-----
Amongst the fixed stars too, Galileo pursued his enquiries. The
Milky-Way, which had so greatly puzzled the ancient Philosophers, and
which Aristotle imagined to be vapours risen to an extraordinary height,
he found to consist of an innumerable multitude of small stars; whose
light appears indistinct and confounded together to the naked eye. And
in every part of the heavens, his telescope shewed him abundance of
stars, not visible without it. In short, with such unabated ardour did
this great man range through the fields of Astronomy, that he seemed to
leave nothing for others to glean after him.
Nevertheless, by prodigiously encreasing the magnifying powers of their
telescopes, his followers made several great discoveries; some of which
I shall briefly mention. Mercury was found to become bisected, and
horned near its inferior conjunction, as well as Venus. Spots were
discovered in Mars, and from their apparent motion, the time of his
revolution on an axis nearly perpendicular to its orbit, was determined.
A sort of belts or girdles, of a variable or fluctuating nature, were
found to surround Jupiter, and likewise certain spots on his surface,
whence he was concluded to make one revolution in about ten hours on his
axis; which is likewise nearly perpendicular to his orbit. Five[A25]
moons or satellites were found to attend Saturn, which Galileo’s
telescope; on account of their prodigious distance, could not
reach:[A26] And the form of his ring was found to be a thin circular
plane, so situated as not to be far from parallel to the plane of our
equator; and always remaining parallel to itself. This ring, as well as
Saturn, evidently derives its light from the sun, as appears by the
shadows they mutually cast on each other.
-----
Footnote A25:
It has been since ascertained that Saturn has seven satellites, as is
more particularly mentioned in the subsequent note. W. B.
Footnote A26:
It was about six years after the delivery of this oration, (viz. on
the 13th of March, 1781,) that Herschel discovered the Georgium Sidus.
And nearly eight years and an half after this first discovery, he made
two others: on the 28th of August, 1789, he was enabled to ascertain,
by means of his telescope of forty feet focal length, that Saturn has
a sixth satellite; and, on the 17th of September following, he found
that he has a seventh. The same celebrated astronomer has since made
several important discoveries. Thus, under the liberal patronage of
his sovereign, has the great Herschel succeeded, by his extraordinary
skill and industry in the making of very large _specula_, in
constructing telescopes, which, in the words of the learned Mr. Vince,
“have opened new views of the heavens, and penetrated into the depths
of the universe; unfolding scenes which excite no less our wonder than
our admiration.”
Many important discoveries (some of which are noticed in the foregoing
pages of these memoirs) have been made by other eminent astronomers,
since the date of Dr. Rittenhouse’s Oration; some of them, indeed,
since his decease; among which are the discoveries of three new
planets. W. B.
-----
Besides several other remarkable appearances, which Hugenius[A27]
discovered amongst the fixed stars, there is one in Orion’s Sword,
which, I will venture to say, whoever shall attentively view, with a
good telescope and experienced eye, will not find his curiosity
disappointed. “Seven small stars, (says he,) of which three are very
close together, seemed to shine through a cloud, so that a space round
them appeared much brighter than any other part of heaven, which being
very serene and black looked here as if there was an opening, through
which one had a prospect into a much brighter region.” Here some have
supposed old night to be entirely dispossessed, and that perpetual
daylight shines amongst numberless worlds without interruption.
-----
Footnote A27:
The celebrated Huygens, who, in his Latin works, is styled _Hugenius_.
W. B.
-----
This is a short account of the discoveries made with the telescope. Well
might Hugenius congratulate the age he lived in, on such a great
acquisition of knowledge: And recollecting those great men, Copernicus,
Regiomontanus, and Tycho, so lately excluded from it by death, what an
immense treasure, says he, would they have given for it. Those ancient
philosophers too, Pythagoras, Democritus, Anaxagoras, Philolaus, Plato,
Hipparchus; would they not have travelled over all the countries of the
world, for the sake of knowing such secrets of nature, and of enjoying
such sights as these?
Thus have we seen the materials collected, which were to compose the
magnificent edifice of astronomical Philosophy; collected, indeed, with
infinite labour and industry, by a few volunteers in the service of
human knowledge, and with an ardour not to be abated by the weaknesses
of human nature, or the threatened loss of sight, one of the greatest of
bodily misfortunes! It was now time for the great master-builder to
appear, who was to rear up this whole splendid group of materials into
due order and proportion. And it was, I make no doubt, by a particular
appointment of Providence, that at this time the immortal Newton
appeared. Much had been done preparatory to this great work by others,
without which if he had succeeded, we should have been ready to
pronounce him something more than human. The doctrine of atoms had been
taught by some of the ancients. Kepler had suspected that the planets
gravitated towards each other, particularly the earth and moon; and that
their motion prevented their falling together: and Galileo first of all
applied geometrical reasoning to the motion of projectiles. But the
solid spheres of the ancients, or the vortices of Des Cartes,[A28] were
still found necessary to explain the planetary motions; or if Kepler had
discarded them, it was only to substitute something else in their stead,
by no means sufficient to account for those grand movements of nature.
It was Newton alone that extended the simple principle of gravity, under
certain just regulations, and the laws of motion, whether rectilinear or
circular, which constantly take place on the surface of this globe,
throughout every part of the solar system; and from thence, by the
assistance of a sublime geometry, deduced the planetary motions, with
the strictest conformity to nature and observation.
-----
Footnote A28:
Among the many eminent astronomers in the sixteenth and seventeenth
centuries, mentioned by Mr. Lalande, in his _Astronomie_, with
interesting particulars concerning most of them, the only notice he
there takes of his ingenious countryman, who endeavoured to establish
the theory of Vortices which he had projected, is in these words:
“Descartes (René,) né en Touraine en 1596, mort à Stockholm en 1650.
Sa vie a été écrite fort au long par Baillet, à Paris, 1691, in 4^o.”
W. B.
-----
Other systems of Philosophy have been spun out of the fertile brain of
some great genius or other; and for want of a foundation in nature, have
had their rise and fall, succeeding each other by turns. But this will
be durable as science, and can never sink into neglect, until “universal
darkness buries all.”
Other systems of Philosophy have ever found it necessary to conceal
their weakness, and inconsistency, under the veil of unintelligible
terms[A29] and phrases, to which no two mortals perhaps ever affixed the
same meaning: But the Philosophy of Newton disdains to make use of such
subterfuges; it is not reduced to the necessity of using them, because
it pretends not to be of nature’s privy council, or to have free access
to her most inscrutable mysteries; but to attend carefully to her works,
to discover the immediate causes of visible effects, to trace those
causes to others more general and simple, advancing by slow and sure
steps towards the great First Cause of all things.
-----
Footnote A29:
The philosophy of Aristotle retained terms so very obscure, that it
seems the Devil himself did not understand, or at least could not
explain them; otherwise we can hardly suppose, that, when the good
patriarch of Venice had summoned his attendance for this very purpose,
he would have been so rude as to put him off with an answer not only
unintelligible but inarticulate. See _Bayle, in Art. Barbaro._
-----
And now the Astronomy of our planetary system seemed compleated. The
telescope had discovered all the globes whereof it is composed, at least
as far as we yet know. Newton with more than mortal sagacity had
discovered those laws by which all their various, yet regular, motions
are governed, and reduced them to the most beautiful simplicity: laws to
which not only their great and obvious variety of motions are
conformable, but even their minute irregularities; and not only planets
but comets likewise. The busy mind of man, never satiated with
knowledge, now extended its views further, and made use of every
expedient that suggested itself, to find the relation that this system
of worlds bears to the whole visible creation. Instruments were made
with all possible accuracy, and the most skilful observers applied
themselves with great diligence to discover an annual parallax, from
which the distances of the fixed stars would be known. They found
unexpected irregularities, and might have been long perplexed with them
to little purpose, had not Dr. Bradley happily accounted for them, by
shewing that light from the heavenly bodies strikes the eye with a
velocity and direction, compounded of the proper velocity and direction
of _light_, and of the _eye_, as carried about with the earth in its
orbit; compared to which, the diurnal motion and all other accidental
motions of the eye, are quite inconsiderable. Thus, instead of what he
aimed at, he discovered something still more curious, the real velocity
of light, in a way entirely new and unthought of.
All Astronomical knowledge being conveyed to us from the remotest
distances, by that subtle, swift and universal messenger of
intelligence, LIGHT; it was natural for the curious to enquire into its
properties, and particularly to endeavour to know with what velocity it
proceeds, in its immeasurable journeys. Experimental Philosophy,
accustomed to conquer every difficulty, undertook the arduous problem;
but confessed herself unequal to the task.[A30] Here, Astronomy itself
revealed the secret; first in the discovery of Roemer, who found that
the farther Jupiter is distant from us, the later the light of his
satellites always reaches us; and afterwards in this of Dr. Bradley,
informed us, that light proceeds from the sun to us in about eight
minutes of time.[A31]
-----
Footnote A30:
Alluding to the experiments made in France, for determining the
velocity of light; which, though unsuccessful, discovered a noble
philosophical spirit.
Footnote A31:
This prodigious velocity of light can be no argument against its
materiality, as will appear from the following considerations. The
greatest velocity which we can communicate to any body, is that of a
cannon-ball, impelled by gun-powder; this may be at the rate of about
20 miles in a minute of time. The planet Saturn moves about 360 miles
in a minute, that is 18 times swifter than a cannon-ball; and the
comet of 1680, in its perihelion, moved near 56.66 times swifter than
Saturn, or 990.5 times swifter than a cannon-ball. Now these are
material bodies, moving with very various, and all of them exceedingly
great velocities; and no reason appears why the last mentioned
velocity should be the utmost limit, beyond which nature cannot
proceed; or that some other body may not move 7 or 8 hundred times
swifter than a comet, as light is found to do.
That the different refrangibility of the rays of light, on which their
colours depend, arises from their different velocities, seems so
natural a conjecture, that it has perhaps occurred to every one who
has thought on this subject. To this there are three principal
objections. The first is, that, according to this hypothesis, when the
satellites of Jupiter are eclipsed, their colour ought to change,
first to a green and then to a blue, before their light becomes
extinct; which is contrary to experience. But this objection appears
to me of no weight; for we do not lose sight of the satellite because
there is no light coming from thence to the eye, but because there is
not light enough to render it visible. Therefore at the time a
satellite disappears, there is still light of all colours arriving at
the eye: and though the blue light should predominate on account of
its slower progress, yet the red may predominate on another account;
for along the edge of Jupiter’s shadow, as it passes over the
satellite, a greater proportion of red light, than of blue, will be
thrown by the refraction of Jupiter’s atmosphere. The second objection
is, that since the velocity of the earth in its orbit, causes an
aberration of about 20 seconds in the place of a star, if the
different colours of light depended on different velocities, the
aberration of blue light ought proportionably to exceed that of red
light, which would give such an oblong form to a fixed star as might
be discovered with a good telescope. This objection is of no more
force than the former. The effect ought indeed to follow, but not in a
sensible quantity; for at the altitude of 70 degrees, the apparent
place of a fixed star is likewise removed 20 seconds by refraction,
and the very same separation of the rays must take place; yet this I
think is not discoverable with the best telescope. Perhaps by uniting
these two equal causes, which may be readily done, and thereby
doubling the effect, it may become sensible.
The third objection arises from that curious discovery of Dollond, by
which we are enabled so greatly to improve refracting telescopes. And
this objection I shall for the present leave in its full force; as
well against the above hypothesis, as against every other which I have
seen for the same purpose.
-----
As the apparent motion of the fixed stars, arising from this cause, was
observed to complete the intire circle of its changes in the space of a
year, it was for some time supposed to arise from an annual parallax,
notwithstanding its inconsistency in other respects with such a
supposition. But this obstacle being removed, there followed the
discovery of another apparent motion in the heavens, arising from the
nutation of the earth’s axis; the period whereof is about nineteen
years. Had it not been so very different from the period of the former,
the causes of both must have been almost inexplicable. This latter
discovery is an instance of the superior advantages of accurate
observation: For it was well known that such a nutation must take place
from the principles of the Newtonian Philosophy; yet a celebrated
astronomer had concluded from hypothetical reasoning, that its quantity
must be perfectly insensible.
The way being cleared thus far, Dr. Bradley assures us, from his most
accurate observations, that the annual parallax cannot exceed two
seconds, he thinks not one; and we have the best reason to confide in
his judgment and accuracy. From hence then we draw this amazing
conclusion; that the diameter of the earth’s orb bears no greater
proportion to the distance of the stars which Bradley observed, than one
second does to the radius; which is less than as one to 200,000.
Prodigiously great as the distance of the fixed stars from our sun
appears to be, and probably their distances from each other are no less,
the Newtonian Philosophy will furnish us with a reason for it: That the
several systems may be sufficiently removed from each other’s
attraction, which we are very certain must require an immense distance;
especially if we consider that the cometic part, of our system at least,
appears to be the most considerable though so little known to us. The
dimensions of the several parts of the planetary system, had been
determined near the truth by the astronomers of the last age, from the
parallax of Mars. But from that rare phenomenon the transit of Venus
over the sun’s disk, which has twice happened within a few years past,
the sun’s parallax is now known beyond dispute to be 8 seconds and an
half, nearly; and consequently, the sun’s distance almost 12,000
diameters of the earth.
If from the distances of the several planets, and their apparent
diameters taken with that excellent instrument, the micrometer, we
compare their several magnitudes, we shall find the Moon, Mercury, and
Mars, to be much less than our Earth, Venus a little less, but Saturn
many hundred times greater, and Jupiter above one thousand times. This
prodigious globe, placed at such a vast distance from the other planets,
that the force of its attraction might the less disturb their motions,
is far more bulky and ponderous than all the other planets taken
together. But even Jupiter, with all his fellows of our system, are as
nothing compared to that amazing mass of matter the Sun. How much are we
then indebted to Astronomy, for correcting our ideas of the visible
creation! Wanting its instruction, we should infallibly have supposed
the earth by far the most important body in the universe, both for
magnitude and use. The sun and moon would have been thought two little
bodies nearly equal in size, though different in lustre, created solely
for the purpose of enlightening the earth; and the fixed stars, so many
sparks of fire, placed in the concave vault of heaven, to adorn it, and
afford us a glimmering light in the absence of the sun and moon.
But how does Astronomy change the scene!—Take the miser from the earth,
if it be possible to disengage him; he whose nightly rest has been long
broken by the loss of a single foot of it, useless perhaps to him; and
remove him to the planet Mars, one of the least distant from us:
Persuade the ambitious monarch to accompany him, who has sacrificed the
lives of thousands of his subjects to an imaginary property in certain
small portions of the earth; and now point it out to them, with all its
kingdoms and wealth, a glittering star “close by the moon,” the latter
scarce visible and the former less bright than our Evening Star:—Would
they not turn away their disgusted sight from it, as not thinking it
worth their smallest attention, and look for consolation in the gloomy
regions of Mars?[A32]
-----
Footnote A32:
Mars appears to be surrounded by a very great and dense atmosphere.
-----
But dropping the company of all those, whether kings or misers, whose
minds and bodies are equally affected by gravitation, let us proceed to
the orb of Jupiter; the Earth and all the inferior planets will vanish,
lost in the sun’s bright rays, and Saturn only remain; He too sometimes
so diminished in lustre, as not to be easily discovered. But a new and
beautiful system will arise. The four moons of Jupiter will become very
conspicuous; some of them perhaps appearing larger, others smaller than
our moon; and all of them performing their revolutions with incredible
swiftness, and the most beautiful regularity:—varying their phases from
full to new and from new to full, and frequently eclipsing the sun and
each other, at least to the equatorial parts of Jupiter; and almost in
every revolution suffering eclipses themselves by falling into Jupiter’s
shadow; excepting that the outermost will seem, like a traveller fond of
the sun-beams, cautiously to avoid the shadow for whole years together.
Since we are advanced so far, if not tired of the journey, let us
proceed a step further; it is but 400 millions of miles to the globe of
Saturn. Here again all will be lost, but Jupiter itself. The Sun will
put on something of a starlike appearance, but with excessive
brightness. The five[A33] satellites of Saturn will exhibit appearances
similar to those of Jupiter, but they will very rarely eclipse the Sun,
or suffer eclipses themselves. The particular phænomena of Saturn’s
ring, we cannot explain, unless we knew the time and plane of Saturn’s
revolution on his axis. But this we know, that it must sometimes appear,
by night, like a prodigious luminous arch, almost equal to one quarter
of the heavens; and at other times, dark, so as to afford no light
itself, but to intercept the light of every star beyond it, by night,
and of the sun itself by day. And to conclude, if borne on the wings of
a comet we should travel with it to the remotest part of its orbit; our
whole planetary system would disappear, and the sun become a star, only
more refulgent than Sirius perhaps, because less distant.
-----
Footnote A33:
Dr. Herschel discovered, in the year 1789, (fourteen years after the
delivery of this Oration,) two other satellites of Saturn. These are
the innermost of his (now) seven secondary planets.