demonstrably untenable, though ingenious, like Professor Tyndall’s, or
altogether and obviously untenable like Professor Tait’s, there are
certain phenomena of comets’ tails which force upon us the belief that
they are phenomena of repulsion, though the repulsive action is of a
kind not yet known to physicists.
1. The curvature of all the cometic tails when not seen from a point in
or near the place of their motion.
2. The existence of more tails than one to the same comet, the
different tails being differently curved.
3. The phenomena of striations athwart the tail.
It is evident that all these phenomena are such as we might fairly
expect if a comet’s tail is caused by the sun’s repulsive action on
molecules, raised by his heating action on the head. The matter thus
swept away would resemble smoke, driven upwards from the funnel of a
moving steamer, and then swept in any given direction by a steady wind;
we should see a curved train of such matter just as we see a curved
streak of smoke. If the matter raised from the head is not all of one
kind (and it is antecedently unlikely that it should be), there would
be more than one trail of matter, if the sun’s repulsive action were
different on these different kinds of matter. Lastly, the striations
seen athwart the tail, as in the well known case of Donati’s great
comet, would be explained, either as due to the observed pulsational
manner in which the envelopes are raised (if matter were raised
uniformly from the head there could be no formation of successive
envelopes), or else as due to the carrying off into the main tail,
where alone such striations are seen, of matter which, had it freed
itself at the beginning, would have been swept off into the smaller
tails, but being as it were entangled in the great outflow of matter
forming the large tail, escapes later, and when it does, gets swept off
at its own more rapid rate, and there forms a streak lying at an angle
with the direction of the principal tail.
Bredichin has shown that where there are three tails to a comet, their
forms correspond with the theory that the envelopes raised from the
head are principally formed of hydrogen, carbon and iron, but this,
which, if established, would be the most important physical discovery
yet made respecting comets, seems open at present to considerable
doubt, though confirmations seem to be given to it, in some respects,
by the results of spectroscopic analysis.
To spectroscopic analysis we must in all probability look for such
information respecting comets, as may hereafter enable us to understand
their nature. On this point let us consider what is said by one who,
if not the greatest living astronomical spectroscopist, is _facile
princeps_ in this country—Dr. W. Huggins. First, however, we must
consider the past of this method of research as applied to comets.
The first successful application of the spectroscope to comets was made
by Donati in 1864—the light of the comet being then divided into three
bright bands, whose position, however, was not exactly determined.
In 1866 Dr. Huggins obtained two kinds of light from a telescopic
comet, part of the comet’s light giving a continuous spectrum, probably
reflected sunlight, the other a spectrum of three bands. In 1868 a
comet was observed (Brorsen’s) with more success. Three bands were seen
in the spectrum of the light from the comet’s head, and a comparison of
these with measures of similar bright bands belonging to the spectra of
various combinations of carbon, showed, or rather seemed to suggest,
that “combinations of carbon might be present in the comet.”
“In conjunction with my friend, the late Dr. W. Allen
Miller,” says Dr. Huggins, “I confronted directly with the
spectroscope attached to the telescope, the comet’s light
with that from inductive sparks passing in olefiant gas.
The sensible identity of the two spectra left no doubt
of the essential oneness of the cometary stuff with the
gas composed of carbon and hydrogen that was employed for
comparison.” “Since that time,” proceeds Dr. Huggins, “the
light from some twenty comets has been examined by different
observers. The general close agreement in all cases,
notwithstanding some small divergences, of the bright bands
in the cometary light with those seen in the spectra of
hydrocarbons, justifies us fully in ascribing the original
light of these comets to matter which contains carbon in
combination with hydrogen.”
Last year photography was applied to this spectroscopic work. The
spectrum of the brightest comet of that year was partly continuous, and
on this continuous spectrum many of the well known Fraunhofer lines
could be traced. This made it certain that part of the comet’s light
was reflected sunlight; though Dr. Huggins considers also that a part
of the continuous spectrum of every comet is due to inherent light. On
this point some doubts may be permitted. It is one thing for special
bands to show themselves, for some substances may become self-luminous
under special conditions at very moderate temperatures; it is quite
another thing that the solid parts of a comet’s substance should become
incandescent. I venture to express my own belief that this can scarcely
happen except in the case of comets which approach very near to the
sun. Besides the continuous spectrum with dark lines, the photograph
showed also a spectrum of bright lines.
“These lines,” says Dr. Huggins, “possessed extreme
interest, for there was certainly contained within this
hieroglyphic writing some new information. A discussion
of the position of these new lines showed them to be
undoubtedly the same lines which appear in certain compounds
of carbon. Not long before, Professors Liveing and Dewar had
found from their laboratory experiments that these lines are
only present when nitrogen is also present, and that they
indicate a nitrogen compound of carbon, namely—cyanogen.
Two other bright groups were also seen in the photograph,
confirming the presence of hydrogen, carbon, and nitrogen.”
It is worthy of notice that, only a few days later, Dr. H. Draper
succeeded in obtaining a photograph of the same comet’s spectrum.
It appeared to him to confirm Dr. Huggins’ statements, except only
that the dark Fraunhofer lines were not visible—the photograph having
probably been taken under less favorable conditions.
So far, then, it seems clear that comets shine in part by reflecting
sunlight, partly with light of their own; the part of the cometic
substance which certainly shines with its own light is gaseous, and
this gas in most comets “contains carbon, hydrogen, and nitrogen,
possibly also oxygen, in the form of hydrocarbons, cyanogen, and
possibly oxygen compounds of carbon.”
But the latest comet has brought with it fresh news. Its spectrum
is not like that given by the comets we have been considering. The
bright lines of sodium are seen in it, and also other bright lines
and groups of lines, which have not yet been shown to be identical
with any belonging to the hydrocarbon groups, but probably are so. Dr.
Huggins’ photograph shows, he considers, “that the original light of
the comet, which gives a continuous spectrum (he means that portion
of the original light which does so), was too strong to allow of the
Fraunhofer lines being recognized in the reflected solar light.”
We demur to this as being _shown_, it may fairly be said to be
_suggested_. The cyanogen groups are not seen.
Such is Dr. Huggins’ account; but it is manifest that this comet
underwent important changes, of which—we are surprised to note—Dr.
Huggins has taken no account. Thus, in April, Professors Tacchini and
Vogel found simply a faint continuous spectrum. In May, Vogel found
that the three bands associated with carbon were present, though faint,
while there was no trace of the sodium band. On the contrary, on the
nights of June 4, 5, and 7, Dr. B. Hasselberg, of the Observatory of
Pulkowa, found that the nucleus of the comet gave a very strong and
extended continuous spectrum, with an “excessively strong” bright
line in the orange yellow, proved by micrometrical measurement to be
identical with the D line—the well known double sodium line of the
solar spectrum. The observation was confirmed by Dunér, Bredichin,
and Vogel. On this Mr. Hind remarks, “It is necessary to conclude
that, during the last fortnight of May, the spectrum of Wells’ comet
had changed in a manner of which the history of science furnishes no
precedent.” It must, however, be remembered that as yet no comets have
been examined under sufficiently favorable conditions, to enable us to
say whether the change thus observed was really exceptional, or only
exceptional in being for the first time noted. Whenever such a comet
as Donati’s comes favorably under spectroscopic scrutiny, we shall
probably learn something about these changes which will throw more
light than anything yet discovered on the physical economy of these
mysterious bodies.
What, then, do we know certainly respecting comets? What may we surmise
with more or less probability? And in what direction may we look with
most hope for future information? We know certainly that, in whatever
way they are formed, the sun excites intense disturbance in them as
they approach him. Prof. Stokes remarks that these effects, so much
greater at a first view than we might fairly expect in the case of
many of the comets observed, which have approached the sun no nearer
than our own earth does, or not so near, may be accounted for by the
circumstance that comets travel in what may be regarded as, to all
intents and purposes, a vacuum. From Dr. Crooke’s experiments on very
high vacua, we may infer that there is very little loss of heat, except
by radiation. Thus the heat received by the meteoric components of
a comet would be much greater than might otherwise be expected. Dr.
Huggins mentions, in the same connection, the remarkable persistence
of the bright trains of meteors in the rare upper air, which sometimes
remain visible for three-quarters of an hour before the light fades, as
the heat is gradually radiated away. “Our reasoning on these points,”
he remarks, in his dry way, “would undergo considerable modification if
we accept the views as to the condition of interplanetary space and of
the sun’s action which have been recently suggested by Dr. Siemens in
his solar theory”—but of course we do not.
Bredichin’s researches, showing that three distinct curvatures in
comets’ tails correspond to the winnowing out by solar repulsive action
of (1) hydrogen, (2) carbon, and (3) iron, seem worthy of careful
study and investigation. It accords well with spectroscopic evidence
as to the condition of the matter raised in gaseous form from the
nucleus; and if as yet we have had no direct spectroscopic evidence
of the existence of iron in comets, we know that meteors are closely
connected with comets, and that many meteors contain iron. Moreover,
as unexpected spectroscopic evidence of the presence of the substance
sodium, common in so many meteors, has been found in the case of one
comet, we may fairly hope that under yet more favorable conditions, the
presence of iron also may be recognized in the same way.
How far electricity may be looked to for an explanation of cometic
phenomena, is a doubtful point among astronomers and physicists.
For my own part, I must confess I share the strong objections which
many physicists have expressed against the mere vague suggestion
that perhaps _this_ is an electrical phenomenon, perhaps _that other
feature_ is electrical too, perhaps _all or most_ of the phenomena of
comets depend on electricity. It is so easy to make such suggestions,
so difficult to obtain evidence in their favor having the slightest
scientific value. Still, I hold the electrical idea to be well worth
careful study. Whatever credit may hereafter be given to any electrical
theory of comets, will be solely and entirely due to those who may
help to establish it upon a basis of sound evidence—none whatever to
the mere suggestion, which has been made time and again since it was
first advanced by Fontenelle. Dr. Huggins says that he finds there is a
rapidly growing feeling among physicists that both the inherent light
(which he prefers to call the self-light) of comets and the phenomena
of their tails belong to the order of electrical phenomena. An
American astronomer recently wrote to him, as to American views of the
self-light of comets, “I can not speak with authority for anyone but
myself; still I think the prevailing impression amongst us is that this
light is due to an electric, or, if I may coin the word, (far better
not) an electric-oid action of some kind.” On this Dr. Huggins himself
remarks:
“The spectroscopic results fail to give conclusive evidence
on this point; still, perhaps, upon the whole, especially
if we consider the photographs of last year, the teachings
of the spectroscope are in favor of the view that the
self-light of comets is due to electric discharges. Those
who are disposed to believe that the truth lies in this
direction, differ from each other in the precise modes
in which they would apply the known laws of electric
action to the phenomena of comets. Broadly, the different
applications of principles of electricity which have been
suggested, group themselves about the common idea, that
great electrical disturbances are set up by the sun’s action
in connection with the vaporization of some of the matter of
the nucleus, and that the tail is probably matter carried
away, possibly in connection with electric discharges,
under an electrical influence of repulsion exerted by the
sun. This view necessitates the supposition that the sun is
strongly electrified, either negatively or positively, and
further, that in the processes taking place in the comet,
either of vaporization or of some other kind, the matter
thrown out by the nucleus has become strongly electrified
in the same way as the sun—that is, negatively if the sun’s
electricity is negative, or positively if the sun’s is
positive. The enormous disturbances which the spectroscope
shows to be always at work in the sun must be accompanied by
electrical changes of equal magnitude, but we know nothing
as to how far these are all, or the great majority of
them, in one direction, so as to cause the sun to maintain
permanently a high electrical state, whether positive or
negative.”
Unless some such state of things exist, Sir John Herschel’s statement,
“That this force” (the repulsive force forming the tail) “can not be
of the nature of electric or magnetic forces,” must be accepted, for,
as he points out, “the center of gravity of each particle would not
be affected; the attraction on one of its sides would precisely equal
the repulsion on the other.” Repulsion of the cometary matter would
only take place if this matter, after it has been driven off from the
nucleus and the sun, have both high electric potentials of the same
kind. Further, it is suggested that luminous jets, streams, halos, and
envelopes belong to the same order of phenomena as the aurora, the
electrical brush, and the stratified discharges of exhausted tubes.
All this, it will be noticed, is at present merely hypothetical. It
is, however, worthy of notice that _outside_ of electricity there is
nothing known to physicists which seems to afford even a promise of
explanation, so far, at least, as the grander and more striking (also
the most mysterious) of cometic phenomena are concerned. It may well
be that with our advancing knowledge of meteors and meteor systems,
the spectroscopic analysis of the next few comets of the larger and
completer types—comets like Donati’s comet, the great comet of 1811,
and the comet of 1861—may throw unexpected light on mysteries which
still remain among the most profound and unpromising problems presented
to modern science.—_The Contemporary Review._
A TOUR ROUND THE WORLD.
By MRS. MARY LOW DICKINSON.
[Continued.]
An early start from Killarney, and a seven-hour journey by rail, ending
in delightful quarters at the pleasant Hotel Shelburne, in Dublin,
gives a comfortable sense of having passed an agreeable day.
“I would like to have run from Limerick Junction up to Limerick,” says
a feminine member of the quartette.
“I don’t think we missed anything in Limerick; I felt like stopping
awhile at Kilkenny,” answered her brother.
“But Kilkenny comes to America,” said the first speaker, who had been
reading about the enormous annual emigration from that town.
“Well, I fancy one can have all one wants of Limerick at home.”
“Not all I want,” said the little woman, perversely.
“What can you want, who have had your full share of domestic torments,
with numberless Bridgets and Norahs?”
There was no answer, but later, in the privacy of her chamber, she
said, “I did so want to go to Limerick, to buy some Irish lace. You
know they make it there, and send it over to Brussels, and then it is
bought back into Ireland for quadruple its cost”—but the air of mild
rebuke with which her companion looked up from the diary in which she
was describing the beauties of Killarney scenery, seemed to act as a
sudden check upon the purely feminine outburst. “Of course I know we
can’t stop to buy things,” she added, apologetically.
“Nor can we buy things when we do stop. It’s dreadful of you to begin
to want things so early.”
“Yes,” chimed in a masculine voice at the door, “It’s unworthy the
spirit of a true Chautauquan. Get on your hats, girls, there’s time
enough for a drive, to get what the books recommend—a general view of
the town.”
Our hotel fronts Stephen’s Green, whose twenty acres of beautiful
grounds form only one of six such ornamented breathing places for
that portion of the city which lies on the right bank of the Liffey,
beyond the confines of the ancient town. Our coachman knows his
business, and halts to point out the equestrian statue of George the
Second, showing through the trees in the center of the square. Then
we must look at the old Mansion House, and at the site of the great
exposition building which stands in twelve acres of garden, and was,
if successful, to have been what the Crystal Palace of Sydenham is to
London. Financially, it was not a success, and the future use of the
building is not yet decided. We halt again to admire the architectural
beauty of “the finest building in Dublin, if not in Ireland,” formerly
the Irish House of Parliament and now used as the Bank of Ireland.
If it were not too late we could go in and see the whole process of
printing the bank notes. As it is, we jog slowly on to the eastern
end of College Green, which is entirely occupied by the imposing
front of Trinity College. The huge Corinthian pile covers an extent
of thirty acres. The income is derived largely from landed estates.
Its library of one hundred and thirty thousand volumes grows rapidly,
for the University is one of the five that has the right to a copy of
every volume published in the United Kingdom. The number of students,
usually about two thousand, is diminished somewhat by the new Queen’s
University, founded by Victoria in 1850, which grants degrees to the
graduates from the Queen’s Colleges at Belfast, Galway, and Cork. To a
company of learners, nothing could be more interesting than to visit
the museums, observatories, botanic gardens, and printing houses of
this great institution, which has had such an influence on education
in Ireland; but, if we could not see all, we confess to a preference
for a day at the Royal Dublin Society, whose professors lecture to
the public gratuitously, and whose schools in the fine arts instruct
worthy pupils without charge. We want also to see the male and female
training schools, under the charge of the Board of Education, the
Deaf and Dumb Asylum, and the Drummond Orphanage, owned and supported
by a merchant of that name. Other societies for the promotion of
science and literature are too numerous to name. No city is more
generously provided with the means of education, or produces more
learned scholars, yet nowhere is the ignorance of the lowest classes
more marked. No place more abounds in charitable institutions. The
charity schools number over two hundred, yet the condition of the poor
is wretched in the extreme. The city is one of striking contrasts,
of grand architectural effects, heightened by the meanness of the
dwellings of the poor. The nine bridges over the Liffey add greatly to
the picturesqueness of the place, but the water of the stream is as
notoriously filthy as ever. Within a few years some efforts have been
made to improve the sanitation of the city and the hygienic condition
of the poor. Dirty alleys have been widened, model tenements built and
drinking fountains supplied. At enormous cost the neighboring streams
were turned into a valley, which made a natural reservoir seventeen
miles from the city. Thence the water is brought through tunnels to
filtering chambers, eight miles from town, from which it is distributed
over the city. From the University we went down Dame Street to the
Castle, used since the time of Elizabeth as the residence of the Lord
Lieutenant. Here there are state apartments to be seen, and music to
be heard in the beautiful chapel, but none of this to-day. The sun is
setting, and as we whirl on past hospital, postoffice, Custom House,
and convents, over the bridge and by the magnificent structure called
the Four Courts, because there are held the courts of Queen’s Bench,
Common Pleas, Chancery, and Exchequer, there is not time to drive
inside the gate of the beautiful Phœnix Park and look at the obelisk
that commemorates the victories of Wellington. That must be left for
the morrow, as must the old Christ Church and St. Patrick’s Cathedral,
whose ancient archepiscopal palace is now used as barracks for the
police. In this cathedral are the tombs of Dean Swift and the Stella of
his poetry. The structure is the most remarkable instance of complete
restoration of our day, Mr. B. L. Guinness, M. P., having spent
£150,000 in its restoration. He was knighted for his generosity, and it
does not become us to suggest that the sum could have been better spent.
We shall have to divide into two parties to-morrow, and while the
artist and teacher go to examine the picture gallery and the schools,
the others will get a look at the docks and the harbor, where, by aid
of dredging machines, the sand is kept at bay, so that ships can now
come up to the quays. Commerce still continues to be important, as
Dublin is the avenue of supply for imports for the midland district,
but manufacturers of woollen, cotton, silk, etc., are nearly extinct,
though the general financial and commercial condition of Ireland is
improved within the last twenty years.
While Dublin has declined, especially in manufactures of linen and
flax, Belfast, the metropolis of the north of Ireland, has steadily
grown in both. It lies on our route to the north, five hours from
London. It offers nothing in the common lines of interest, such as
churches, museums, public buildings, and parks that would induce a
stay; but its immense manufactories are well worth any loss of time a
visit may involve. We were shown all through the largest steam-mill,
which employs nearly three thousand men; and through the immense
establishment at Ardoyne, where the finest linen—that intended for the
noblest houses, whose coats of arms are woven in the web—is made by
hand. It was interesting indeed, to see, as in the former mill, the
process from the beginning with the raw flax to the beautiful completed
fabric, and more interesting to watch the workers, many of them women,
young girls and little children, who live their lives out day after
day in the dust and din of machinery. Their pinched and haggard faces,
their dull, spiritless eyes, and the constant monotonous motion of
their hands made them seem a part of the machines, and brought to mind
with great force Mrs. Browning’s “Cry of the Children.”