We will now return to the narrative; and in due course discuss the
condemnation of Galileo by the Inquisition sixteen years after the
events just described.
It may be mentioned, as illustrating the feeling in Rome towards
Galileo personally, that on the 11th March, 1616, he had an audience,
lasting three-quarters of an hour, of Pope Paul V. He assured the Pope
of the rectitude of his intentions, and complained of the persecutions
of his adversaries. Paul V. answered very kindly, saying that both
himself and the Cardinals of the Index had formed a high personal
opinion of him, and did not believe his calumniators.
In the year 1620 there appeared a monitum of the Congregation of the
Index, permitting the reading of the great work of Copernicus after
certain specified corrections had been made.
Not long after this, in 1622, if I mistake not, Pope Paul V. died, and
Galileo’s friend, Cardinal Barberini, succeeded him, taking the name
of Urban VIII. Another of his friends, Monsignor Ciampoli, became
secretary of briefs to the new Pope.
Our philosopher having ascertained that he would be well received,
went to Rome in April, 1624, and was treated by the new Pope with
all possible consideration. He had, in fact, several conversations
with him; and we may well conjecture it was on these occasions that
Urban VIII., discussing the Copernican theory, used some of those
arguments which Galileo afterwards put in the mouth of Simplicio in his
celebrated Dialogue, thereby deeply offending the Pope.
But there was, about this time, a sort of moderate reaction in favour
of Galileo among the authorities at Rome. For instance, a work of his
published since the decree of the Index, and entitled “Il Saggiatore,”
in which he had favoured the theory of the Earth’s motion, was
attacked, and an attempt was made to have it prohibited or at least
corrected, but the attempt was a failure.
The reports of casual or unofficial conversations are always to be
received with caution and with some qualification; yet at least they
are “straws which show how the wind blows.”
Thus we are told that Cardinal Hohen-Zollern, in a conversation with
the Pope (Urban VIII.) on the subject of Copernicus, endeavoured to
show the necessity of proceeding with great circumspection on that
point, to which it is said the Pope replied that the Church had not
condemned and would not condemn that opinion as heretical, but only as
temerarious. So again the Master of the Sacred Palace, himself resting
neutral between Ptolemy and Copernicus, is reported to have said that
there was no matter of faith in question, the great point being that
one must not in any way mix up the Holy Scriptures with it.
We may suppose that when the Pope spoke of the opinion having been
condemned as temerarious, what he meant was not that it had been
explicitly censured as such--using the word in the technical sense
which it bears when applied as a censure--for that it plainly had not
been, but that the general effect of the prohibition issued by the
Index was to stamp the mark of rashness upon it. This, I may observe,
if it be the right interpretation, is quite consistent with the theory
that the prohibition was of a disciplinary and a provisional character.
We have also another reputed conversation of the Pope with
Campanella--resting on the authority of Prince Cesi, who related it to
Father Castelli--and it is important if true. Campanella had said that
certain Germans, ready to embrace the Catholic faith, had hesitated on
account of the condemnation of Copernicus, to which Pope Urban VIII.
had replied that this was not his intention, and if he had had the
arrangement of matters the decree would never have been made. “Non fu
mai nostra intenzione, e se fosse toccato a noi, non si sarebbe fatto
quel decreto.”
As already remarked, we must not attach too great weight to reports
of private conversations; but it is probable that some such scene
took place as here represented, and, if it did, it is surely wholly
incompatible with the idea that the decree was a decision in matters
of faith. No Pope, no well-informed ecclesiastic of any rank, would
express himself so in such a case; but it is quite consistent with what
we might expect in a question of simple discipline.
It will now be convenient, before discussing the matter further, to
resume the narrative, and to touch upon the questions connected with
the condemnation of Galileo by the Inquisition, and his enforced
abjuration. It is, indeed, these latter proceedings that have left so
deep an impression upon the popular mind, though, strictly speaking,
they were of less importance than the decree of the Index--of less
importance, that is, to all others besides Galileo himself.
It seems that our philosopher overrated the effect of the reaction that
had taken place in his favour, real though it was so far as it went.
He thought he might now safely publish the work on which he had been
labouring, and on which he probably relied as likely to influence the
minds of learned men, ecclesiastical as well as lay, in the direction
of Copernicanism.
He came in May in the year 1630 to Rome, and had a very long audience
with the Pope, who treated him with great kindness and even increased
a pension he had already bestowed upon him; but we do not know what
passed as to other matters on this occasion. He had also an interview
with Father Riccardi, who had now become Master of the Sacred Palace,
with a view of obtaining authority to print his book. Father Riccardi
upon this engaged Father Visconti, who was a professor of mathematics,
to read the work and mark such passages as he thought necessary.
Father Visconti reported that there were some passages which required
correction, and many points that he would like to discuss with the
author. However, the Master of the Sacred Palace gave leave for the
printing of the work, expressing at the same time a wish to see it once
more himself; consequently it was arranged that Galileo should return
to Rome in the autumn, in order to add the preface, and to insert in
the body of the work certain passages, calculated to show that the
question was being treated purely as a hypothesis.
Two untoward events, however, now occurred: one was the death of Prince
Cesi, a powerful and devoted friend of Galileo, which took place on
the 1st May; and the other was the outbreak of the plague at Florence,
a circumstance which interrupted communications, and caused delays
resulting in mistakes and misunderstandings. With a view of having
the Dialogue printed at Florence, it was arranged that the revision
required by the ecclesiastical authorities should take place there
instead of at Rome. Father Hyacinthe Stephani, a Dominican, who acted
as reviser, marked several passages in the work, thinking that they
should be explained before the final permission for publication was
conceded.
Then followed mutual delays: the author was tardy in sending to Rome
the corrections to which he had in principle agreed, and the Master of
the Sacred Palace was late in sending to Florence the preface and the
conclusion, so the impatient philosopher began to print his book. The
plague still continued, and the result was that communications were
still interrupted.
The Inquisitor of Florence however received from Rome the power to
approve officially the copy of Galileo’s work that would be submitted
to him, with instructions specially added by Father Riccardi that he
must bear in mind the wishes of the Pope to the following effect:
The title of the work must indicate that it dealt only with the
mathematical question connected with Copernicanism, also that the
Copernican opinion must not be put forward as a positive truth, but
merely as a hypothesis, and this without alluding to the interpretation
of Scripture; moreover, that it should be stated that the work was only
written to show that if the decree (_i.e._ of 1616) was made at Rome,
nevertheless the authorities knew all the reasons against it that could
be urged, and were not ignorant of one of them--an idea conformable to
the words of the preface and the conclusion, which he would send from
Rome corrected. With this precaution, it was intimated the book would
meet with no obstacle at Rome, and thus satisfaction might be given
to the author, and also to the Grand Duke of Tuscany, who had shown
himself to be so eager in the matter.
This remarkable letter points towards a conclusion which has been
drawn by some writers, that the preface to the Dialogue was written for
Galileo by Father Riccardi or some other person, and was not his own
composition; for the above is precisely what was said in the preface
as it afterwards appeared, and it seems to me almost incredible that
Galileo should have spontaneously written any such words, exposing him
to the charge, which has really been made against him, of transparent
irony, thereby giving offence in the very quarters where conciliation
was desirable.
And it must be remarked that when Father Riccardi on the 19th July of
this year sent the preface to Florence, he allowed Galileo the liberty
of making verbal alterations only; so that whether he composed it or
only revised it, it is Father Riccardi rather than the author of the
Dialogue who must be held responsible for the contents, and the same
remark applies at least partially to the conclusion also, it having
been specially revised by the same hand.
The preface is addressed to the discreet reader, and the words to
which I have just alluded are as follows: “Some years ago, a wholesome
edict was promulgated in Rome which, in order to check the dangerous
scandals of the present age, imposed an opportune silence upon the
Pythagorean opinion of the motion of the earth. There were not wanting
some who rashly asserted that that decree resulted, not from judicious
examination, but from ill-informed passion; and there were heard
complaints that Consultors, wholly inexperienced in astronomical
observations, ought not to be allowed, with a hasty prohibition, to
clip the wings of speculative intellects. My zeal could not keep
silence on hearing the temerity of the complaints so made. As one fully
informed of that most prudent decision, I judged it right to appear
publicly in the theatre of the world, as a witness of pure truth. I
happened then to be present in Rome; I had not only audiences, but
approbations from the most eminent prelates of that Court, and it was
not without my own previous information that the publication of that
decree then followed.” The author goes on to say that he wished to
show to foreign nations how much was known in Italy, and particularly
in Rome, on this subject; and that from this climate there proceed not
only dogmas for the salvation of the soul, but ingenious devices for
the delight of the mind.
This last clause certainly savours of bitter irony, and probably did
not proceed from Father Riccardi’s pen. He then states that for the
purpose in hand he had taken the Copernican part in the Dialogue
as a pure mathematical hypothesis, endeavouring by every artifice
to represent it as superior, not to that of the stability of the
Earth absolutely speaking, but to the doctrine as defended by the
Peripatetics, to whom he alludes with some contempt.
He adds that he will treat of three principal heads: under the first
he would show that all our experience was insufficient to prove
conclusively the motion of the Earth, but that it adapted itself
equally to either theory; he hoped also to produce many observations
unknown to antiquity. In the second place, the celestial phenomena
would be examined, by which the Copernican hypothesis would be so
reinforced as if it ought to come out of the contest absolutely
victorious. In the third place he would propound his theory about
the tides: “proporrò una fantasia ingegnosa,” he says. He had long
been of opinion that the unknown problem of the tides would receive
some light on the assumption of the Earth’s motion. Other persons had
adopted his statement on this point as if it had been their own; he
therefore thought it desirable to expound it himself. He hints, too,
that the willingness to admit the stability of the Earth, and to take
the contrary side solely for mathematical caprice, is partly based
on piety, religion, the knowledge of the Divine omnipotence, and the
consciousness of human weakness.
He had thought it well to cast these thoughts into the form of a
dialogue, which gave a certain amount of freedom to digressions.
He then introduces the personages who sustain the discussion, and who
are supposed to meet at Venice at the palace of one of their number,
Sagredo by name.
This preface, if one may judge by internal evidence, was probably the
joint composition of Galileo and Father Riccardi, the former having
written the original draft, the latter having altered the draft and
supplemented it with important additions.
The body of the Dialogue--which I suspect that many persons who
consider themselves competent to give an opinion on the Galileo
case have not so much as even seen--is divided into four portions,
each being supposed to be one day’s dialogue. The interlocutors are
Salviati, Sagredo, and Simplicio. Great offence was taken at the rôle
attributed to this last-named personage--the true doctrine put into the
mouth of a simpleton! It has been said that Pope Urban VIII. considered
it as an insult directed against himself, because, in conversation with
Galileo, he had used some of the very arguments employed by Simplicio.
This, however, may have happened without the author intending thereby
to offer any personal affront to His Holiness; some character was bound
to appear on the anti-Copernican side, and it was inevitable that the
arguments that Galileo had heard, whether from ignorant or enlightened
antagonists, should be put into the mouth of such character. The name
Simplicio is of course not meant as a compliment; moreover, he is made
to say some very unwise things, and is occasionally treated with a
sort of polite contempt by the scientific and mathematical Salviati;
and yet he is not at all a simpleton in our sense of the word, he is
a devoted follower of Aristotle, whom he constantly quotes, and is in
fact a type--probably exaggerated--of the school of the Peripatetics,
as they were, and still are, called; he does not know much of geometry
or arithmetic, and so is at no small disadvantage when arguing with
Salviati, but he is far from being a mere fool. Our author, in his
preface, introduces Salviati and Sagredo--the former a Florentine,
the latter a Venetian--as real personages, deceased friends of his
own, though this may be a mere conventional form of expression; but
he expressly states that Simplicio is not the true name of the “buon
Peripatetico.”
The friends are supposed to meet in the palace of Sagredo, at Venice,
as before stated.
The first day’s dialogue deals with a good deal of what one may term
preliminary matter: that bodies have three dimensions and no more; that
circular motion is the most perfect and the most natural; showing by
this that Galileo had not at that time arrived at a true comprehension
of the first law of motion, as we now hold it. The motion of weights
on an inclined plane finds also a place in the discussion; and so does
what we now term the law of accelerating force, which Galileo had
grasped so well as to be able to explain how the velocity increases by
infinitely small steps gradually, and not, as it were, by sudden jumps.
Much of the matter disputed on--as, for example, whether the heavenly
bodies being incorruptible differ in that respect from the Earth,
liable as it is to corruption and decay--which seems to us either
erroneous in conception or irrelevant to the question at issue, or
both--arose out of the old Aristotelian philosophy; and in those days a
dissertation which neglected points of this kind would have been looked
upon probably with contempt, as evading subjects that it ought to have
grappled with. The distinction between natural and artificial motion,
which occurs repeatedly in the Dialogue, is an instance of an utterly
mistaken notion, having its origin in Aristotle, who, great philosopher
though he was in other ways, failed in his investigations of physical
science, partly from being misled by verbal fallacies.[7]
Another point that our author endeavours to establish in the first
day’s dialogue is that the Moon is not a polished surface, as Simplicio
and others thought, but much like our own Earth, with mountains and
plains and seas--this last being a mistake, as subsequent observation
has shown. The solar spots are also discussed, and so, incidentally, is
the question whether the heavenly bodies are inhabited, the affirmative
opinion finding little favour with any one.
During the second day the great subject is the revolution of the Earth
on its axis; and Salviati urges forcibly the improbability of the
motion of the whole celestial sphere round the Earth in twenty-four
hours, including such a number of vast bodies, and with such an immense
velocity, while one single body (the Earth), turning round on itself,
would produce the same effect. He argues also that if you believe in
this motion of the celestial sphere, you must suppose the planets to
be moving in two opposite directions at the same time, the diurnal
one from east to west, and the annual one from west to east--using
the word _annual_ in its extended sense, as applied to the periodical
revolutions of all the planets. To this Simplicio makes the sapient
answer that Aristotle proves that circular motions are not contrary
to each other; upon which the third interlocutor, Sagredo, asks him
whether when two knights meet one another in the open field, or two
fleets at sea--in the latter case sinking each other--such motions
can be called contrary? This Simplicio is obliged to admit; he uses,
however, another argument, which did not seem so absurd in the then
existing state of science, namely, that there may be another sphere
beyond that of the stars, and itself starless, to which belongs the
property of the diurnal revolution, and that this sphere may carry
along with it the inferior spheres, these latter participating in its
movement. Ideas such as these were part of the pre-telescopic notions
of astronomy. Simplicio’s argument is in reply to some powerful reasons
drawn from the motions of the planets, the nearer revolving in a
shorter, and the more remote in a longer period; it being extremely
unlikely that they would be all whirled round the Earth in one day; and
also from considerations connected with the stars.
It took a long time to disabuse the human mind of the antiquated
opinion that the stars and planets were set in vast movable spheres, as
lamps might be set in a large revolving cupola.
One of the objections made at that time against the axial rotation of
the Earth was that, if it were really the case, any weight dropped from
a high tower would fall some way to the west of the tower, on account
of the latter having been carried on eastward by the revolution of the
Earth during the few seconds the weight takes in falling,[8] and that
such a result was contrary to experience. In those days, when even
the first law of motion had been barely guessed at, the second law,
that of the action of combined forces on any body, was of course not
generally understood; and a considerable debate as to this point occurs
in this same day’s dialogue. Simplicio has the hardihood to assert that
if a stone be let fall from the mast of a vessel, the vessel being in
motion, it falls behind the mast. Salviati, after making a foolish
distinction--in accordance, however, with the philosophical ideas then
prevalent--between the natural motion of the Earth on its axis, and
the artificial motion of the vessel, asks Simplicio if he has ever
tried the experiment, which, of course, he had not. He then tells him,
and most truly so, that the experiment, if made, would show a very
different result, and that the stone would fall at the foot of the
mast, whether the vessel were in motion or not. Further on, Simplicio
maintained that a projectile thrown from the hand, according to
Aristotle’s argument, is carried on by the air, itself set in motion by
the hand of the projector; and if the stone let fall from the mast of a
ship falls at the foot of the mast, it must be the effect of the air.
So again he imagines that a ball dropped from the hand of a man, riding
fast on horseback, falls some way behind, and does not partake of the
horse’s speed. Salviati, however, tells him that he deceives himself,
and that experience would teach him the contrary.
Various difficulties are discussed in this dialogue well known to the
disputants of that day. It being questioned why a projectile shot from
a gun point-blank towards the east does not fall above the mark aimed
at; or shot westwards fall below it? How it is that birds, when flying,
are not left behind by the revolving Earth, since they at any rate are
completely detached from the ground above which they are soaring? Why
it is that light objects do not fly off at a tangent?
One sees throughout the power of the master-mind of Galileo. He knew
many things in mechanics which no subsequent research or experiment
has ever corrected; but here and there, as may naturally be supposed,
he is at fault. It must ever be remembered that a dialogue, though
a convenient form of argument in some respects, does not always give
one a clear insight into the author’s real convictions. You are not
sure whether he quite agrees with any of the spokesmen, and, indeed,
Galileo, in his defence before the Inquisition, practically assumes
that he did not so agree. It is, however, a good form of discussion
for a man whose opinions are intended to be expressed in a _tentative_
shape, and perhaps Galileo’s mind was in a state congenial to such
expression. But, at any rate, it makes it rather more difficult to do
justice to the author, as one is never sure what he intends to be taken
as the expression of his own deliberate belief; indeed, whatever may
have been the amount of indecision in which in this case our author’s
mind was involved, it is scarcely possible, notwithstanding his
disclaimer, to ignore the fact of his strong Copernican opinions.
I think one may say that Galileo did not, at the time when he wrote
the dialogue, know the gravity of the air. I say at that time, because
it is quite possible that he knew it before his death, since he lived
some ten or twelve years after writing this work. It is maintained that
he knew it because there is extant a letter from Baliani, the date
of which I believe to be about 1631, in which the latter expresses
his acknowledgments to Galileo for having taught him this truth. May
it not, however, be that what is here meant is the _pressure_ of the
air? If any one thinks Galileo understood at that time the principle
of the gravity of the atmosphere, I refer him to the second day’s
dialogue. He was aware, no doubt, that the air was carried round by
the Earth in its diurnal motion, but why it was so carried round I do
not think he quite understood; indeed, as may well be supposed, he did
not _clearly_ understand what gravity was; it was a mysterious force,
drawing heavy bodies towards the centre of the Earth, a force to which
we, indeed, give the name of gravity, but of the essence of which we
know nothing, as, in fact, we know nothing of the nature of the force
that moves the heavenly bodies. This passage is remarkable because it
looks as if Galileo half suspected that the force which acted on the
Moon and the planets might be akin to that which attracted terrestrial
objects towards the centre of the Earth. If he really had arrived at
such a conclusion, he would have anticipated the great discovery made
thirty or forty years later. I think, however, that he only wished to
illustrate the one by the other, and that the allusion means no more.
I give, however, the passage in a note,[9] so that any reader may form
his own judgment; and I may add that according to an opinion commonly
held by the Copernican school of that age, the adherence of the
atmosphere to the Earth as it revolved was the effect of _friction_.
Our philosopher, wise as he was, had not freed himself from the
antiquated notion that some bodies were essentially heavy and others
light, which latter had no tendency to descend; not thereby meaning
comparatively light substances, but such as were absolutely free from
the action of gravity; the fact not being then understood that it is
only the resistance of the air that prevents the smallest feather from
falling to the ground as quickly as a cannon-shot.
Another mistake into which he falls is that of maintaining, in answer
to the argument that the diurnal rotation of the Earth would cause
objects to fly off from the surface at a tangent, that _no amount_ of
velocity of rotation would be sufficient for such a result to follow;
whereas, it is well known to modern students of mechanics that if a
certain very high velocity of rotation were reached, the centrifugal
force would overcome that of gravity, and objects would be projected
from the surface of the Earth in the direction of the tangent at that
point.
Some irrelevant arguments occur, of which, no doubt, many were
employed at that time on both sides; I think it was the late Professor
de Morgan who (in an article written for a popular periodical) made
a list of these; and it must in all fairness be said, that this
circumstance ought to be taken into account, as palliating the apparent
obstinacy of the anti-Copernican party in denying the motion of the
Earth. The argument drawn from the tides is, of course, the most
striking instance of these scientific fallacies; but it was by no means
the only one; in this particular dialogue there is another, which is
worth noticing because it confirms what I have just said as to Galileo
knowing nothing of the doctrine of universal gravitation. He puts
into the mouth of Salviati the argument that bodies which emit light,
as do the Sun and fixed stars, are essentially different from those
which, like the Earth and planets, have no such property--a distinction
which modern astronomy does not endorse--and that, as the Earth in
this respect resembles the planets, and the planets are undoubtedly
moving, so probably the Earth also is like them in motion, whilst the
Sun and the stars remain at rest. It is obvious that ideas of this
kind, however plausible they may seem, are utterly at variance with the
theory of universal gravitation, according to which, even if the Sun
were a dark, cold body and the Earth glowing with heat and light, the
Earth would revolve about the Sun just as it does now, _provided the
mass of the two bodies remained the same_ as at present.
Another suggestion, and a rather amusing one, on the opposition side,
was that all things in motion require occasional rest, as we see to
be the fact with animals; therefore the Earth, if it were constantly
moving, would stand in need of rest--an argument, I suppose, which
needs no very elaborate answer.
In the third day’s dialogue a question is raised, and sifted at
great length, as to whether a certain newly observed star in the
constellation Cassiopeia was in the firmament among the distant fixed
stars, or “sublunar,” _i.e._ nearer to the Earth than the Moon. This
star was probably the same as the very remarkable one first observed
by Tycho Brahé in 1572, which attained a brilliancy so extraordinary,
that it is said to have been equal to the planet Venus, and to have
been visible to good eyes in full daylight; in about a month’s time it
appeared to grow smaller, and gradually faded away until it disappeared
entirely--about six months after it was first discovered. This was some
years before the invention of the telescope, and the observations were
deprived of any assistance they might have gained from that source. The
star was one of the most noteworthy of all the variable stars on record.
There followed upon the mention of this star, a dissertation on the
method of finding the distances of the heavenly bodies by parallax. The
principle of this method was, as we may suppose, well known to Galileo;
but he probably did not allow _sufficiently_ for the great difficulty
in taking accurate observations, especially with the imperfect
instruments then in use; I say sufficiently, because that there were
such errors he knew, and he insists on the fact in the Dialogue.
Much discourse is spent on the distance of this new star; the
apparent reason of which is that it had created some sensation among
the astronomers of that day, and therefore the subject received an
attention out of proportion to its real importance--I mean importance
so far as the Copernican controversy was concerned.
The conversation is then brought back to the objections made by
contemporary philosophers to the Copernican system. Aristotle’s idea
of the universe was that of a vast sphere, or number of concentric
hollow spheres, with the Earth in the centre; if that were shown to be
probably untrue, his system broke down.[10] Coming, however, to our own
immediate portion of the universe, the question is now raised whether
the Earth or the Sun is the centre of revolution. Galileo, by the mouth
of Salviati, explains forcibly the argument for the Sun being so. That
Mercury and Venus revolve round the Sun he takes for certain; the
phases of Venus, which he had himself observed, proved it as regards
that planet; and the fact of neither of these bodies ever being seen
far apart from the Sun, greatly strengthened the conclusion in respect
of both of them. A transit of Mercury over the Sun’s disc had, in fact,
been observed in the year 1631, by Gassendi; but Galileo was doubtless
not aware of it when he wrote the Dialogue.
It being clear then that Venus and Mercury revolve round the Sun,
Galileo shows what strong ground there is for inferring that the
superior planets, Mars, Jupiter, and Saturn (the others not being then
known), do so also; this he judges from the greater size of these
latter, and particularly of Mars, when in opposition than when in
conjunction; whence we may conclude that the Earth, which as well as
the Sun is contained within their orbits, is not in the centre of them,
or nearly so. It is remarkable that Galileo treats all the planets as
revolving in _circles_, though one would think he must at that time
have been aware of Kepler’s discovery--that they move in _ellipses_. He
makes Simplicio grant these last-mentioned points, which is curious;
and he also explains how the telescope showed phenomena, such as the
phases of Venus, which were unknown to Copernicus. Simplicio has
hitherto had no confidence in this new instrument, and following in the
footsteps of his friends the Peripatetic philosophers, has supposed the
appearances in question to be optical illusions arising from the lenses
used; he will, however, gladly be corrected if in error. Simplicio’s
mathematical acquirements are not very great, and it is necessary to
explain to him that the areas of circles vary in proportion, not to
their diameters simply but to the squares of the diameters, a point
which arises in reference to the false judgment formed by the naked eye
as to the size of the celestial bodies, an error which is corrected
by the telescope. Then to those who made it a difficulty that the
Earth should move round the Sun, not alone, but accompanied by the
Moon, Salviati is made to reply that Jupiter revolves round the Sun
accompanied by four moons.
Again the greater simplicity of the Copernican theory, in accounting
for the planetary motions, as they appear to us, is expounded by the
same personage.
Galileo occasionally makes the interlocutors allude to himself as “il
nostro amico comune,” “il nostro Accademico Linceo,” etc., and thus
claims credit for having been the first to discover the solar spots, a
credit which ought not to belong exclusively to him, as Fabricius and
the Jesuit Father Scheiner saw the spots at about the same time.
An argument is here attempted to be drawn in favour of the Earth’s
annual motion from the apparent course of the Sun-spots, and the
curves they sometimes describe (as viewed from hence), owing to the
inclination of the Sun’s axis to an axis perpendicular to the plane of
the ecliptic--an inclination of about 7°; there is nothing, however, at
all conclusive in such argument, because the appearances in question
result from the different _relative_ positions of the Earth and Sun at
different seasons of the year, and would be the same whichever of the
two bodies were in motion.
There follows some conversation arising from one of the anti-Copernican
books of that day; one of the difficulties suggested, being the vast
distance at which you must suppose the fixed stars to be placed, if
Copernicus be right. We who are accustomed to the idea of these immense
distances, can scarcely understand the prejudices of the philosophers
of that age against admitting them. And it is worth noting that Galileo
takes for granted, while answering these theoretical objections, the
calculation of his predecessors--that the distance of the Sun is that
of 1,208 semi-diameters of the Earth, that is something more than
4,800,000 miles, about one-nineteenth part of what we now know it to
be. So also he supposes the size of the Sun to be much less than what
is really the case. He was also under the erroneous impression, arising
doubtless from the imperfection of the instruments he used, that the
stars really had an apparent diameter, though less than Tycho Brahé and
other astronomers had supposed, and estimates the angular diameter of a
star of the first magnitude at about 5″; consequently he imagined the
stars to be much nearer than is actually the fact. It is well known to
modern observers, that the apparent size of a star is the effect of an
optical illusion, and that greatly as the stars vary in brightness,
they present no appreciable diameter at all to the eye; not even those
classed as being of the first magnitude.
Another and more weighty objection to Copernicus is, however, urged by
the mouth of Simplicio, and it is this--if the Earth really makes an
annual revolution round the Sun, why do not the fixed stars, viewed as
they must be at different seasons of the year from points so widely
distant, change their apparent positions in the heavens? We have just
seen that the true distance of the Sun was not known at that time;--if
it had been known, and if the men of that age had been aware that the
diameter of the Earth’s orbit was about 184,000,000 miles in length,
the objection would have been still more forcible. But the modern
answer to it is conclusive: the stars, or rather a certain number
of them, do actually undergo a small displacement in their apparent
position every year, or in the technical language of astronomy, they
have an annual parallax, a fact which not merely disposes of the
objection, but actually confirms the truth of the Copernican theory.
Galileo’s reply (by the mouth of Salviati) is to the effect that
the followers of Ptolemy admit that it takes 36,000 years to effect
a complete revolution of the starry sphere; then, judging from the
planets, the length of time required for the orbit is in proportion
to the distance, and we suppose the distance of the starry sphere to
be, on such assumption, 10,800 semi-diameters of the Earth’s orbit (or
Sun’s orbit, as they called it). At so great a distance as that, the
change of position caused by the Earth’s annual motion round the Sun
would not be appreciable.
The principle of this reply is of course quite sound, and we, who know
the stars to be considerably farther from us than the above estimate
supposes, can well understand that the vast majority of them have no
annual parallax whatever, that the finest instruments can discover.
To further objections drawn from the enormous distances of the stars,
and the difficulty of perceiving the use which such remote bodies can
be to the Earth, it is replied that such speculations are useless and
presumptuous, and also that words like small, very small, immense,
etc., are relative rather than absolute.
Some pains are taken in the course of the dialogue to explain how the
stars, according to their different positions, would be affected by
annual parallax, supposing such to be discoverable, and assuming the
motion of the Earth. And a minute explanation is also given, on this
latter assumption, of the length of day and night varying in different
latitudes according to the seasons; illustrating the fact that details
which appear to us elementary and are taught to schoolboys, were
strange to the minds even of educated and learned men in those days.
One remark, arising from the questions connected with stellar parallax,
is most striking, as showing how far Galileo was advanced in his
knowledge of pure mathematics as well as of mechanics and astronomy.
Salviati is made to say that the circumference of an infinite circle
is identical with a straight line: “sono l’istessa cosa.” This idea,
familiar though it be to modern mathematicians, is one that we
should not have expected to find enunciated in the early part of the
seventeenth century; even the intelligent Sagredo cannot understand or
believe it, and it is not further discussed; but the fact of its being
here stated is especially noteworthy.[11]
Another (less felicitous) guess is hazarded by the same interlocutor
Salviati, who, as I have already remarked, appears to be the one that
most nearly represents the author’s own mind,--to account for the
Earth keeping her axis pointed (approximately, that is to say) in the
same direction during each annual revolution round the Sun. Salviati
suggests that it may be due to some magnetic influence, and that the
interior of the Earth may be a vast loadstone. This is strange, because
it is evident from what immediately preceded, that the author was aware
of the true reason, which in fact he illustrates by the well-known
experiment of a light ball floating in a bucket of water, to which a
revolving motion is imparted. It seems, however, that a work by William
Gilbert on the subject of magnetism had had some influence on the
scientific thought of the period, and that Galileo had considered it
worthy of his attention. The writer had maintained the probability of
this theory, of the Earth’s interior being an enormous loadstone--not
an unnatural idea in the then-existing state of science--and Galileo
was evidently somewhat fascinated by the hypothesis. Magnetism was
attracting the notice of the philosophers of that day, and the property
of the needle, which is termed the _dip_, had been recently discovered.
There is not much else worthy of special mention in the third day’s
dialogue; which in fact, as a whole, is not equal to that of the second
day.
The fourth day is mainly devoted to the argument drawn from the tides.
It was in handling this branch of the subject that Galileo’s great
sagacity and power of discernment seem to have deserted him. It is a
curious thing that the inhabitant of a Mediterranean country, who, for
all that one knows, never saw a really great tide in his life, should
have seized upon this topic, and so utterly misused and perverted it.
If, instead of living in Italy, he had resided at an English seaport,
he would probably have never fallen into the mistakes he thus made. In
the Mediterranean there are currents, arising from other causes, which
he, however, attributed to tidal action; but for the most part there
is little, if any, appreciable ebb and flow of the tides, scarcely any
perceptible rise and fall of the sea, a fact which he particularly
notices. But in some few places, and notably at Venice, there is
a sensible tide, so it is said, causing a difference of a few feet
between high and low water.
Now Galileo was under the impression that the ebb and flow took each
about six hours, following the ordinary solar day; whereas, if he had
observed the phenomenon on the shores of any sea, where the tidal wave
of the ocean made its full force to be felt, or again, at the mouth of
a great tidal river, he never could have failed to perceive that the
rise and fall of the water follow approximately the _lunar_, and not
the solar day, the former being fifty minutes longer than the latter.
It must of course be understood that the theory of the tides was first
investigated fully and scientifically by the same great genius to whom
we owe the theory of universal gravitation; and Galileo, who lived
half a century earlier, may well be excused for not having grasped
it. But it had long been known that the Sun and Moon had an influence
upon the tides, and as I have just stated, any one who watched the
movements of the sea from day to day, and from week to week, at a place
where there is a great rise and fall--as for instance, in the Bristol
Channel--could not fail to perceive that the Moon had the principal
share in the work, however unable he might be to comprehend the theory.
Besides which, the theory, however obvious to us (at least in its main
outlines), was not by any means so intelligible to the men of Galileo’s
age. They might just guess that the Sun exercised some attractive
influence over the Earth, and the Earth again over the Moon, but they
did not know that the Moon attracted the Earth exactly in the same way,
though with far inferior potency, owing to her much smaller mass; and
consequently they were not aware of the Moon’s power to raise the great
tidal wave in the ocean, to which are due the remarkable phenomena so
familiar to the inhabitants of the English coasts.
Galileo would have been wise if he had not touched on a point which
he neither understood in theory, nor had properly acquainted himself
with by practical observation. Good causes are often damaged by bad
arguments, and such was the case on this occasion.[12] There was,
however, something ingenious in his argument. If you take a basin of
water, and move it along quite smoothly and evenly, no great commotion
in the water takes place; but suppose some stoppage or jerk to occur,
the result will be, as we know, very different. Now the Earth has two
motions, one round its axis in twenty-four hours, and the other round
the Sun in one year; every point, then, on the Earth’s surface moves
through space more rapidly while on that side of the globe which is
turned away from the Sun, than on that side which by the diurnal
revolution is turned round in the contrary direction. Here, then, with
the sea lying in its vast basin, and revolving with other things on the
surface of the Earth from west to east every day, and thus accelerated
in its motion through space during twelve hours and retarded during
the other twelve hours, you have on a large scale the same result
that a basin, half full of water, held in your hands and checked by
some retarding obstacle, gives you on a very small and minute scale.
Strange indeed it is that a man who was acquainted with the laws of
motion sufficiently to know that anything thrown or dropped in a
vessel or a vehicle, partook of the motion of the latter and followed
its course (so long as it remained within the vehicle) just as if the
whole were at rest--that he should have failed to perceive that the
ocean, lying in its bed in that mighty vehicle the Earth, would be
carried round in the daily rotation with an uniform velocity, unless
interfered with by the attraction of other bodies. Simplicio, who for
once is right, puts the difficulty, that if the sea behaved in the
way supposed, the air would do so in the same way: the reply to which
is that the air being thin and light is less adherent to the Earth
than the water which is heavier, and does not accommodate itself to
the Earth’s movements as water does; further, that where the air is
not hemmed in, as it were, by mountains and other inequalities on the
Earth’s surface, it really is partially left behind by the diurnal
rotation, and in the neighbourhood of the tropics, where the effect is
chiefly felt, a constant wind blows accordingly from east to west. Our
philosopher had evidently heard of the trade winds, though he had not
acquired an accurate knowledge of their course or of their origin. It
is undoubtedly true that they do help strongly to prove the revolution
of the Earth, because they arise from cold currents of air flowing in
from the north and from the south respectively towards the tropics,
to supply the place of the atmosphere rarefied by the sun’s heat, and
consequently ascending, as is the case in those regions. Then these
cold currents, coming from latitudes where there is a less velocity of
rotation, tend to preserve that velocity and lag behind the Earth as
it revolves, so that they have the effect of north-easterly winds in
the northern hemisphere, and south-easterly in the southern hemisphere.
Galileo’s imperfect information prevented him from using this important
argument.
However, to return to the tides. He had to account for other phenomena,
besides the daily rise and fall, namely, for the much greater rise and
fall which take place soon after new and full moon, and which are known
as the spring-tides. Unable to deny that these were in some way due
to lunar influence, he took refuge in the supposition that the Moon,
when at the full, retarded the motion of the Earth in its orbit, since
as the two travel together round the Sun at those particular times,
they form, as it were, a lengthened pendulum, longer than at other
times by the semi-diameter of the lunar orbit; and therefore (like any
other pendulum) must vibrate more slowly. I should say that he does
not appear to have been aware of the existence of _two_ spring-tides
in each lunation, and therefore only tries to account for one; and it
is obvious that this method of explaining them is not only utterly
inadequate, but even absurd. The Moon truly enough exercises a certain
disturbing influence on the orbital motion of the Earth, but that has
nothing to do with the spring-tides.
There remained the necessity of accounting for the annual, or, more
properly, semi-annual increase of the ebb and flow of the sea. Galileo
suggests that this arises from the angle made by the plane of the
equator with the ecliptic at the equinoxes, owing to which there would
not be the same counteraction exercised by the Earth’s motion in its
orbit on the waters of the ocean at those periods as there would at
the solstices. But it seems that this would rather tend to diminish
the tides than to increase them, as, indeed, would be the case as
regards the last-mentioned explanation with respect to the ordinary
spring-tides. What really does happen at the equinoxes is, that the Sun
and the full or new Moon being at those times vertical to the equator
(or nearly so), they have a greater attractive force than at other
spring-tides over the vast expanse of the ocean, and the tides are
consequently greater. There is also another increase which sometimes
occurs when the Moon happens to be at its least distance from the
Earth at the time of spring-tides, but that was unknown to Galileo.
He touches, however, and very properly so, on the great modifications
in the tides caused by various gulfs, by the forms of the great
continents, and the shapes of different seas--modifications, in fact,
which are well known to be almost innumerable, and have been learnt
only by careful observation and experience.
One of the worst features of this Dialogue is the contempt which the
author shows for those opinions on the subject which differ from his
own; and it is difficult to suppress a feeling of disgust when he
alludes in this way to Kepler, who had partly guessed the true cause of
the tides, and of whom he otherwise speaks in terms of respect.[13]
If a man of science, when he wishes to publish to the world a discovery
or a hypothesis, adopts the form of a dialogue as a method of stating
his case, he ought in all reason to do full justice to the antagonistic
side, and state his opponent’s case as well as his own. I fear that
Galileo failed to do this, not only in this particular dialogue, but
also to some extent in those of the three preceding days. Simplicio,
as I said above, is not a fool, but as a personage in a scientific
argument he is lamentably deficient.
Simplicio at the end of the Dialogue urges that God could, in His
infinite power, cause the tides by some other means than those
suggested by Salviati, to which true and pious (though, perhaps, rather
irrelevant) argument the latter respectfully and devoutly assents.
The concluding sentences are said, as I have remarked elsewhere, to
have been recast or retouched by Father Riccardi.
It is worth noticing that there is a passage in the fourth day’s
dialogue, in which the author alludes to the fact of the Sun being
apparently longer by about nine days in passing along the ecliptic from
the spring to the autumn equinox, than in passing from the autumnal to
the vernal; that is to say, of the northern hemisphere having so much
longer summer than winter, and he treats it as one of the recondite
problems of astronomy not as yet understood. This is an additional
proof that for some reason or another he had not made himself
acquainted with Kepler’s researches; for as soon as it became known
that the planets move, not in circles, but in ellipses, with the Sun
in one of the foci, it was obvious that there would be in every case
(though in some more than others) this inequality to which allusion has
been made, and the Earth, if a planet, would be subject to the same
rule as the rest.
Such, then, is a somewhat imperfect _précis_ of this famous work of
Galileo, which owes its importance to the historical circumstances
connected with its publication quite as much, to say the least of it,
as to its own intrinsic merit.