to the place, just to see whether any foreign object did happen to be in
that neighbourhood; but it is no light matter to derange the work of an
Observatory, and alter the work mapped out for the staff into a sudden
sweep for a new planet, on the strength of a mathematical investigation
just received by post. If observatories were conducted on these
unsystematic and spasmodic principles, they would not be the calm,
accurate, satisfactory places they are.
Of course, if any one could have known that a new planet was to be had
for the looking, _any_ course would have been justified; but no one
could know this. I do not suppose that Mr. Adams himself could feel all
that confidence in his attempted prediction. So there the matter
dropped. Mr. Adams's communication was pigeon-holed, and remained in
seclusion for eight or nine months.
Meanwhile, and quite independently, something of the same sort was going
on in France. A brilliant young mathematician, born in Normandy in 1811,
had accepted the post of Astronomical Professor at the Ecole
Polytechnique, then recently founded by Napoleon. His first published
papers directed attention to his wonderful powers; and the official head
of astronomy in France, the famous Arago, suggested to him the
unexplained perturbations of Uranus as a worthy object for his fresh and
well-armed vigour.
At once he set to work in a thorough and systematic way. He first
considered whether the discrepancies could be due to errors in the
tables or errors in the old observations. He discussed them with minute
care, and came to the conclusion that they were not thus to be explained
away. This part of the work he published in November, 1845.
He then set to work to consider the perturbations produced by Jupiter
and Saturn, to see if they had been with perfect accuracy allowed for,
or whether some minute improvements could be made sufficient to destroy
the irregularities. He introduced several fresh terms into these
perturbations, but none of them of sufficient magnitude to do more than
slightly lessen the unexplained perturbations.
He next examined the various hypotheses that had been suggested to
account for them:--Was it a failure in the law of gravitation? Was it
due to the presence of a resisting medium? Was it due to some unseen but
large satellite? Or was it due to a collision with some comet?
All these he examined and dismissed for various reasons one after the
other. It was due to some steady continuous cause--for instance, some
unknown planet. Could this planet be inside the orbit of Uranus? No, for
then it would perturb Saturn and Jupiter also, and they were not
perturbed by it. It must, therefore, be some planet outside the orbit of
Uranus, and in all probability, according to Bode's empirical law, at
nearly double the distance from the sun that Uranus is. Lastly he
proceeded to examine where this planet was, and what its orbit must be
to produce the observed disturbances.
[Illustration: FIG. 94.--Uranus's and Neptune's relative positions.
The above diagram, drawn to scale by Dr. Haughton, shows the paths of
Uranus and Neptune, and their positions from 1781 to 1840, and
illustrates the _direction_ of their mutual perturbing force. In 1822
the planets were in conjunction, and the force would then perturb the
radius vector (or distance from the sun), but not the longitude (or
place in orbit). Before that date Uranus had been hurried along, and
after that date it had been retarded, by the pull of Neptune, and thus
the observed discrepancies from its computed place were produced. The
problem was first to disentangle the outstanding perturbations from
those which would be caused by Jupiter and Saturn and all other known
causes, and then to assign the place of an outer planet able to produce
precisely those perturbations in Uranus.]
Not without failures and disheartening complications was this part of
the process completed. This was, after all, the real tug of war. So many
unknown quantities: its mass, its distance, its excentricity, the
obliquity of its orbit, its position at any time--nothing known, in
fact, about the planet except the microscopic disturbance it caused in
Uranus, some thousand million miles away from it.
Without going into further detail, suffice it to say that in June, 1846,
he published his last paper, and in it announced to the world his
theoretical position for the planet.
Professor Airy received a copy of this paper before the end of the
month, and was astonished to find that Leverrier's theoretical place for
the planet was within 1 deg. of the place Mr. Adams had assigned to it eight
months before. So striking a coincidence seemed sufficient to justify a
Herschelian "sweep" for a week or two.
But a sweep for so distant a planet would be no easy matter. When seen
in a large telescope it would still only look like a star, and it would
require considerable labour and watching to sift it out from the other
stars surrounding it. We know that Uranus had been seen twenty times,
and thought to be a star, before its true nature was by Herschel
discovered; and Uranus is only about half as far away as Neptune is.
Neither in Paris nor yet at Greenwich was any optical search undertaken;
but Professor Airy wrote to ask M. Leverrier the same old question as he
had fruitlessly put to Mr. Adams: Did the new theory explain the errors
of the radius vector or not? The reply of Leverrier was both prompt and
satisfactory--these errors were explained, as well as all the others.
The existence of the object was then for the first time officially
believed in.
The British Association met that year at Southampton, and Sir John
Herschel was one of its Sectional Presidents. In his inaugural address,
on September 10th, 1846, he called attention to the researches of
Leverrier and Adams in these memorable words:--
"The past year has given to us the new [minor] planet Astraea; it
has done more--it has given us the probable prospect of another.
We see it as Columbus saw America from the shores of Spain. Its
movements have been felt trembling along the far-reaching line of
our analysis with a certainty hardly inferior to ocular
demonstration."
It was about time to begin to look for it. So the Astronomer-Royal
thought on reading Leverrier's paper. But as the national telescope at
Greenwich was otherwise occupied, he wrote to Professor Challis, at
Cambridge, to know if he would permit a search to be made for it with
the Northumberland Equatoreal, the large telescope of Cambridge
University, presented to it by one of the Dukes of Northumberland.
Professor Challis said he would conduct the search himself; and shortly
commenced a leisurely and dignified series of sweeps round about the
place assigned by theory, cataloguing all the stars which he observed,
intending afterwards to sort out his observations, compare one with
another, and find out whether any one star had changed its position;
because if it had it must be the planet. He thus, without giving an
excessive time to the business, accumulated a host of observations,
which he intended afterwards to reduce and sift at his leisure.
The wretched man thus actually saw the planet twice--on August 4th and
August 12th, 1846--without knowing it. If only he had had a map of the
heavens containing telescopic stars down to the tenth magnitude, and if
he had compared his observations with this map as they were made, the
process would have been easy, and the discovery quick. But he had no
such map. Nevertheless one was in existence: it had just been completed
in that country of enlightened method and industry--Germany. Dr.
Bremiker had not, indeed, completed his great work--a chart of the whole
zodiac down to stars of the tenth magnitude--but portions of it were
completed, and the special region where the new planet was expected
happened to be among the portions already just done. But in England
this was not known.
Meanwhile, Mr. Adams wrote to the Astronomer-Royal several additional
communications, making improvements in his theory, and giving what he
considered nearer and nearer approximations for the place of the planet.
He also now answered quite satisfactorily, but too late, the question
about the radius vector sent to him months before.
Let us return to Leverrier. This great man was likewise engaged in
improving his theory and in considering how best the optical search
could be conducted. Actuated, probably, by the knowledge that in such
matters as cataloguing and mapping Germany was then, as now, far ahead
of all the other nations of the world, he wrote in September (the same
September as Sir John Herschel delivered his eloquent address at
Southampton) to Berlin. Leverrier wrote, I say, to Dr. Galle, head of
the Observatory at Berlin, saying to him, clearly and decidedly, that
the new planet was now in or close to such and such a position, and that
if he would point his telescope to that part of the heavens he would see
it; and, moreover, that he would be able to tell it from a star by its
having a sensible magnitude, or disk, instead of being a mere point.
Galle got the letter on the 23rd of September, 1846. That same evening
he did point his telescope to the place Leverrier told him, and he saw
the planet that very night. He recognized it first by its appearance. To
his practised eye it did seem to have a small disk, and not quite the
same aspect as an ordinary star. He then consulted Bremiker's great star
chart, the part just engraved and finished, and sure enough on that
chart there was no such star there. Undoubtedly it was the planet.
The news flashed over Europe at the maximum speed with which news could
travel at that date (which was not very fast); and by the 1st of October
Professor Challis and Mr. Adams heard it at Cambridge, and had the
pleasure of knowing that they were forestalled, and that England was
out of the race.
It was an unconscious race to all concerned, however. Those in France
knew nothing of the search going on in England. Mr. Adams's papers had
never been published; and very annoyed the French were when a claim was
set up on his behalf to a share in this magnificent discovery.
Controversies and recriminations, excuses and justifications, followed;
but the discussion has now settled down. All the world honours the
bright genius and mathematical skill of Mr. Adams, and recognizes that
he first solved the problem by calculation. All the world, too,
perceives clearly the no less eminent mathematical talents of M.
Leverrier, but it recognizes in him something more than the mere
mathematician--the man of energy, decision, and character.
LECTURE XVI
COMETS AND METEORS
We have now considered the solar system in several aspects, and we have
passed in review something of what is known about the stars. We have
seen how each star is itself, in all probability, the centre of another
and distinct solar system, the constituents of which are too dark and
far off to be visible to us; nothing visible here but the central sun
alone, and that only as a twinkling speck.
But between our solar system and these other suns--between each of these
suns and all the rest--there exist vast empty spaces, apparently devoid
of matter.
We have now to ask, Are these spaces really empty? Is there really
nothing in space but the nebulae, the suns, their planets, and their
satellites? Are all the bodies in space of this gigantic size? May there
not be an infinitude of small bodies as well?
The answer to this question is in the affirmative. There appears to be
no special size suited to the vastness of space; we find, as a matter of
fact, bodies of all manner of sizes, ranging by gradations from the most
tremendous suns, like Sirius, down through ordinary suns to smaller
ones, then to planets of all sizes, satellites still smaller, then the
asteroids, till we come to the smallest satellite of Mars, only about
ten miles in diameter, and weighing only some billion tons--the smallest
of the regular bodies belonging to the solar system known.
But, besides all these, there are found to occur other masses, not much
bigger and some probably smaller, and these we call comets when we see
them. Below these, again, we find masses varying from a few tons in
weight down to only a few pounds or ounces, and these when we see them,
which is not often, we call meteors or shooting-stars; and to the size
of these meteorites there would appear to be no limit: some may be
literal grains of dust. There seems to be a regular gradation of size,
therefore, ranging from Sirius to dust; and apparently we must regard
all space as full of these cosmic particles--stray fragments, as it
were, perhaps of some older world, perhaps going to help to form a new
one some day. As Kepler said, there are more "comets" in the sky than
fish in the sea. Not that they are at all crowded together, else they
would make a cosmic haze. The transparency of space shows that there
must be an enormous proportion of clear space between each, and they are
probably much more concentrated near one of the big bodies than they are
in interstellar space.[30] Even during the furious hail of meteors in
November 1866 it was estimated that their average distance apart in the
thickest of the shower was 35 miles.
Consider the nature of a meteor or shooting-star. We ordinarily see them
as a mere streak of light; sometimes they leave a luminous tail behind
them; occasionally they appear as an actual fire-ball, accompanied by an
explosion; sometimes, but very seldom, they are seen to drop, and may
subsequently be dug up as a lump of iron or rock, showing signs of rough
treatment by excoriation and heat. These last are the meteorites, or
siderites, or aerolites, or bolides, of our museums. They are popularly
spoken of as thunderbolts, though they have nothing whatever to do with
atmospheric electricity.
[Illustration: FIG. 95.--Meteorite.]
They appear to be travelling rocky or metallic fragments which in their
journey through space are caught in the earth's atmosphere and
instantaneously ignited by the friction. Far away in the depths of space
one of these bodies felt the attracting power of the sun, and began
moving towards him. As it approached, its speed grew gradually quicker
and quicker continually, until by the time it has approached to within
the distance of the earth, it whizzes past with the velocity of
twenty-six miles a second. The earth is moving on its own account
nineteen miles every second. If the two bodies happened to be moving in
opposite directions, the combined speed would be terrific; and the
faintest trace of atmosphere, miles above the earth's surface, would
exert a furious grinding action on the stone. A stream of particles
would be torn off; if of iron, they would burn like a shower of filings
from a firework, thus forming a trail; and the mass itself would be
dissipated, shattered to fragments in an instant.
[Illustration: FIG. 96.--Meteor stream crossing field of telescope.]
[Illustration: FIG. 97.--Diagram of direction of earth's orbital
motion, showing that after midnight, _i.e._ between midnight and noon,
more asteroids are likely to be swept up by any locality than between
noon and midnight. [From Sir R.S. Ball.]]
Even if the earth were moving laterally, the same thing would occur. But
if earth and stone happened to be moving in the same direction, there
would be only the differential velocity of seven miles a second; and
though this is in all conscience great enough, yet there might be a
chance for a residue of the nucleus to escape entire destruction, though
it would be scraped, heated, and superficially molten by the friction;
but so much of its speed would be rubbed out of it, that on striking
the earth it might bury itself only a few feet or yards in the soil, so
that it could be dug out. The number of those which thus reach the earth
is comparatively infinitesimal. Nearly all get ground up and dissipated
by the atmosphere; and fortunate it is for us that they are so. This
bombardment of the exposed face of the moon must be something
terrible.[31]
Thus, then, every shooting-star we see, and all the myriads that we do
not and cannot see because they occur in the day-time, all these bright
flashes or streaks, represent the death and burial of one of these
flying stones. It had been careering on its own account through space
for untold ages, till it meets a planet. It cannot strike the actual
body of the planet--the atmosphere is a sufficient screen; the
tremendous friction reduces it to dust in an instant, and this dust then
quietly and leisurely settles down on to the surface.
Evidence of the settlement of meteoric dust is not easy to obtain in
such a place as England, where the dust which accumulates is seldom of a
celestial character; but on the snow-fields of Greenland or the
Himalayas dust can be found; and by a Committee of the British
Association distinct evidence of molten globules of iron and other
materials appropriate to aerolites has been obtained, by the simple
process of collecting, melting, and filtering long exposed snow.
Volcanic ash may be mingled with it, but under the microscope the
volcanic and the meteoric constituents have each a distinctive
character.
The quantity of meteoric material which reaches the earth as dust must
be immensely in excess of the minute quantity which arrives in the form
of lumps. Hundreds or thousands of tons per annum must be received; and
the accretion must, one would think, in the course of ages be able to
exert some influence on the period of the earth's rotation--the length
of the day. It is too small, however, to have been yet certainly
detected. Possibly, it is altogether negligible.
It has been suggested that those stones which actually fall are not the
true cosmic wanderers, but are merely fragments of our own earth, cast
up by powerful volcanoes long ago when the igneous power of the earth
was more vigorous than now--cast up with a speed of close upon seven
miles a second; and now in these quiet times gradually being swept up by
the earth, and so returning whence they came.
I confess I am unable to draw a clear distinction between one set and
the other. Some falling stars may have had an origin of this sort, but
certainly others have not; and it would seem very unlikely that one set
only should fall bodily upon the earth, while the others should always
be rubbed to powder. Still, it is a possibility to be borne in mind.
We have spoken of these cosmic visitors as wandering masses of stone or
iron; but we should be wrong if we associated with the term "wandering"
any ideas of lawlessness and irregularity of path. These small lumps of
matter are as obedient to the law of gravity as any large ones can be.
They must all, therefore, have definite orbits, and these orbits will
have reference to the main attracting power of our system--they will, in
fact, be nearly all careering round the sun.
Each planet may, in truth, have a certain following of its own. Within
the limited sphere of the earth's predominant attraction, for instance,
extending some way beyond the moon, we may have a number of satellites
that we never see, all revolving regularly in elliptic orbits round the
earth. But, comparatively speaking, these satellite meteorites are few.
The great bulk of them will be of a planetary character--they will be
attendant upon the sun.
It may seem strange that such minute bodies should have regular orbits
and obey Kepler's laws, but they must. All three laws must be as
rigorously obeyed by them as by the planets themselves. There is nothing
in the smallness of a particle to excuse it from implicit obedience to
law. The only consequence of their smallness is their inability to
perturb others. They cannot appreciably perturb either the planets they
approach or each other. The attracting power of a lump one million tons
in weight is very minute. A pound, on the surface of such a body of the
same density as the earth, would be only pulled to it with a force equal
to that with which the earth pulls a grain. So the perturbing power of
such a mass on distant bodies is imperceptible. It is a good thing it is
so: accurate astronomy would be impossible if we had to take into
account the perturbations caused by a crowd of invisible bodies.
Astronomy would then approach in complexity some of the problems of
physics.
But though we may be convinced from the facts of gravitation that these
meteoric stones, and all other bodies flying through space near our
solar system, must be constrained by the sun to obey Kepler's laws, and
fly round it in some regular elliptic or hyperbolic orbit, what chance
have we of determining that orbit? At first sight, a very poor chance,
for we never see them except for the instant when they splash into our
atmosphere; and for them that instant is instant death. It is unlikely
that any escape that ordeal, and even if they do, their career and orbit
are effectually changed. Henceforward they must become attendants on the
earth. They may drop on to its surface, or they may duck out of our
atmosphere again, and revolve round us unseen in the clear space between
earth and moon.
Nevertheless, although the problem of determining the original orbit of
any given set of shooting-stars before it struck us would seem nearly
insoluble, it has been solved, and solved with some approach to
accuracy; being done by the help of observations of certain other
bodies. The bodies by whose help this difficult problem has been
attacked and resolved are comets. What are comets?
I must tell you that the scientific world is not entirely and completely
decided on the structure of comets. There are many floating ideas on the
subject, and some certain knowledge. But the subject is still, in many
respects, an open one, and the ideas I propose to advocate you will
accept for no more than they are worth, viz. as worthy to be compared
with other and different views.
Up to the time of Newton, the nature of comets was entirely unknown.
They were regarded with superstitious awe as fiery portents, and were
supposed to be connected with the death of some king, or with some
national catastrophe.
Even so late as the first edition of the _Principia_ the problem of
comets was unsolved, and their theory is not given; but between the
first and the second editions a large comet appeared, in 1680, and
Newton speculated on its appearance and behaviour. It rushed down very
close to the sun, spun half round him very quickly, and then receded
from him again. If it were a material substance, to which the law of
gravitation applied, it must be moving in a conic section with the sun
in one focus, and its radius vector must sweep out equal areas in equal
times. Examining the record of its positions made at observatories, he
found its observed path quite accordant with theory; and the motion of
comets was from that time understood. Up to that time no one had
attempted to calculate an orbit for a comet. They had been thought
irregular and lawless bodies. Now they were recognized as perfectly
obedient to the law of gravitation, and revolving round the sun like
everything else--as members, in fact, of our solar system, though not
necessarily permanent members.
But the orbit of a comet is very different from a planetary one. The
excentricity of its orbit is enormous--in other words, it is either a
very elongated ellipse or a parabola. The comet of 1680, Newton found
to move in an orbit so nearly a parabola that the time of describing it
must be reckoned in hundreds of years at the least. It is now thought
possible that it may not be quite a parabola, but an ellipse so
elongated that it will not return till 2255. Until that date arrives,
however, uncertainty will prevail as to whether it is a periodic comet,
or one of those that only visit our system once. If it be periodic, as
suspected, it is the same as appeared when Julius Caesar was killed, and
which likewise appeared in the years 531 and 1106 A.D. Should it appear
in 2255, our posterity will probably regard it as a memorial of Newton.
[Illustration: FIG. 98.--Parabolic and elliptic orbits. The _a b_
(visible) portions are indistinguishable.]