_Discovery of Jupiter's
satellites—Kepler—Sizzi—Astrologers—Mæstlin—Horky—Mayer._
AS soon as Galileo had provided himself with a second instrument, he
began a careful examination of the heavenly bodies, and a series of
splendid discoveries soon rewarded his diligence. After considering the
beautiful appearances which the varied surface of the moon presented to
this new instrument, he turned his telescope towards Jupiter, and his
attention was soon arrested by the singular position of three small
stars, near the body of that planet, which appeared almost in a straight
line with it, and in the direction of the ecliptic. The following
evening he was surprised to find that two of the three which had been to
the eastward of the planet, now appeared on the contrary side, which he
could not reconcile with the apparent motion of Jupiter among the fixed
stars, as given by the tables. Observing these night after night, he
could not fail to remark that they changed their relative positions. A
fourth also appeared, and in a short time he could no longer refuse to
believe that these small stars were four moons, revolving round Jupiter
in the same manner in which our earth is accompanied by its single
attendant. In honour of his patron Cosmo, he named them the Medicæan
stars. As they are now hardly known by this appellation, his doubts,
whether he should call them Medicæan, after Cosmo's family, or Cosmical,
from his individual name, are become of less interest.
An extract from a letter which Galileo received on this occasion from
the court of France, will serve to show how highly the honour of giving
a name to these new planets was at that time appreciated, and also how
much was expected from Galileo's first success in examining the heavens.
"The second request, but the most pressing one which I can make to you,
is, that you should determine, if you discover any other fine star, to
call it by the name of the great star of France, as well as the most
brilliant of all the earth; and, if it seems fit to you, call it rather
by his proper name of Henri, than by the family name of Bourbon: thus
you will have an opportunity of doing a thing just and due and proper in
itself, and at the same time will render yourself and your family rich
and powerful for ever." The writer then proceeds to enumerate the
different claims of Henri IV. to this honour, not forgetting that he
married into the family of the Medici, &c.
The result of these observations was given to the world, in an Essay
which Galileo entitled _Nuncius Sidereus_, or the Intelligencer of the
Stars; and it is difficult to describe the extraordinary sensation which
its publication produced. Many doubted, many positively refused to
believe, so novel an announcement; all were struck with the greatest
astonishment, according to their respective opinions, either at the new
view of the universe thus offered to them, or at the daring audacity of
Galileo in inventing such fables. We shall proceed to extract a few
passages from contemporary writers relative to this book, and the
discoveries announced in it.
Kepler deserves precedence, both from his own celebrity, and from the
lively and characteristic account which he gives of his first receiving
the intelligence:—"I was sitting idle at home, thinking of you, most
excellent Galileo, and your letters, when the news was brought me of the
discovery of four planets by the help of the double eye-glass.
Wachenfels stopped his carriage at my door to tell me, when such a fit
of wonder seized me at a report which seemed so very absurd, and I was
thrown into such agitation at seeing an old dispute between us decided
in this way, that between his joy, my colouring, and the laughter of
both, confounded as we were by such a novelty, we were hardly capable,
he of speaking, or I of listening. My amazement was increased by the
assertion of Wachenfels, that those who sent this news from Galileo were
celebrated men, far removed by their learning, weight, and character,
above vulgar folly; that the book was actually in the press, and would
be published immediately. On our separating, the authority of Galileo
had the greatest influence on me, earned by the accuracy of his
judgment, and excellence of his understanding; so I immediately fell to
thinking how there could be any addition to the number of the planets
without overturning my Mysterium Cosmographicum, published thirteen
years ago, according to which Euclid's five regular solids do not allow
more than six planets round the sun."
This was one of the many wild notions of Kepler's fanciful brain, among
which he was lucky enough at length to hit upon the real and principal
laws of the planetary motions. His theory may be briefly given in his
own words:—"The orbit of the earth is the measure of the rest. About it
circumscribe a dodecahedron. The sphere including this will be that of
Mars. About Mars' orbit describe a tetrahedron: the sphere containing
this will be Jupiter's orbit. Round Jupiter's describe a cube: the
sphere including this will be Saturn's. Within the earth's orbit
inscribe an icosahedron: the sphere inscribed in it will be Venus's
orbit. In Venus inscribe an octahedron: the sphere inscribed in it will
be Mercury's. You have now the reason of the number of the planets:" for
as there are no more than the five regular solids here enumerated,
Kepler conceived this to be a satisfactory reason why there could be
neither more nor less than six planets. His letter continues:—"I am so
far from disbelieving the existence of the four circumjovial planets,
that I long for a telescope to anticipate you, if possible, in
discovering two round Mars, (as the proportion seems to me to require,)
six or eight round Saturn, and perhaps one each round Mercury and
Venus."
The reader has here an opportunity of verifying Galileo's observation,
that Kepler's method of philosophizing differed widely from his own. The
proper line is certainly difficult to hit between the mere theorist and
the mere observer. It is not difficult at once to condemn the former,
and yet the latter will deprive himself of an important, and often
indispensable assistance, if he neglect from time to time to consolidate
his observations, and thence to conjecture the course of future
observation most likely to reward his assiduity. This cannot be more
forcibly expressed than in the words of Leonardo da Vinci:[46] "Theory
is the general, experiments are the soldiers. The interpreter of the
works of nature is experiment; that is never wrong; it is our judgment
which is sometimes deceived, because we are expecting results which
experiment refuses to give. We must consult experiment, and vary the
circumstances, till we have deduced general rules, for it alone can
furnish us with them. But you will ask, what is the use of these general
rules? I answer, that they direct us in our inquiries into nature and
the operations of art. They keep us from deceiving ourselves and others,
by promising ourselves results which we can never obtain."
In the instance before us, it is well known that, adopting some of the
opinions of Bruno and Brutti, Galileo, even before he had seen the
satellites of Jupiter, had allowed the possibility of the discovery of
new planets; and we can scarcely suppose that they had weakened his
belief in the probability of further success, or discouraged him from
examining the other heavenly bodies. Kepler on the contrary had taken
the opposite side of the argument; but no sooner was the fallacy of his
first position undeniably demonstrated, than, passing at once from one
extreme to the other, he framed an unsupported theory to account for the
number of satellites which were round Jupiter, and for those which he
expected to meet with elsewhere. Kepler has been styled the legislator
of the skies; his laws were promulgated rather too arbitrarily, and they
often failed, as all laws must do which are not drawn from a careful
observation of the nature of those who are to be governed by them.
Astronomers have reason to be grateful for the theorems which he was the
first to establish; but so far as regards the progress of the science of
inductive reasoning, it is perhaps to be regretted, that the seventeen
years which he wasted in random and unconnected guesses should have been
finally rewarded, by discoveries splendid enough to shed deceitful
lustre upon the method by which he arrived at them.
Galileo himself clearly perceived the fallacious nature of these
speculations on numbers and proportions, and has expressed his
sentiments concerning them very unequivocally. "How great and common an
error appears to me the mistake of those who persist in making their
knowledge and apprehension the measure of the apprehension and knowledge
of God; as if that alone were perfect, which they understand to be so.
But I, on the contrary, observe that Nature has other scales of
perfection, which we cannot comprehend, and rather seem disposed to
class among imperfections. For instance, among the relations of
different numbers, those appear to us most perfect which exist between
numbers nearly related to each other; as the double, the triple, the
proportion of three to two, &c.; those appear less perfect which exist
between numbers remote from, and prime to each other; as 11 to 7, 17 to
13, 53 to 37, &c.; and most imperfect of all do those appear which exist
between incommensurable quantities, which by us are nameless and
inexplicable. Consequently, if the task had been given to a man, of
establishing and ordering the rapid motions of the heavenly bodies,
according to his notions of perfect proportions, I doubt not that he
would have arranged them according to the former rational proportions;
but, on the contrary, God, with no regard to our imaginary symmetries,
has ordered them in proportions not only incommeasurable and irrational,
but altogether inappreciable by our intellect. A man ignorant of
geometry may perhaps lament, that the circumference of a circle does not
happen to be exactly three times the diameter, or in some other
assignable proportion to it, rather than such that we have not yet been
able to explain what the ratio between them is; but one who has more
understanding will know that if they were other than they are, thousands
of admirable conclusions would have been lost, and that none of the
other properties of the circle would have been true: the surface of the
sphere would not be quadruple of a great circle, nor the cylinder be to
the sphere as three to two: in short, no part of geometry would be true,
and as it now is. If one of our most celebrated architects had had to
distribute this vast multitude of fixed stars through the great vault of
heaven, I believe he would have disposed them with beautiful
arrangements of squares, hexagons, and octagons; he would have dispersed
the larger ones among the middle sized and the less, so as to correspond
exactly with each other; and then he would think he had contrived
admirable proportions: but God, on the contrary, has shaken them out
from His hand as if by chance, and we, forsooth, must think that He has
scattered them up yonder without any regularity, symmetry, and
elegance."
It is worth remarking that the dangerous ideas of aptitude and
congruence of numbers had taken such deep and general root, that long
afterwards, when the reality of Jupiter's satellites was incontestably
established, and Huyghens had discovered a similar satellite near
Saturn, he was so rash as to declare his belief, (unwarned by the vast
progress which astronomy had made in his own time,) that no more
satellites would be discovered, since the one which he discovered near
Saturn, with Jupiter's four, and our moon, made up the number six,
exactly equal to the number of the principal planets. Every reader knows
that this notion, so unworthy the genius of Huyghens, has been since
exploded by the discovery both of new planets, and new satellites.
Francesco Sizzi, a Florentine astronomer, took the matter up in a
somewhat different strain from Kepler.[47]—"There are seven windows
given to animals in the domicile of the head, through which the air is
admitted to the rest of the tabernacle of the body, to enlighten, to
warm, and nourish it, which are the principal parts of the μικροκοσμος
(or little world); two nostrils, two eyes, two ears, and a mouth; so in
the heavens, as in a μακροκοσμος (or great world), there are two
favourable stars, two unpropitious, two luminaries, and Mercury alone
undecided and indifferent. From which and many other similar phenomena
of nature, such as the seven metals, &c., which it were tedious to
enumerate, we gather that the number of planets is necessarily seven.
Moreover, the satellites are invisible to the naked eye, and therefore
can exercise no influence on the earth, and therefore would be useless,
and therefore do not exist. Besides, as well the Jews and other ancient
nations as modern Europeans have adopted the division of the week into
seven days, and have named them from the seven planets: now if we
increase the number of the planets this whole system falls to the
ground." To these remarks Galileo calmly replied, that whatever their
force might be, as a reason for believing beforehand that no more than
seven planets would be discovered, they hardly seemed of sufficient
weight to destroy the new ones when actually seen.
Others, again, took a more dogged line of opposition, without venturing
into the subtle analogies and arguments of the philosopher just cited.
They contented themselves, and satisfied others, with the simple
assertion, that such things were not, and could not be, and the manner
in which they maintained themselves in their incredulity was
sufficiently ludicrous. "Oh, my dear Kepler,"[48] says Galileo, "how I
wish that we could have one hearty laugh together. Here, at Padua, is
the principal professor of philosophy, whom I have repeatedly and
urgently requested to look at the moon and planets through my glass,
which he pertinaciously refuses to do. Why are you not here? what shouts
of laughter we should have at this glorious folly! and to hear the
professor of philosophy at Pisa labouring before the grand duke with
logical arguments, as if with magical incantations, to charm the new
planets out of the sky."
Another opponent of Galileo deserves to be named, were it only for the
singular impudence of the charge he ventures to bring against him. "We
are not to think," says Christmann, in the Appendix to his _Nodus
Gordius_, "that Jupiter has four satellites given him by nature, in
order, by revolving round him, to immortalize the name of the Medici,
who first had notice of the observation. These are the dreams of _idle
men_, who love ludicrous ideas better than our laborious and industrious
correction of the heavens.—Nature abhors so horrible a chaos, and to
the truly wise such vanity is detestable."
Galileo was also urged by the astrologers to attribute some influence,
according to their fantastic notions, to the satellites, and the account
which he gives his friend Dini of his answer to one of this class is
well worth extracting, as a specimen of his method of uniting sarcasm
with serious expostulation; "I must," says he, "tell you what I said a
few days back to one of those nativity-casters, who believe that God,
when he created the heavens and the stars, had no thoughts beyond what
they can themselves conceive, in order to free myself from his tedious
importunity; for he protested, that unless I would declare to him the
effect of the Medicæan planets, he would reject and deny them as
needless and superfluous. I believe this set of men to be of Sizzi's
opinion, that astronomers discovered the other seven planets, not by
seeing them corporally in the skies, but only from their effects on
earth,—much in the manner in which some houses are discovered to be
haunted by evil spirits, not by seeing them, but from the extravagant
pranks which are played there. I replied, that he ought to reconsider
the hundred or thousand opinions which, in the course of his life, he
might have given, and particularly to examine well the events which he
had predicted with the help of Jupiter, and if he should find that all
had succeeded conformably to his predictions, I bid him prophecy merrily
on, according to his old and wonted rules; for I assured him that the
new planets would not in any degree affect the things which are already
past, and that in future he would not be a less fortunate conjuror than
he had been: but if, on the contrary, he should find the events
depending on Jupiter, in some trifling particulars not to have agreed
with his dogmas and prognosticating aphorisms, he ought to set to work
to find new tables for calculating the constitution of the four Jovial
circulators at every bygone moment, and, perhaps, from the diversity of
their aspects, he would be able, with accurate observations and
multiplied conjunctions, to discover the alterations and variety of
influences depending upon them; and I reminded him, that in ages past
they had not acquired knowledge with little labour, at the expense of
others, from written books, but that the first inventors acquired the
most excellent knowledge of things natural and divine with study and
contemplation of the vast book which nature holds ever open before those
who have eyes in their forehead and in their brain; and that it was a
more honourable and praiseworthy enterprize with their own watching,
toil, and study, to discover something admirable and new among the
infinite number which yet remain concealed in the darkest depths of
philosophy, than to pass a listless and lazy existence, labouring only
to darken the toilsome inventions of their neighbours, in order to
excuse their own cowardice and inaptitude for reasoning, while they cry
out that nothing can be added to the discoveries already made."
The extract given above from Kepler, is taken from an Essay, published
with the later editions of the _Nuncius_, the object and spirit of which
seem to have been greatly misunderstood, even by some of Kepler's
intimate friends.—They considered it as a covert attack upon Galileo,
and, accordingly, Maestlin thus writes to him:—"In your Essay (which I
have just received) you have plucked Galileo's feathers well; I mean,
that you have shown him not to be the inventor of the telescope, not to
have been the first who observed the irregularities of the moon's
surface, not to have been the first discoverer of more worlds than the
ancients were acquainted with, &c. One source of exultation was still
left him, from the apprehension of which Martin Horky has now entirely
delivered me." It is difficult to discover in what part of Kepler's book
Maestlin found all this, for it is one continued encomium upon Galileo;
insomuch that Kepler almost apologizes in the preface for what may seem
his intemperate admiration of his friend. "Some might wish I had spoken
in more moderate terms in praise of Galileo, in consideration of the
distinguished men who are opposed to his opinions, but I have written
nothing fulsome or insincere. I praise him, for myself; I leave other
men's judgments free; and shall be ready to join in condemnation when
some one wiser than myself shall, by sound reasoning, point out his
errors." However, Maestlin was not the only one who misunderstood
Kepler's intentions: the Martin Horky of whom he speaks, a young German,
also signalized himself by a vain attack upon the book which he thought
his patron Kepler condemned. He was then travelling in Italy, whence he
wrote to Kepler his first undetermined thoughts about the new
discoveries. "They are wonderful; they are stupendous; whether they are
true or false I cannot tell."[49] He seems soon to have decided that
most reputation was to be gained on the side of Galileo's opponents, and
his letters accordingly became filled with the most rancorous abuse of
him. At the same time, that the reader may appreciate Horky's own
character, we shall quote a short sentence at the end of one of his
letters, where he writes of a paltry piece of dishonesty with as great
glee as if he had solved an ingenious and scientific problem. After
mentioning his meeting Galileo at Bologna, and being indulged with a
trial of his telescope, which, he says, "does wonders upon the earth,
but represents celestial objects falsely;"[50] he concludes with the
following honourable sentence:—"I must confide to you a theft which I
committed. I contrived to take a mould of the glass in wax, without the
knowledge of any one, and, when I get home, I trust to make a telescope
even better than Galileo's own."
Horky having declared to Kepler, "I will never concede his four new
planets to that Italian from Padua though I die for it," followed up
this declaration by publishing a book against Galileo, which is the one
alluded to by Maestlin, as having destroyed the little credit which,
according to his view, Kepler's publication had left him. This book
professes to contain the examination of four principal questions
touching the alleged planets; 1st, Whether they exist? 2nd, What they
are? 3rd, What they are like? 4th, Why they are? The first question is
soon disposed of, by Horky's declaring positively that he has examined
the heavens with Galileo's own glass, and that no such thing as a
satellite about Jupiter exists. To the second, he declares solemnly,
that he does not more surely know that he has a soul in his body, than
that reflected rays are the sole cause of Galileo's erroneous
observations. In regard to the third question, he says, that these
planets are like the smallest fly compared to an elephant; and, finally,
concludes on the fourth, that the only use of them is to gratify
Galileo's "thirst of gold," and to afford himself a subject of
discussion.[51]
Galileo did not condescend to notice this impertinent folly; it was
answered by Roffini, a pupil of Magini, and by a young Scotchman of the
name of Wedderburn, then a student at Padua, and afterwards a physician
at the Court of Vienna. In the latter reply we find it mentioned, that
Galileo was also using his telescope for the examination of insects,
&c.[52] Horky sent his performance triumphantly to Kepler, and, as he
returned home before receiving an answer, he presented himself before
his patron in the same misapprehension under which he had written, but
the philosopher received him with a burst of indignation which rapidly
undeceived him. The conclusion of the story is characteristic enough to
be given in Kepler's own account of the matter to Galileo, in which,
after venting his wrath against this "scum of a fellow," whose
"obscurity had given him audacity," he says, that Horky begged so hard
to be forgiven, that "I have taken him again into favour upon this
preliminary condition, to which he has agreed:—that I am to shew him
Jupiter's satellites, AND HE IS TO SEE THEM, and own that they are
there."
In the same letter Kepler writes, that although he has himself perfect
confidence in the truth of Galileo's assertions, yet he wishes he could
furnish him with some corroborative testimonies, which Kepler could
quote in arguing the point with others. This request produced the
following reply, from which the reader will also learn the new change
which had now taken place in Galileo's fortunes, the result of the
correspondence with Florence, part of which we have already
extracted.[53] "In the first place, I return you my thanks that you
first, and almost alone, before the question had been sifted (such is
your candour and the loftiness of your mind), put faith in my
assertions. You tell me you have some telescopes, but not sufficiently
good to magnify distant objects with clearness, and that you anxiously
expect a sight of mine, which magnifies images more than a thousand
times. It is mine no longer, for the Grand Duke of Tuscany has asked it
of me, and intends to lay it up in his museum, among his most rare and
precious curiosities, in eternal remembrance of the invention: I have
made no other of equal excellence, for the mechanical labour is very
great: I have, however, devised some instruments for figuring and
polishing them which I am unwilling to construct here, as they could not
conveniently be carried to Florence, where I shall in future reside. You
ask, my dear Kepler, for other testimonies:—I produce, for one, the
Grand Duke, who, after observing the Medicæan planets several times with
me at Pisa during the last months, made me a present, at parting, worth
more than a thousand florins, and has now invited me to attach myself to
him with the annual salary of one thousand florins, and with the title
of Philosopher and Principal Mathematician to His Highness; without the
duties of any office to perform, but with the most complete leisure; so
that I can complete my Treatises on Mechanics, on the Constitution of
the Universe, and on Natural and Violent Local Motion, of which I have
demonstrated geometrically many new and admirable phenomena. I produce,
for another witness, myself, who, although already endowed in this
college with the noble salary of one thousand florins, such as no
professor of mathematics ever before received, and which I might
securely enjoy during my life, even if these planets had deceived me and
should disappear, yet quit this situation, and betake me where want and
disgrace will be my punishment should I prove to have been mistaken."
It is difficult not to regret that Galileo should be thus called on to
resign his best glasses, but it appears probable that on becoming more
familiar with the Grand Duke, he ventured to suggest that this telescope
would be more advantageously employed in his own hands, than pompously
laid up in a museum; for in 1637 we find him saying, in answer to a
request from his friend Micanzio to send him a telescope—"I am sorry
that I cannot oblige you with the glasses for your friend, but I am no
longer capable of making them, and I have just parted with two tolerably
good ones which I had, reserving only my old discoverer of celestial
novelties which is already promised to the Grand Duke." Cosmo was dead
in 1637, and it is his son Ferdinand who is here meant, who appears to
have inherited his father's love of science. Galileo tells us, in the
same letter, that Ferdinand had been amusing himself for some months
with making object-glasses, and always carried one with him to work at
wherever he went.
When forwarding this telescope to Cosmo in the first instance, Galileo
adds, with a very natural feeling—"I send it to his highness unadorned
and unpolished, as I made it for my own use, and beg that it may always
be left in the same state; for none of the old parts ought to be
displaced to make room for new ones, which will have had no share in the
watchings and fatigues of these observations." A telescope was in
existence, though with the object glass broken, at the end of the last
century, and probably still is in the Museum at Florence, which was
shewn as the discoverer of Jupiter's satellites. Nelli, on whose
authority this is mentioned, appears to question its genuineness. The
first reflecting telescope, made with Newton's own hands, and scarcely
possessing less interest than the first of Galileo's, is preserved in
the library of the Royal Society.
By degrees the enemies of Galileo and of the new stars found it
impossible to persevere in their disbelief, whether real or pretended,
and at length seemed resolved to compensate for the sluggishness of
their perception, by its acuteness when brought into action. Simon Mayer
published his "Mundus Jovialis" in 1614, in which he claims to have been
an original observer of the satellites, but, with an affectation of
candour, allows that Galileo observed them probably about the same time.
The earliest observation which he has recorded is dated 29th December,
1609, but, not to mention the total want of probability that Mayer would
not have immediately published so interesting a discovery, it is to be
observed, that, as he used the old style, this date of 29th December
agrees with the 8th January, 1610, of the new style, which was the date
of Galileo's second observation, and Galileo ventured to declare his
opinion, that this pretended observation was in fact a plagiarism.
Scheiner counted five, Rheita nine, and other observers, with increasing
contempt for Galileo's imperfect announcements, carried the number as
high as twelve.[54] In imitation of Galileo's nomenclature, and to
honour the sovereigns of the respective observers, these supposed
additional satellites were dignified with the names of Vladislavian,
Agrippine, Urbanoctavian, and Ferdinandotertian planets; but a very
short time served to show it was as unsafe to exceed as to fall short of
the number which Galileo had fixed upon, for Jupiter rapidly removed
himself from the neighbourhood of the fixed stars, which gave rise to
these pretended discoveries, carrying with him only his four original
attendants, which continued in every part of his orbit to revolve
regularly about him.
Perhaps we cannot better wind up this account of the discovery of
Jupiter's satellites, and of the intense interest they have at all times
inspired, than in the words of one who inherits a name worthy to be
ranked with that of Galileo in the list of astronomical discoverers, and
who takes his own place among the most accomplished mathematicians of
the present times. "The discovery of these bodies was one of the first
brilliant results of the invention of the telescope; one of the first
great facts which opened the eyes of mankind to the system of the
universe, which taught them the comparative insignificance of their own
planet, and the superior vastness and nicer mechanism of those other
bodies, which had before been distinguished from the stars only by their
motion, and wherein none but the boldest thinkers had ventured to
suspect a community of nature with our own globe. This discovery gave
the holding turn to the opinions of mankind respecting the Copernican
system; the analogy presented by these little bodies (little however
only in comparison with the great central body about which they revolve)
performing their beautiful revolutions in perfect harmony and order
about it, being too strong to be resisted. This elegant system was
watched with all the curiosity and interest the subject naturally
inspired. The eclipses of the satellites speedily attracted attention,
and the more when it was discerned, as it speedily was, by Galileo
himself, that they afforded a ready method of determining the difference
of longitudes of distant places on the earth's surface, by observations
of the instants of their disappearances and reappearances,
simultaneously made. Thus the first astronomical solution of the great
problem of the longitude, the first mighty step which pointed out a
connection between speculative astronomy and practical utility, and
which, replacing the fast dissipating dreams of astrology by nobler
visions, showed how the stars might really, and without fiction, be
called arbiters of the destinies of empires, we owe to the satellites of
Jupiter, those atoms imperceptible to the naked eye, and floating like
motes in the beam of their primary—itself an atom to our sight, noticed
only by the careless vulgar as a large star, and by the philosophers of
former ages as something moving among the stars, they knew not what, nor
why: perhaps only to perplex the wise with fruitless conjectures, and
harass the weak with fears as idle as their theories."[55]
FOOTNOTES:
[46] Venturi. Essai sur les ouvrages de Leo. da Vinci.
[47] Dianoia Astronomica, Venetiis, 1610.
[48] Kepleri Epistolæ.
[49] Kepleri Epistolæ.
[50] It may seem extraordinary that any one could support an argument by
this partial disbelief in the instrument, which was allowed on all hands
to represent terrestrial objects correctly. A similar instance of
obstinacy, in an almost identical case though in a more unpretending
station, once came under the writer's own observation. A farmer in
Cambridgeshire, who had acquired some confused notions of the use of the
quadrant, consulted him on a new method of determining the distances and
magnitudes of the sun and moon, which he declared were far different
from the quantities usually assigned to them. After a little
conversation, the root of his error, certainly sufficiently gross,
appeared to be that he had confounded the angular measure of a degree,
with 69½ miles, the linear measure of a degree on the earth's surface.
As a short way of showing his mistake, he was desired to determine, in
the same manner, the height of his barn which stood about 30 yards
distant; he lifted the quadrant to his eye, but perceiving, probably,
the monstrous size to which his principles were forcing him, he said,
"Oh, Sir, the quadrant's only true for the sky." He must have been an
objector of this kind, who said to Galileo,—"Oh, Sir, the telescope's
only true for the earth."
[51] Venturi.
[52] Quatuor probl. confut. per J. Wedderbornium, Scotobritannum.
Patavii, 1610.
[53] See page 18.
[54] Sherburne's Sphere of Manilius. London, 1675.
[55] Herschel's Address to the Astronomical Society, 1827.