of the senses were trustworthy under all conditions, familiar or
otherwise; whereas, in point of fact, we know that the senses often
deceive, even under familiar conditions, and almost always deceive under
conditions, which are not familiar. A person, for example, accustomed to
the mist and haze of our British air, is told by the sense of sight,
when he is travelling where a clearer atmosphere prevails, that a
mountain forty miles from him is a hill a few miles away. On the other
hand, an Italian travelling through the Highlands is impressed with the
belief that all the features of the scenery are much larger (because he
supposes them much more remote) than they really are. A hundred such
instances of deception might easily be cited. The conditions under which
the aeronaut observes the earth are certainly less familiar than those
under which the Briton views the Alps and Apennines, or the Italian
views Ben Lomond or Ben Lawers. It would be rash, therefore, even if no
other evidence were available, to reject the faith that the earth is a
globe because, as seen from a balloon, it looks like a basin. Indeed, to
be strictly logical, the followers of Parallax ought on this account to
adopt the faith that the earth is not flat, but basin-shaped, which
hitherto they have not been ready to do.
We have seen that Parallax describes a certain experiment on the Bedford
Level, which, if made as he states, would have shown certainly that
something was wrong in the accepted system--for a six-mile straight-edge
along water would be as severe a blow to the belief in a round earth, as
a straight line on the sea-surface from Queenstown to New York. Another
curious experiment adorns his little book, which, if it could be
repeated successfully before a dozen trustworthy witnesses, would rather
astonish men of science. Having, he says, by certain
reasoning--altogether erroneous, but that is a detail--convinced himself
that, on the accepted theory, a bullet fired vertically upwards ought to
fall far to the west of the place whence it was fired, he carefully
fixed an air-gun in a vertical position, and fired forty bullets
vertically upwards. All these fell close to the gun--which is not
surprising, though it must have made such an experiment rather
dangerous; but two fell back into the barrel itself--which certainly was
very surprising indeed. One might fairly challenge the most experienced
gunner in the world to achieve one such vertical shot in a thousand
trials; two in forty bordered on the miraculous.
The earth-flatteners I have been speaking of claim, as one of their
objects, the defence of Scripture. But some of the earth-flatteners of
the last generation (or a little farther back) took quite another view
of the matter. For instance, Sir Richard Phillips, a more vehement
earth-flattener than Parallax, was so little interested in defending
the Scriptures, that in 1793 he was sentenced to a year's imprisonment
for selling a book regarded as atheistic. In 1836 he attempted the
conversion of Professor De Morgan, opening the correspondence with the
remark that he had 'an inveterate abhorrence of all the pretended wisdom
of philosophy derived from the monks and doctors of the Middle Ages, and
not less those of higher name who merely sought to make the monkish
philosophy more plausible, or so to disguise it as to mystify the mob of
small thinkers.' He seems himself to have succeeded in mystifying many
of those whom he intended to convert. Admiral Smyth gives the following
account of an interview he had with Phillips: 'This pseudo-mathematical
knight once called upon me at Bedford, without any previous
acquaintance, to discuss "those errors of Newton, which he almost
blushed to name," and which were inserted in the "Principia" to "puzzle
the vulgar." He sneered with sovereign contempt at the "Trinity of
Gravitating Force, Projectile Force, and Void Space," and proved that
all change of place is accounted for by motion.' [Startling hypothesis!]
'He then exemplified the conditions by placing some pieces of paper on a
table, and slapping his hand down close to them, thus making them fly
off, which he termed applying the momentum. All motion, he said, is in
the direction of the forces; and atoms seek the centre by "terrestrial
centripetation"--a property which causes universal pressure; but in what
these attributes of pushing and pulling differ from gravitation and
attraction was not expounded. Many of his "truths" were as mystified as
the conundrums of Rabelais; so nothing was made of the motion.'
A favourite subject of paradoxical ideas has been the moon's motion of
rotation. Strangely enough, De Morgan, who knew more about past
paradoxists than any man of his time, seems not to have heard of the
dispute between Keill and Bentley over this matter in 1690. He says,
'there was a dispute on the subject, in 1748, between James Ferguson and
an anonymous opponent; and I think there have been others;' but the
older and more interesting dispute he does not mention. Bentley, who was
no mathematician, pointed out in a lecture certain reasons for believing
that the moon does not turn on her axis, or has no axis on which she
turns. Keill, then only nineteen years old, pointed out that the
arguments used by Bentley proved that the moon does rotate instead of
showing that she does not. (Twenty years later Keill was appointed
Savilian Professor of Astronomy at Oxford. He was the first holder of
that office to teach the Newtonian astronomy.)
In recent times, as most of my readers know, the paradox that the moon
does not rotate has been revived more than once. In 1855 it was
sustained by Mr. Jellinger Symons, one of whose staunchest supporters,
Mr. H. Perigal, had commenced the attack a few years earlier. Of course,
the gist of the argument against the moon's rotation lies in the fact
that the moon always keeps the same face turned towards the earth, or
very nearly so. If she did so exactly, and if her distance from the
earth were constantly the same, then her motion would be exactly the
same as though she were rigidly connected with the earth, and turned
round an axis at the earth. The case may be thus illustrated: Through
the middle of a large orange thrust one short rod vertically, and
another long rod horizontally; thrust the further end of the latter
through a small apple, and now turn the whole affair round the short
vertical rod as an axis. Then the apple will move with respect to the
orange as the moon would move with respect to the earth on the
suppositions just made. No one in this case would say that the apple was
turning round on its axis, since its motion would be one of rotation
round the upright axis through the orange. Therefore, say the opponents
of the moon's rotation, no one should say that the moon turns round on
her axis.
Of course, the answer would be obvious even if the moon's motions were
as supposed. The moon is not connected with the earth as the apple is
with the orange in the illustrative case. If the apple, without rigid
connection with the orange, were carried round the orange so as to move
precisely as if it were so connected, it would unquestionably have to
rotate on its axis, as any one will find who may try the experiment.
Thus for the straight rod thrust through the apple substitute a straight
horizontal bar carrying a small basin of water in which the apple
floats. Sway the bar steadily and slowly round, and it will be found (if
a mark is placed on the apple) that the apple no longer keeps the same
face towards the centre of motion; but that, to cause it to do so, a
slow motion of rotation must be communicated to the apple in the same
direction and at the same rate (neglecting the effects of the friction
of the water against the sides of the basin) as the bar is rotating. In
my 'Treatise on the Moon' I have described and pictured a simple
apparatus by which this experiment may easily be made.
But, of course, such experiments are not essential to the argument by
which the paradox is overthrown. This argument simply is, that the moon
as she travels on her orbit round the sun--the real centre of her
motion--turns every part of her equator in succession towards him once
in a lunar month. At the time of new moon the sun illuminates the face
of the moon turned from us; at the time of full moon he illuminates the
face which has been gradually brought round to him as the moon has
passed through her first two quarters. As she passes onwards to new
moon again, the face we see is gradually turned from him until he
shines full upon the other face. And so on during successive lunations.
This could not happen unless the moon rotated. Again, if we lived on the
moon we should find the heaven of the fixed stars turning round from
east to west once in rather more than twenty-seven days; and unless we
supposed, as we should probably do for a long time, that our small world
was the centre of the universe, and that the stars turned round it, we
should be compelled to admit that it was turning on its own axis from
west to east once in the time just named. There would be no escape. The
mere fact that all the time the stars thus seemed to be turning round
the moon, the earth would not so seem to move, but would lie always in
the same direction, would in no sort help to remove the difficulty.
Lunarian paradoxists would probably argue that she was in some way
rigidly connected with the moon; but even they would never think of
arguing that their world did not turn on its axis, _unless_ they
maintained that it was the centre of the universe. This, I think, they
would very probably do; but as yet terrestrial paradoxists have not, I
believe, maintained this hypothesis. I once asked Mr. Perigal whether
that was the true theory of the universe--the moon central, the earth,
sun, and heavens carried round her. He admitted that his objections to
accepted views were by no means limited to the moon's rotation; and, if
I remember rightly, he said that the idea I had thrown out in jest was
nearer the truth than I thought, or used words to that effect. But as
yet the theory has not been definitely enunciated that the moon is the
boss of the universe.
Comets, as already mentioned, have been the subjects of paradoxes
innumerable; but as yet comets have been so little understood, even by
astronomers, that paradoxes respecting them cannot be so readily dealt
with as those relating to well-established facts. Among thoroughly
paradoxical ideas respecting comets, however, may be mentioned one whose
author is a mathematician of well-deserved repute--Professor Tait's
'Sea-Bird Theory' of Comets' Tails. According to this theory, the rapid
formation of long tails and the rapid changes of their position may be
explained on the same principle that we explain the rapid change of
appearance of a flight of sea-birds, when, from having been in a
position where the eye looks athwart it, the flight assumes a position
where the eye looks at it edgewise. In the former position it is
scarcely visible (when at a distance), in the latter it is seen as a
well-defined streak; and as a very slight change of position of each
bird may often suffice to render an extensive flight thus visible
throughout its entire length, which but a few moments before had been
invisible, so the entire length of a comet's tail may be brought into
view, and apparently be formed in a few hours, through some
comparatively slight displacement of the individual meteorites composing
it.
This paradox--for paradox it unquestionably is--affords a curious
illustration of the influence which mathematical power has on the minds
of men. Every one knows that Professor Tait has potential mathematical
energy competent to dispose, in a very short time, of all the
difficulties involved in his theory; therefore few seem to inquire
whether this potential energy has ever been called into action. It is
singular, too, that other mathematicians of great eminence have been
content to take the theory on trust. Thus Sir W. Thomson, at the meeting
of the British Association at Edinburgh, described the theory as
disposing easily of the difficulties presented by Newton's comet in
1680. Glashier, in his translation of Guillemin's 'Les Cometes,' speaks
of the theory as one not improbably correct, though only to be
established by rigid investigation of the mathematical problems
involved.
In reality, not five minutes' inquiry is needed to show any one
acquainted with the history of long-tailed comets that Tait's theory is
quite untenable. Take Newton's comet. It had a tail ninety millions of
miles long, extending directly from the sun as the comet approached him,
and seen, four days later, extending to the same distance, and still
directly from the sun, as the comet receded from him in an entirely
different direction. According to Tait's sea-bird theory, the earth was
at both these epochs in the plane of a sheet of meteorites forming the
tail; but on each occasion the sun also was in the same plane, for the
edge of the sheet of meteorites was seen to be directly in a line with
the sun. The comet's head, of course, was in the same plane; but three
points, not in a straight line, determine a plane. Hence we have, as the
definite result of the sea-bird theory, that the layer or stratum of
meteorites, forming the tail of Newton's comet, lay in the same plane
which contained the sun, the earth, and the comet. But the comet crossed
the ecliptic (the plane in which the earth travels round the sun)
between the epochs named, crossing it at a great angle. When crossing
it, then, the great layer of meteorites was in the plane of the
ecliptic; before crossing it the layer was greatly inclined to that
plane one way, and after crossing it the layer was greatly inclined to
that plane another way. So that we have in no way escaped the difficulty
which the sea-bird theory was intended to remove. If it was a startling
and, indeed, incredible thing that the particles along a comet's tail
should have got round in four days from the first to the second position
of the tail considered above, it is as startling and incredible that a
mighty layer of meteorites should have shifted bodily in the way
required by the sea-bird theory. Nay, there is an element in our result
which is still more startling than any of the difficulties yet
mentioned; and that is, the singular care which the great layer of
meteorites would seem to have shown to keep its plane always passing
through the earth, with which it was in no way connected. Why should
this preference have been shown by the meteor flock for our earth above
all the other members of the solar system?--seeing that the sea-bird
theory _requires_ that this comet, and not Newton's comet alone but all
others having tails, should not only be thus complaisant with respect to
our little earth, but should behave in a totally different way with
respect to every other member of the sun's family.
We can understand that, while several have been found who have applauded
the sea-bird paradox for what it _might_ do in explaining comets' tails,
its advocates have as yet not done much to reconcile it with cometic
observation.
The latest astronomical paradox published is perhaps still more
startling. It relates to the planet Venus, and is intended to explain
the appearance presented by this planet when crossing the sun's face,
or, technically, when in transit. At this time she is surrounded by a
ring of light, which appears somewhat brighter than the disc of the sun
itself. Before fully entering on the sun's face, also, the part of
Venus's globe as yet outside the sun's disc is seen to be girt round by
a ring of exceedingly bright light--so bright, indeed, that it has left
its record in photographs where the exposure was only for the small
fraction of a second allowable in the case of so intensely brilliant a
body as the sun. Astronomers have not found it difficult to explain
either peculiarity. It has been proved clearly in other ways that Venus
has an atmosphere like our own, but probably denser. As the sun is
raised into view above the horizon (after he has really passed below
the horizon plane) by the bending power of our air upon his rays, so the
bending power of Venus's air brings the sun into our view round the dark
body of the planet. But the new paradox advances a much bolder theory.
Instead of an atmosphere such as ours, Venus has a glass envelope; and
instead of a surface of earth and water, in some cases covered with
clouds, Venus has a surface shining with metallic lustre.[53]
The author of this theory, Mr. Jos. Brett, startled astronomers by
announcing, a few years ago, that with an ordinary telescope he could
see the light of the sun's corona without the aid of an eclipse, though
astronomers had observed that the delicate light of the corona fades out
of view with the first returning rays of the sun after total eclipse.
The latest paradoxist, misled by the incorrect term 'centrifugal force,'
proposes to 'modify, if not banish,' the old-fashioned astronomy. What
is called centrifugal force is in truth only inertia. In the familiar
instance of a body whirled round by a string, the breaking of the string
no more implies that an active force has pulled away the body, than the
breaking of a rope by which a weight is pulled implies that the weight
has exerted an active resistance. Of course, here again the text-books
are chiefly in fault.
Such are a few among the paradoxes of various orders by which
astronomers, like the students of other sciences, have been from time to
time amused. It is not altogether, as it may seem at first sight, 'a sin
against the twenty-four hours' to consider such matters; for much may be
learned not only from the study of the right road in science, but from
observing where and how men may go astray. I know, indeed, few more
useful exercises for the learner than to examine a few paradoxes, when
leisure serves, and to consider how, if left to his own guidance, he
would confute them.
XI.
_ON SOME ASTRONOMICAL MYTHS._
The expression 'astronomical myth' has recently been used, on the
title-page of a translation from the French, as synonymous with false
systems of astronomy. It is not, however, in that sense that I here use
it. The history of astronomy presents the records of some rather
perplexing observations, not confirmed by later researches, but yet not
easily to be explained away or accounted for. Such observations Humboldt
described as belonging to the myths of an uncritical period; and it is
in that sense that I employ the term 'astronomical myth' in this essay.
I propose briefly to describe and comment on some of the more
interesting of these observations, which, in whatever sense they are to
be interpreted, will be found to afford a useful lesson.
It is hardly necessary, perhaps, to point out that the cases which I
include here I regard as really cases in which astronomers have been
deceived by illusory observations. Other students of astronomy may
differ from me as respects some of these instances. I do not wish to
dogmatise, but simply to describe the facts as I see them, and the
impressions which I draw from them. Those who view the facts differently
will not, I think, have to complain that I have incorrectly described
them.
At the outset, let me point out that some observations which were for a
long time regarded as mythical have proved to be exact. For instance,
when as yet very few telescopes existed, and those very feeble,
Galileo's discovery of moons travelling round Jupiter was rejected as an
illusion for which Satan received the chief share of credit. There is an
amusing and yet in one aspect almost pathetic reference to this in his
account of his earlier observations of Saturn. He had seen the planet
apparently attended on either side by two smaller planets, as if helping
old Saturn along. But on December 4, 1612,[54] turning his telescope on
the planet, he found to his infinite amazement not a trace of the
companion planets could be seen; there in the field of view of his
telescope was the golden-tinted disc of the planet as smoothly rounded
as the disc of Mars or Jupiter. 'What,' he wrote, 'is to be said
concerning so strange a metamorphosis? Are the two lesser stars consumed
after the manner of the solar spots? Have they vanished or suddenly
fled? Has Saturn, perhaps, devoured his children? Or were the
appearances, indeed, illusion or fraud with which the glasses have so
long deceived me as well as many others to whom I have shown them? Now,
perhaps, is the time come to revive the well-nigh withered hopes of
those who, guided by more profound contemplations, have discovered the
fallacy of the new observations, and demonstrated the utter
impossibility of the existence of those things which the telescope
appears to show. I do not know what to say in a case so surprising, so
unlooked for, and so novel. The shortness of the time, the unexpected
nature of the event, the weakness of my understanding, and the fear of
being mistaken, have greatly confounded me.' We now know that these
observations, as well as those made soon after by Hevelius, though
wrongly interpreted, were correct enough. Nay, we know that if either
Galileo or Hevelius had been at the pains to reason out the meaning of
the alternate visibility and disappearance of objects looking like
attendant planets, they must have anticipated the discovery made in 1656
by Huyghens, that Saturn's globe is girdled about by a thin flat ring so
vast that, if a score of globes like our earth were set side by side,
the range of that row of worlds would be less than the span of the
Saturnian ring system.
There is a reference in Galileo's letter to the solar spots; 'Are the
two lesser stars,' he says, 'consumed after the manner of the solar
spots?' When he thus wrote the spots were among the myths or fables of
astronomy, and an explanation was offered, by those who did not reject
them utterly, which has taken its place among forsaken doctrines, those
broken toys of astronomers. It is said that when Scheiner, himself a
Jesuit, communicated to the Provincial of the Jesuits his discovery of
the spots on the sun, the latter, a staunch Aristotelian, cautioned him
not to see these things. 'I have read Aristotle's writings from
beginning to end many times,' he said, 'and I can assure you I have
nowhere found in them anything similar to what you mention' [amazing
circumstances!] 'Go, therefore, my son, tranquillise yourself; be
assured that what you take for spots on the sun are the faults of your
glasses or your eyes.' As the idea was obviously inadmissible that a
celestial body could be marked by spots, the theory was started that the
dark objects apparently seen on the sun's body were in reality small
planets revolving round the sun, and a contest arose for the possession
of these mythical planets. Tarde maintained that they should be called
_Astra Borbonia_, in honour of the royal family of France; but C.
Malapert insisted that they should be called _Sidera Austriaca_.
Meantime the outside world laughed at the spots, and their names, and
the astronomers who were thought to have invented both. 'Fabritius puts
only three spots,' wrote Burton in his 'Anatomy of Melancholy,' 'and
those in the sun; Apelles 15, and those without the sun, floating like
the Cyanean Isles in the Euxine Sea. Tarde the Frenchman hath observed
33, and those neither spots nor clouds as Galileus supposed, but planets
concentric with the sun, and not far from him, with regular motions.
Christopher Schemer' [a significant way of spelling Scheiner's name], 'a
German Suisser Jesuit, divides them _in maculas et faculas_, and will
have them to be fixed _in solis superficie_ and to absolve their
periodical and regular motions in 27 or 28 dayes; holding withall the
rotation of the sun upon his centre, and are all so confident that they
have made schemes and tables of their motions. The Hollander censures
all; and thus they disagree among themselves, old and new,
irreconcilable in their opinions; thus Aristarchus, thus Hipparchus,
thus Ptolomaeus, thus Albategnius, etc., with their followers, vary and
determine of these celestial orbs and bodies; and so whilst these men
contend about the sun and moon, like the philosophers in Lucian, it is
to be feared the sun and moon will hide themselves, and be as much
offended as she was with those, and send another message to Jupiter, by
some new-fangled Icaromenippus, to make an end of all these curious
controversies, and scatter them abroad.'