five several Magnitudes. Nor are these all,
but a longer Glass does yet further, both more nicely distinguish the
Magnitudes of those already noted, and also discover several other of
smaller Magnitudes, not discernable by the twelve foot Glass: Thus have I
been able, with a good thirty six foot Glass, to discover many more Stars
in the _Pleiades_ then are here delineated, and those of three or four
distinct Magnitudes less then any of those spots of the fourteenth
Magnitude. And by the twinkling of divers other places of this _Asterisme_,
when the Sky was very clear, I am apt to think, that with longer Glasses,
or such as would bear a bigger _aperture_, there might be discovered
multitudes of other small Stars, yet inconspicuous. And indeed, for the
discovery of small Stars, the bigger the _aperture_ be, the better adapted
is the Glass; for though perhaps it does make the several specks more
radiant, and glaring, yet by that means, uniting more Rays very near to one
point, it does make many of those radiant points conspicuous, which, by
putting on a less _aperture_, may be found to vanish; and therefore, both
for the discovery of the fixt Star, and for finding the _Satellites_ of
_Jupiter_, before it be out of the day, or twilight, I alwayes leave the
Object-glass as clear without any _aperture_ as I can, and have thereby
been able to discover the _Satellites_ a long while before; I was able to
discern them, when the smaller _apertures_ were put on; and at other times,
to see multitudes of other smaller Stars, which a smaller _aperture_ makes
to disappear.
In that notable _Asterism_ also of the Sword of _Orion_, where the
ingenious Monsieur _Hugens van Zulichem_ has discovered only three little
Stars in a cluster, I have with a thirty six foot Glass, without any
_aperture_ (the breadth of the Glass being about some three inches and a
half) discover’d five, and the twinkling of divers others up and down in
divers parts of that small milky Cloud.
So that ’tis not unlikely, but that the meliorating of _Telescopes_ will
afford as great a variety of new Discoveries in the Heavens, as better
_Microscopes_ would among small terrestrial Bodies, and both would give us
infinite cause, more and more to admire the omnipotence of the Creator.
* * * * *