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Micrographia by Robert Hooke

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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
sphaerical, 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 terrae_, 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 Sphaere
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 sphaerical
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 Eclypse, 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 _Coelestial_ 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.

* * * * *


Observ. LX. _Of the _Moon_._

Having a pretty large corner of the Plate for the seven Starrs, void, for
the filling it up, I have added one small _Specimen_ of the appearance of
the parts of the Moon, by describing a small spot of it, which, though
taken notice of, both by the Excellent _Hevelius_, and called _Mons
Olympus_ (though I think somewhat improperly, being rather a vale) and
represented by the Figure X. of the 38. _Scheme_, and also by the Learn'd
_Ricciolus_, who calls it _Hipparchus_, and describes it by the Figure Y,
yet how far short both of them come of the truth, may be somewhat perceiv'd
by the draught, which I have here added of it, in the Figure Z, (which I
drew by a thirty foot Glass, in _October_ 1664. just before the Moon was
half inlightned) but much better by the Reader's diligently observing it
himself, at a convenient time, with a Glass of that length, and much better
yet with one of threescore foot long, for through these it appears a very
spacious Vale, incompassed with a ridge of Hills, not very high in
comparison of many other in the Moon, nor yet very steep. The Vale it self
ABCD, is much of the figure of a Pear, and from several appearances of it,
seems to be some very fruitful place, that is, to have its surface all
covered over with some kinds of vegetable substances; for in all positions
of the light on it, it seems to give a much fainter reflection then the
more barren tops of the incompassing Hills, and those a much fainter then
divers other cragged, chalky, or rocky Mountains of the Moon. So that I am
not unapt to think, that the Vale may have Vegetables _analogus_ to our
Grass, Shrubs, and Trees; and most of these incompassing Hills may be
covered with so thin a vegetable Coat, as we may observe the Hills with us
to be, such as the short Sheep pasture which covers the Hills of
_Salisbury_ Plains.

Up and down in several parts of this place here describ'd (as there are
multitudes in other places all over the surface of the Moon) may be
perceived several kinds of pits, which are shap'd almost like a dish, some
bigger, some less, some shallower, some deeper, that is, they seem to be a
hollow _Hemisphere_, incompassed with a round rising bank, as if the
substance in the middle had been digg'd up, and thrown on either side.
These seem to me to have been the effects of some motions within the body
of the Moon, _analogus_ to our Earthquakes, by the eruption of which, as it
has thrown up a brim, or ridge, round about, higher then the Ambient
surface of the Moon, so has it left a hole, or depression, in the middle,
proportionably lower; divers places resembling some of these, I have
observ'd here in _England_, on the tops of some Hills, which might have
been caus'd by some Earthquake in the younger dayes of the world. But that
which does most incline me to this belief, is, first, the generality and
diversity of the Magnitude of these pits all over the body of the Moon.
Next, the two experimental wayes, by which I have made a representation of
them.

The first was with a very soft and well temper'd mixture of Tobacco-pipe
clay and Water, into which, if I let fall any heavy body, as a Bullet, it
would throw up the mixture round the place, which for a while would make a
representation, not unlike these of the Moon; but considering the state and
condition of the Moon, there seems not any probability to imagine, that it
should proceed from any cause _analogus_ to this; for it would be difficult
to imagine whence those bodies should come; and next, how the substance of
the Moon should be so soft; but if a Bubble be blown under the surface of
it, and suffer'd to rise, and break; or if a Bullet, or other body, sunk in
it, be pull'd out from it, these departing bodies leave an impression on
the surface of the mixture, exactly like these of the Moon, save that these
also quickly subside and vanish. But the second, and most notable,
representation was, what I observ'd in a pot of boyling Alabaster, for
there that powder being by the eruption of vapours reduc'd to a kind of
fluid consistence, if, whil'st it boyls, it be gently remov'd besides the
fire, the Alabaster presently ceasing to boyl, the whole surface,
especially that where some of the last Bubbles have risen, will appear all
over covered with small pits, exactly shap'd like these of the Moon, and by
holding a lighted Candle in a large dark Room, in divers positions to this
surface, you may exactly represent all the _Phaenomena_ of these pits in
the Moon, according as they are more or less inlightned by the Sun.

And that there may have been in the Moon some such motion as this, which
may have made these pits, will seem the more probable, if we suppose it
like our Earth, for the Earthquakes here with us seem to proceed from some
such cause, as the boyling of the pot of Alabaster, there seeming to be
generated in the Earth from some subterraneous fires, or heat, great
quantities of vapours, that is, of expanded aerial substances, which not
presently finding a passage through the ambient parts of the Earth, do, as
they are increased by the supplying and generating principles, and thereby
(having not sufficient room to expand themselves) extreamly condens'd, at
last overpower, with their _elastick_ properties, the resistence of the
incompassing Earth, and lifting it up, or cleaving it, and so shattering of
the parts of the Earth above it, do at length, where they find the parts of
the Earth above them more loose, make their way upwards, and carrying a
great part of the Earth before them, not only raise a small brim round
about the place, out of which they break, but for the most part
considerable high Hills and Mountains, and when they break from under the
Sea, divers times, mountainous Islands; this seems confirm'd by the
_Vulcans_ in several places of the Earth, the mouths of which, for the most
part, are incompassed with a Hill of a considerable height, and the tops of
those Hills, or Mountains, are usually shap'd very much like these pits, or
dishes, of the Moon: Instances of this we have in the descriptions of
_AEtna_ in _Sicily_, of _Hecla_ in _Iceland_, of _Tenerif_ in the
_Canaries_, of the several _Vulcans_ in _New-Spain_, describ'd by _Gage_,
and more especially in the eruption of late years in one of the _Canary_
Islands. In all of which there is not only a considerable high Hill raised
about the mouth of the _Vulcan_, but, like the spots of the Moon, the top
of those Hills are like a dish, or bason. And indeed, if one attentively
consider the nature of the thing, one may find sufficient reason to judge,
that it cannot be otherwise; for these eruptions, whether of fire, or
smoak, alwayes raysing great quantities of Earth before them, must
necessarily, by the fall of those parts on either side, raise very
considerable heaps.

Now, both from the figures of them, and from several other circumstances;
these pits in the Moon seem to have been generated much after the same
manner that the holes in Alabaster, and the _Vulcans_ of the Earth are
made. For first, it is not improbable, but that the substance of the Moon
may be very much like that of our Earth, that is, may consist of an earthy,
sandy, or rocky substance, in several of its superficial parts, which parts
being agitated, undermin'd, or heav'd up, by eruptions of vapours, may
naturally be thrown into the same kind of figured holes, as the small dust,
or powder of Alabaster. Next, it is not improbable, but that there may be
generated, within the body of the Moon, divers such kind of internal fires
and heats, as may produce such Exhalations; for since we can plainly enough
discover with a _Telescope_, that there are multitudes of such kind of
eruptions in the body of the Sun it self, which is accounted the most noble
AEtherial body, certainly we need not be much scandaliz'd at such kind of
alterations, or corruptions, in the body of this lower and less
considerable part of the universe, the Moon, which is only secundary, or
attendant, on the bigger, and more considerable body of the Earth. Thirdly,
'tis not unlikely, but that supposing such a sandy or mouldring substance
to be there found, and supposing also a possibility of the generation of
the internal _elastical_ body (whether you will call it air or vapours)
'tis not unlikely, I say, but that there is in the Moon a principle of
gravitation, such as in the Earth. And to make this probable, I think, we
need no better Argument, then the roundness, or globular Figure of the body
of the Moon it self, which we may perceive very plainly by the _Telescope_,
to be (bating the small inequality of the Hills and Vales in it, which are
all of them likewise shap'd, or levelled, as it were, to answer to the
center of the Moons body) perfectly of a Sphaerical figure, that is, all
the parts of it are so rang'd (bating the comparitively small ruggedness of
the Hills and Dales) that the outmost bounds of them are equally distant
from the Center of the Moon, and consequently, it is exceedingly probable
also, that they are equidistant from the Center of gravitation; and indeed,
the figure of the superficial parts of the Moon are so exactly shap'd,
according as they should be, supposing it had a gravitating principle as
the Earth has, that even the figure of those parts themselves is of
sufficient efficacy to make the gravitation, and the other two suppositions
probable: so that the other suppositions may be rather prov'd by this
considerable Circumstance, or Observation, then this suppos'd Explication
can by them; for he that shall attentively observe with an excellent
_Telescope_, how all the Circumstances, notable in the shape of the
superficial parts, are, as it were, exactly adapted to suit with such a
principle, will, if he well considers the usual method of Nature in its
other proceedings, find abundant argument to believe it to have really
there also such a principle; for I could never observe, among all the
mountainous or prominent parts of the Moon (whereof there is a huge
variety) that any one part of it was plac'd in such a manner, that if there
should be a gravitating, or attracting principle in the body of the Moon,
it would make that part to fall, or be mov'd out of its visible posture.
Next, the shape and position of the parts is such, that they all seem put
into those very shapes they are in by a gravitating power: For first, there
are but very few clifts, or very steep declivities in the ascent of these
Mountains; for besides those Mountains, which are by _Hevelius_ call'd the
_Apennine_ Mountains, and some other, which seem to border on the Seas of
the Moon, and those only upon one side, as is common also in those Hills
that are here on the Earth; there are very few that seem to have very steep
ascents, but, for the most part, they are made very round, and much
resemble the make of the Hills and Mountains also of the Earth; this may be
partly perceived by the Hills incompassing this Vale, which I have here
describ'd; and as on the Earth also, the middlemost of these Hills seems
the highest, so is it obvious also, through a good _Telescope_, in those of
the Moon; the Vales also in many are much shap'd like those of the Earth,
and I am apt to think, that could we look upon the Earth from the Moon,
with a good _Telescope_, we might easily enough perceive its surface to be
very much like that of the Moon.

Now whereas in this small draught, (as there would be multitudes if the
whole Moon were drawn after this manner) there are several little
Ebullitions, or Dishes, even in the Vales themselves, and in the
incompassing Hills also; this will, from this supposition, (which I have, I
think, upon very good reason taken) be exceeding easily explicable; for, as
I have several times also observ'd, in the surface of Alabaster so ordered,
as I before describ'd, so may the later eruptions of vapours be even in the
middle, or on the edges of the former; and other succeeding these also in
time may be in the middle or edges of these, &c. of which there are
Instances enough in divers parts of the body of the Moon, and by a boyling
pot of Alabaster will be sufficiently exemplifi'd.

To conclude therefore, it being very probable, that the Moon has a
principle of gravitation, it affords an excellent distinguishing Instance
in the search after the cause of gravitation, or attraction, to hint, that
it does not depend upon the diurnal or turbinated motion of the Earth, as
some have somewhat inconsiderately supposed and affirmed it to do; for if
the Moon has an attractive principle, whereby it is not only shap'd round,
but does firmly contain and hold all its parts united, though many of them
seem as loose as the sand on the Earth, and that the Moon is not mov'd
about its Center; then certainly the turbination cannot be the cause of the
attraction of the Earth, and therefore some other principle must be thought
of, that will agree with all the secundary as well as primary Planets. But
this, I confess, is but a probability, and not a demonstration, which (from
any Observation yet made) it seems hardly capable of, though how successful
future indeavours (promoted by the meliorating of Glasses, and observing
particular circumstances) may be in this, or any other, kind, must be with
patience expected.

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