The comet of Donati, which presented so imposing a
spectacle in our northern hemisphere in 1858, strongly revived the
popular interest in such appearances. Few of the present generation had
an opportunity of gazing on any comet of remarkable splendour and
magnitude. Our fathers told us of the wonderful comet of 1811, spanning
half the circuit of the heavens with its tail, and hanging over the
earth for weeks together like a drawn sword; we listened, however, as to
a legend of the olden time, and despaired of our seeing such sights in
our day. The comet of 1858 burst upon our view, and proclaimed
that there were still wonders in nature, surpassing any that the eye of
man had yet seen. Notwithstanding the tendency of the old to exaggerate
the wonders of their youth, the privileged few who remembered the comet
of 1811, confessed that Donati's comet was altogether a finer
object, though it did not stretch so far across the heavens.
Though comets served no other object than to awaken
fresh interest in celestial phenomena, their visits to our
regions of space would not be in vain. Such startling phenomena serve to
dispel the indifference with which we are prone to gaze upon the
unvarying page of the "Book of God," presented to us by the ordinary
aspect of the heavens. It was a bright star in the sixteenth century
suddenly bursting forth and as suddenly dying out, that gave to the
world the great astronomer Tycho Brahe. This imposing object determined
his tastes and fixed his future career. Mr. Bond, the distinguished
American astronomer, who has just been gathered to his fathers, dates
his astronomical career from the total eclipse of 1806. This sublime and
awe-inspiring spectacle took possession of his mind to such a degree,
that he gazed with ceaseless and inquiring wonder on the face of the
heavens ; this eager prying into the depths of space, gave him, while
yet a boy, almost supernatural power of vision with the naked eye. As a
watchmaker's apprentice he could not command the aid of instruments,
but, with the faith of genius, he believed that he would not be denied a
sight of nature's secrets, if he made good use of the advantages within
his reach. He employed various arts to increase the sensitiveness of his
retina. He practised looking into dark wells, with his eyes shaded from
the light, to catch a glimpse of the almost invisible objects at the
bottom. By this means, his eye acquired such keenness of vision, that he
was the first in the American continent to discover the great comet of
1811. Little did the young apprentice think that he was, in this way,
most effectually qualifying himself for the management, as head of an
observatory, of some of the finest instruments in the world. His finely
trained eye enabled him, in after life, to make the independent
discovery of the dark ring of Saturn, and of a new satellite belonging
to the same system. His early mechanical training enabled him to devise
and perfect one of the greatest improvements in observation in modern
times, viz., the substitution, by means of electricity, of touch instead
of hearing to mark the instant of any astronomical event. To him, also,
are we indebted for the art of celestial photography, which promises to
revolutionize the system of astronomical observations. May we not hope
that Donati's comet, in like manner, may have aroused the dormant powers
of many a young astronomer who may live to wring from nature her deepest
and most wonderful secrets?
One great charm of the comet, both in a popular and
scientific view, is the illustration of formative processes visibly
going on in the grandest scale. What would not the geologist give to see
illustrated, in a conveniently brief space, the molten globe and the
subsequent induration ; and the various upheavals and submergences,
which his theories require? Now, the comet exhibits corresponding
organic changes in the course of a few days or weeks, and on a scale
vastly greater than what our earth could exhibit. What is it that
renders the sight of the moon's surface so disappointing ? Is it not
that all is motionless? There are no moving clouds, no fluctuating seas,
no fleeting snow-covering, no active volcanoes:—all is motionless and
deathlike. Why does the surface of the sun attract such deep interest at
this moment? Because we discover agencies working with intense activity.
There are elements of change detected, and these are watched with the
most anxious interest. In the moon, we see a frozen torrent; in the sun,
we see the stream dashing on with impetuous speed.
Measured by this source of interest, the comet
arrests our attention much more powerfully than even the sun, the
changes being much more rapid and comprehensive. We see the whole orb
transformed before our eyes. We are apt to think that the full-sized
comet is merely the same as the cloudy speck which we descried some
weeks before, and that the whole difference of aspect is due to
difference of distance; just as the white speck on the distant horizon,
differs from the frigate entering the harbour with all her sails set.
This, however, is far from being the case : no doubt increasing
proximity magnifies the object; but apart from this, there is a
wonderfully rapid expansion and change of form. The comet at a distance,
is the rose in bud; in the proximity of the sun, it is the rose in full
blossom. In the latter position, there is an astonishing development in
size and shape. Perhaps a more apt comparison would be to the sea
anemone, in which we have, with the same substance, an alteration of
bulk and form. The comet in its remote position, is the shrunken
anemone; with its bright nucleus and flaming tail, it is the anemone
with its tentacles expanded, and its brilliant colours displayed.
With the naked eye we can only see the bright petals
of the flower coming forth from the calyx; we do not detect the
formative processes going on within the germ. But just as with the
microscope we can dissect a flower, and detect the most secret processes
of nature in the germ, so can we, with the telescope, penetrate the
nucleus of the comet, and discover the plan of evolution by which the
tail of the comet springs from this centre. As we watch with interest
the potter fashioning an artistic vase from a mass of clay, so do we
witness with a higher interest the comet, with all its symmetry and
beauty, emerging from a mass of cloud, fashioned by great laws which are
only the hands of the Divine Artist, by which it is moulded into the
requisite form.
The illustrative figure at the head of this article
is intended to have only a typical significance. We have been kindly
favoured by Mr. Cooper with views of Donati's comet, as seen through his
great refractor at Markree Observatory. His admirable sketches represent
different stages of the development of the comet. We have combined them,
so as to give in one view a clear idea of the structure of the comet. We
have added another envelope to bring out more fully the stratified
character, and to comprehend the observations of M. Chagornac at the
Observatory of Paris. The particles of which the comet is composed are
never motionless; they are in a ceaseless flow, impelled by forces which
communicate to them an inconceivable rapidity of motion. The deepest
mystery hangs over the nature of these forces, but the phenomena, as
witnessed with the aid of the telescope, are easily described. When a
comet is first descried in the heavens, it appears as a faint star, with
a haze around it. It is often impossible to tell by its mere physical
aspect whether it is really a comet. To solve the doubt, the observer
must wait a little and see whether it is in motion. This assures him
that is not a new nebula, and he concludes that it must be a comet. He
traces for a short way the curve described, and having got a sample, he
readily deduces the shape, dimensions, and position of the whole orbit.
He proceeds, just as one would do in cutting out figures in the sward of
a garden. When a portion of a regular figure is given, it is easy to cut
out the whole figure. In this way the astronomer can readily tell how
near it will approach the sun, and at what time. As the comet approaches
the sun, internal changes can be detected. The globular mass is drawn
out, the nucleus or brightest point being at the end nearest the sun.
The other end soon assumes the appearance of a tail.
The forms of most large comets conform to one general
type. The tail is slightly convex on one side, and has very much the
appearance of a scimitar, the nucleus representing the hand with which
it is grasped. The Chinese call comets broom-stars; and the appearance
of many is very much that of a broom, bent and worn with much use. The
outline of the convex is generally better defined than that of the
concave side. There is a comparatively dark band running lengthwise
through the middle of the comet, so that they have often the appearance
of two tails; the two bright streaks on each side of the tail being
taken for two distinct tails.
This is the appearance to the naked eye. When the
nucleus is watched with the telescope, very remarkable changes are
witnessed. The luminous matter of the comet rushes forth from the
nucleus as the stream of fiery particles issues from a rocket. They rush
out towards the sun with great velocity, but soon they reach a certain
point, when they are driven back to form the tail. The appearance is not
unlike that of a lighted torch held against the wind. The tendency of
the flame to rise is counteracted by the action of the wind, which
drives it back. The most perfect illustration we have seen of the
internal form and movement of the comet, was the playing of the fountain
in the garden of the Tuileries, close to the Place de la Concorde. The
water rose to a considerable height, and fell back in an umbrella form.
There was a slight wind, and the jet was slightly curved along its
length, as in the case of the comet. The windward side was also best
defined, the loose spray being all thrown to the leeward side. The
descending canopy being hollow, the outline of both side's was more
condensed than the central part, the eye having to look through a
greater mass of water. The luminous particles shoot forth from the
nucleus just as the water spouts forth from the jet, and are thrown back
to form a hollow envelope. The curvature of the comet, and the sharper
definition of the convex side, are usually explained on the supposition
that the comet is moving through a resisting medium, so that an effect
similar to the influence of the wind on a jet-d'eau is produced.
In the case of Donati's comet, the telescope detected
the formation of numerous envelopes, so that the head presented the
appearance of distinct stratification. The forces brought into action,
seemed to be intermittent, and this intermission is marked by the
boundaries of the strata. There is no continuous flow in nature, and the
play of a fountain gives a very good illustration of this. However
steady the pressure of the water may be, the fountain plays by jerks,
and not by a steady continuous stream; and the development of the comet
illustrates this on a grand scale, the different envelopes being the
result. What is the structure that results from this view of the comet's
formation? It may be represented by a series of hollow cones packed the
one within the other. The grocer's paper cones may serve as a familiar
illustration, or a nest of flower-pots, the smallest being in the
centre, and the others forming concentric envelopes of increasing size.
This will give a striated appearance to the tail, and when there is a
space between the envelopes, the successive intervals will appear
comparatively dark, so that the comet may seem to have many tails.
Almost all the perplexing shapes comets assume, may be explained on this
assumption of hollow concentric cones.
The next inquiry is, How is this extraordinary
development produced? What are the forces brought into play? The jet of
luminous matter from the centre is naturally explained by the action of
the sun's heat, for the action is energetic in proportion to the
nearness of the comet to the sun. The grand difficulty is the throwing
back of the matter into the tail. It is evident that there is some
repulsive power, and a power capable of communicating an inconceivable
velocity to the repelled particles. The usual way is to ascribe it to
electric or magnetic action. It is ascertained that the sun and the
moon, as well as the earth, are magnets; and the great probability is,
that all the heavenly bodies are magnetic. This seemed to afford a
plausible explanation of the apparently polar forces at play in the
evolution of the comet. Another, however, has recently been started,
which has, for the time, put all others in abeyance, and is in process
of being tested by the French savans. We allude to the hypothesis of M.
Faye, who was commissioned by government to undertake the expedition to
Spain, to observe the total eclipse on the 18th July, but who, from a
misunderstanding with Leverrier, has thrown up the appointment, which
has now been conferred upon the latter. The hypothesis is, that there is
a repulsive force radiated from the sun, as well as the attractive force
of gravitation, and he holds that this is only a single case of a
universal law of matter. He holds that every incandescent body exercises
this repulsion, but that it is so feeble, that it can be detected, only
in the case of matter of such extreme tenuity as that of comets. He
supposes that the force emanates only from the surface of the
sun, and not, like gravitation, from its mass. With this simple
assumption, he endeavours to explain the formation and all the phenomena
of comets. The envelopes are produced by the expansive power of the
sun's heat, balanced by this special repelling power. The shortening of
the period of the comet of Encke is also explained by the same means. He
has agreed to submit the law to the test of experiment. It was objected,
that if the incandescent surface of the sun repelled the rarefied matter
of comets, the same thing should happen in the case of incandescent
bodies on the surface of the earth. He at once accepted the consequence,
and agreed to put the thing to the test, if matter rare enough could be
found and made visible. The extremely rarefied air left in the vacuum of
an air-pump, was taken as the test; and a heated plate was employed to
repel this matter. The difficulty was to make the repulsion visible.
This was effected by passing an electric discharge through the exhausted
receiver. The air was made palpable by means of the stratified light
produced by a powerful coil. Several distinguished physicists hold the
point demonstrated, though, as yet, most withhold their assent.
If this new law be established, it will be the most
important advance since the time of Newton. It will, indeed, dethrone
gravitation from the sole sway which it has hitherto exercised. Newton's
law has been repeatedly put to the test, and more than once, astronomers
trembled lest the whole Newtonian structure should totter and fall; but
gravitation with its simple ratio, has come forth triumphant from the
severest ordeals. One trial more awaits it, and all the astronomers of
European fame are ranged on one side or other of the question. The two
principal parties are M. Hansen of Altona, and M. Delauny, and with one
or other of these the various combatants side. The controversy raged for
some time in the Academy of Sciences between Leverrier and Delauny, but
gradually all the great astronomers of the day have been caught in the
vortex. Adams is almost the only astronomer of note that stands by
Delauny; Pontecoulant and Plana are leagued with Leverrier in support of
the results of Hansen. Delauny has elaborated a theory, by which he
shows that the moon's actual motion does not accord with the law of
gravitation. Hansen, on the other hand, asserts that his theory is
correct, as his tables, adopted by the British Government, accord
completely with observation. The precise point is, what is termed, the
acceleration of the moon's mean motion, one result of which is, that the
moon, in the course of the year, nears the earth by about an inch. Yet
on this inch, depends the fate of the Newtonian theory. Adams and his
French associate, hold that their results point clearly to some other
cause than the law of gravitation, and M. Faye indicates his new law of
repulsion as the true cause to explain the residual phenomenon. Though
the controversy has not assumed a national aspect, the temper displayed
on both sides ill befits the dignity of science. The comet in the hand
of Newton broke through and shivered the crystal spheres of antiquity,
and annihilated the vortices of Descartes. Strange if it be destined
also to shake the foundations of even his own system itself!
When Newton failed to show how his law could prevent
the perturbations of the planets introducing confusion into the system,
he suggested that the direct interference of the Almighty might be
employed occasionally to rectify matters. He and the theologians of his
day seemed to think that there must be a defect in the celestial
machine, unless it was proved to be capable of lasting for ever, and
great was the rejoicing when the stability of the system was
established. But suppose there is a resisting medium, accounting for the
destructive course of Encke's comet towards the sun, or that the law of
gravitation requires to be modified by some new law, which may
compromise the present order of things by introducing an element of
decay, are we to conclude that there is a defect of wisdom in the
constitution of the celestial machine? We might as reasonably deny the
traces of wisdom in the structure of our bodies, because they have not
the stamp of eternity. Theologians have been too much led away with the
idea of the solar system being a rigid machine, with unvarying
adjustments, instead of a single phase of the mighty evolutions of the
material universe. There is nothing fixed and rigid in the material
world except the laws by which the all-wise Ruler governs it. Every star
in the heavens, and every molecule in the earth, is in incessant
movement. There is a constant flow, and the wisdom of the Creator is
seen, rather in the exquisite rhythm of movement than in the rigid
collocation of the material world. We see only a few spokes of the great
wheel of the universe; and what appears to be a destructive breach of
continuity, may be only the completed part of a mighty cycle.
As to the purpose served by comets, the odd fancy of
Newton, that they are intended to serve as fuel to the sun, is again
revived, and revived by our ablest philosophers. It is held, in
connexion with the dynamical theory of heat, that the sun's heat and
light must be exhausted, unless supplied with fuel, and comets are held
as part of the fuel destined to keep the central fire burning. It is
supposed that there is a zone of aerolites, constituting the zodiacal
light, which supplies the sun with the needful heat. And some careful
observers state, that they have actually seen large bodies plunge into
the luminous atmosphere, just as a large block of coal is thrown into
the furnace by the stoker. These bodies are not supposed to act
precisely as fuel, though they keep up the heat. In the practice of the
Armstrong guns, it was found when a bolt was lodged in the sides of the
gun-boat, that it could not be touched with the hand on account of the
great heat—the arrested motion being converted into heat. So the
aerolites and comets are supposed to communicate the requisite heat by
having their prodigious velocity arrested, and converted into this other
form of force. This is, however, a point of so much interest, and sheds
so much light on the wise balancing of forces throughout the material
world, that we reserve it for a separate paper.
Some advocates for the doctrine of a plurality of
worlds, driven by an inexorable logic, do not hesitate to maintain, that
the purpose of comets is to be inhabited. They start on the principle,
that it is not necessary to prove any probable conditions of life in
order to hold that any body of the system is inhabited; and,
consequently, they can assign no limits to their theory. They do not
hesitate to hold, although the heat is so great as to burn up diamonds
like tinder, and although the substance of the comet is rarer than the
most perfect vacuum of the air-pump, that there may be living beings on
every particle of its matter. The argument with which this view is
supported is, that it is quite possible for God to make beings capable
of existing in such conditions. But, is it right to make our notions of
possibility the basis of a theory of God's providence? The
question is one of probability, not of possibility. And if
we are to proceed reverentially, we must argue from the known to the
unknown, from our experience on this globe to what is probably the
Divine procedure in worlds the physical conditions of which are only
partially revealed to us.