or eight miles to less than a quarter of a mile, even if there be not
some far smaller and beyond the range of the most powerful telescopes
man can construct, we find ourselves among objects resembling those
with which the study of our own earth has rendered us familiar. When
Sartorius's map of Etna and the surrounding region was first seen at
the Geological Society's rooms, many supposed that it represented
lunar features. The Vesuvian volcanic region, again, is presented
side by side with a lunar region of similar extent in Nasmyth's fine
treatise on the moon, and the resemblance is very close. Considering
the part which water plays in producing terrestrial volcanic phenomena,
it may reasonably be doubted whether there is in reality so close a
resemblance as a superficial comparison (and we can make no other)
would suggest. There are those, indeed, who believe that some of the
multitudinous small craters of the moon have had their origin in the
downfall of meteoric masses on her once plastic surface; and strange
though the thought may seem, there would be considerable difficulty
in showing how the surface of the moon could have remained without
traces of the meteoric downfall to which during myriads of centuries
she was exposed undefended by that atmospheric shield which protects
our earth from millions of meteors yearly falling upon her. We could
only attribute the smallest lunar craters, however, to this cause.
It may be noticed in passing that Professor Newcomb, apparently
referring to this suggestion, which some had thought too fanciful to
be seriously advanced, says that 'the figures of these inequalities
(the small craters) can be closely imitated by throwing pebbles upon
the surface of some smooth plastic mass, as mud or mortar.' Craters,
however, larger than a mile or so in diameter, and many also of smaller
dimensions, must be regarded as due to the same process of contraction
which produced the great craters, but as belonging to an era when this
process went on less actively. In like manner another feature of the
moon's surface, the existence of narrow furrows called _rilles_, which
sometimes extend to a considerable distance, passing across levels,
intersecting crater walls, and reappearing beyond mountain-ranges as
though carried under like tunnels, must be regarded as due to the
cracking of the crust thus slowly shrinking.
It is noteworthy that the signs of change which have been suspected
during recent years belong to these smaller and probably more recent
lunar formations. In November, 1866, Dr. Schmidt, chief of the
Athens Observatory, announced that the crater Linné in the lunar
Sea of Serenity was missing. To understand the importance of this
announcement, let it simply be noted that the quantity of matter
necessary to fill that crater up would be at least equal to that
which would be required to form a mountain covering the whole area
of London to a height of two miles! The crater was described by
former lunar observers as at least five miles in diameter and very
deep. It is not now actually missing, as Schmidt supposed, but it is
certainly no longer deep. It is, in fact, exceedingly shallow. Sir
J. Herschel's opinion was that the crater had been filled up from
beneath by an effusion of viscous lava, which, overflowing the rim on
all sides, poured down the outer slope so as to efface its ruggedness
and convert it into a gradual declivity casting no stray shadows. But
the stupendous nature of the disturbing forces necessary to produce
such an overflow of molten matter has led most astronomers to adopt
in preference the theory that the wall surrounding the crater has
been overthrown, either in consequence solely of the processes of
contraction and expansion described above, or from the reinforcement
of their action by the effects due to sublunarian energies. Some
consider that the descriptions of the crater by Mädler and Lohrmann
(which slightly differ) were erroneous, and that there has been no real
change. Others deny that any change has occurred, on the ground that
Linné varies in aspect according to the manner of its illumination.
This I perceive is Professor Newcomb's explanation, who considers
such variations 'sufficient to account for the supposed change.' But
since the time of Schmidt's announcement Linné has several times been
observed under nearly the same conditions as by Mädler and Lohrmann, as
the great shadows formerly seen in its interior have not reappeared.
There seems to be great reason for believing that a change has really
occurred there.
The discovery announced by Dr. Klein is of a different nature. Near the
middle of the visible half of the moon there is a well-known though
small crater called Hyginus, the neighbourhood of which has been often
and carefully examined. While examining this part of the moon's surface
with an excellent 5-1/2in. telescope, in May, 1877, Dr. Klein observed
a small crater full of shadow, and apparently nearly three miles in
diameter. It formed a conspicuous object on the Sea of Vapours. Having
frequently observed this region during the last few years, he felt
certain that no such crater existed there in 1876. He communicated
his discovery to Dr. Schmidt, who stated, in reply, that in all the
numerous drawings he had made of this lunar region no such crater was
indicated. It is not shown in the great chart by Beer and Mädler, or in
Lohrmann's map. Further observation showed that the crater is a deep,
conical opening in the moon's surface. Soon after the sun has risen
at that part of the moon, and, as later observations confirm, shortly
before sunset there, the opening is entirely in shadow, and appears
black. But when the sun is rather higher it appears grey, and with a
yet higher sun it can no longer be distinguished. It can, however, be
seen when the sun is very high on that part of the moon, appearing then
somewhat brighter than the surrounding region, a circumstance which
does not hitherto seem to have been noticed by either Klein or Schmidt.
The moon's surface has been so long and so carefully studied, that it
is almost impossible to understand how such a crater as now certainly
exists in the Sea of Vapours near Hyginus could have escaped detection.
Craters of the kind exist, indeed, in hundreds on the moon's surface.
But many astronomers have given years of their life to the study of
such objects; and the centre of the moon's disc, for reasons which
astronomers will understand, has been studied with exceptional care.
It seems so unlikely that a deep crater three miles in diameter could
escape recognition, that some astronomers have not hesitated to regard
the newly-detected crater as certainly a new formation. For my own
part, though it seems almost impossible to explain how such a crater
could have remained so long unnoticed, I can regard the evidence of
change as amounting only to extreme probability so far as it depends on
the result of past telescopic scrutiny of the moon.
Admitting that a change had occurred, it would not follow that it had
been produced by volcanic forces. It seems far more likely that a
floor originally covering the conical hole now existing in the Sea
of Vapours has given way at last under the effect of long-continued
processes of expansion and contraction, which would operate with
special energy over a region, like the Sea of Vapours, near the moon's
equator.
But there remains to be mentioned a form of evidence respecting
lunar features which could not be effectively applied to the case
of the crater Linné, because the moon had only been subject to the
necessary method of examination during a few years before that crater
was missed. I refer to lunar photography. Many objects less than two
miles in diameter are shown in the best photographs of our satellite
by Rutherfurd, De la Rue, Ellery, and Draper; and as the moon has been
photographed in every phase, some among the views might fairly be
expected to show Klein's crater if it really existed before 1877. I do
not find that in any lunar photographs the crater is shown as a black
or dark gray spot. But in Rutherfurd's splendid photograph of the moon
on March 6, 1865 (when the moon was about nine days five hours old),
the place of Klein's crater is occupied by a small spot lighter than
the surrounding 'sea.' This is the usual appearance of a small crater
under a high sun; and though it may indicate only the existence of a
flat crater floor in 1865 where now a great conical hole exists, it
throws some degree of doubt on the occurrence of any change at all
there. The case strongly suggests the necessity for continuing the
work of lunar photography, which seems of late years to have flagged.
Photographs of the moon should be taken in every aspect and in every
stage of her librational swayings. Possessing such a series, we should
be able to decide at once whether any newly-recognised crater was in
reality a new formation or not.
_THE NOVEMBER METEORS._
During November 13 and 14 the earth is passing through the region along
which lies the course of the family of meteors called the Leonides,
sometimes familiarly known as the November meteors. When at this time
of the year the meteor region thus traversed by the earth is densely
strewn with meteors, there occurs a display of falling stars, one of
the most beautiful, and, rightly understood, one of the most remarkable
of all celestial phenomena. Of old, indeed, when it was supposed that
these meteors were purely meteorological phenomena, they were not
thought specially interesting objects. They were held by some as mere
weather-portents. It was only when a storm of wind was approaching,
_vento impendente_, according to Virgil, that a shower of meteors was
to be seen. Gross ignorance, indeed, has given to showers of falling
stars an interest surpassing even that which has become attached to
them through the discoveries of modern science, for they have been
regarded as portending the end of the world. The shower of November
13, 1833, which was seen in great splendour in America, frightened the
negroes of the Southern States nearly out of their wits. A planter
of South Carolina relates that he was awakened by shrieks of horror
and cries for mercy from 600 or 700 negroes. When he went out to see
what was the matter, he found the negroes prostrate on the ground,
'some speechless, some with bitterest cries imploring God to spare the
world and them.' There is, however, a grandeur in the interpretation
placed by modern science upon these beautiful displays which dwarfs
into littleness even such ideas as have been suggested by the terrors
of superstition. We perceive that meteors are not mere terrestrial
phenomena, nor of brief existence. They speak to us of domains in space
compared with which the volume of our earth--nay, even the volume of
the sun himself--is a mere point: of time-intervals compared with
which the millions of years spoken of by geologists appear but as mere
seconds.
The special meteor family whose track the earth crosses on November
13-14 forms a mighty ellipse round the sun, extending more than 19
times farther from him than the track of our earth, which yet, as
we know, lies more than 92,000,000 miles from the sun. Along this
tremendous orbit the meteors speed with planetary but varying velocity,
crossing the track of our earth with a velocity exceeding by more than
a third her own swift motion of about 19 miles in every second of time.
Coming down somewhat aslant, but otherwise meeting the earth almost
full tilt, the meteors rush into our air at the rate of more than 40
miles per second. They are so intensely heated as they rush through
it that they are turned into the form of vapour, insomuch that we
never make acquaintance with the members of this particular meteoric
family in the solid form. In this respect they resemble the greater
number of our meteoric visitants. It is, indeed, a somewhat fortunate
circumstance for us that this is so, for if Professor Newton, of Yale
College (United States), is right in estimating the total number of
meteors, large and small, which the earth encounters per annum at
400,000,000, it would be rather a serious matter if all or most of
these bodies were not warded off. The least of them, even though a mere
grain perhaps in weight, would yet, arriving with planetary velocity
exceeding a hundredfold or more the velocity of a cannon-ball, prove
an awkward missile if it struck man or animal. But the air effectually
saves us from all save a few fire-balls which are large enough to
remain in great part solid until they actually strike the earth itself.
The importance of the meteors in the planetary system will be
recognised when we remember that the November group alone extends along
its oval course in one complete system of meteors for a length of more
than 1,700 millions of miles, with an average thickness of about a
million miles (determined by noting the average time occupied by the
earth in passing through the system on November 13-14), and an unknown
cross breadth which probably does not fall short of three or four
millions of miles. Other systems are, no doubt, far more important,
for it has been found that meteors follow in the track of comets. Now
the November meteors follow in the track of a comet (Tempel's comet of
1866), which was so small when last favourably placed for observation
that it escaped detection by the naked eye. If so small a comet as
this is followed by so large a meteoric system, in which also meteors
are strewn so richly that during the passage of the earth through it,
tens of thousands of meteors have been counted, how vast must be the
numbers and how large probably the individual bodies following in the
track of such splendid comets as Newton's, Donati's (1858), the comets
of 1811, 1847, 1861, and others! For it should be remembered that we
become cognisant of the existence of a meteoric system only when the
earth threads its way through one, when those which she encounters may
become visible as falling stars if it so chances that she encounters
them on the dark or night half of her surface. But the earth is far
smaller compared with a system like the November meteor-flight than a
rifle-ball compared with the largest flight of birds ever yet seen.
Such a ball fired into a very dense and widely extending flock of
birds might encounter here and there along its course some five or six
birds--not one in 10,000, perhaps, of the entire flight; and if the
flock continued flying with unchanging course, a hundred rifle balls
might be fired through it without seemingly reducing its numbers.
Our earth has passed hundreds of times through the November meteor
system, yet its meteoric wealth has scarcely been reduced at all, so
exceedingly minute is the track of the earth through the meteor system
compared with the extension of the system itself. The region through
which the earth has passed is less than a billionth part of the entire
region occupied by the system. But the November system is but one among
several hundreds through which the earth passes--in other words, the
systems which chance to be traversed by that mere thread-like ring in
space traversed each year by the earth, are not a millionth, not a
billionth, of the total number of such systems. It will be conceived,
therefore, that the total amount of meteoric matter, travelling on
orbits of all degrees of eccentricity and extension from the sun and
inclined at all angles to the general plane of the solar system, must
be enormously great. The idea once advanced by an eminent astronomer
that the total quantity of unattached matter, so to speak, existing
within the solar domain must be estimated rather by pounds than by tons
is now altogether exploded. It would be truer to say that the totality
of matter thus freely travelling around the sun must be estimated by
billions of tons rather than by millions.
Whether it is likely that there will be a display of meteors to-night
(or, rather, to-morrow morning), is a question to which most
astronomers would be disposed, we believe, to reply definitely in the
negative. The display of November 13-14, 1866, was very brilliant; that
of 1867 (best seen in the United States) was almost equally so; but
successive showers steadily diminished. In other words, the part of the
system crossed by the earth in 1866 and 1867 was very rich, but the
part which she crossed afterwards (the rich part having passed far on
towards the remote aphelion of the system outside the orbit of Uranus)
was less rich. For the last few years very few November meteors have
been seen, though the few stragglers which have been seen, and have
been identified as belonging to the family by their paths athwart
the star-depths, have been almost as interesting to astronomers as
the showers of such bodies seen in 1799, 1833, 1866, and 1867. But it
is not altogether impossible that in the small hours 'ayont the twal'
to-morrow morning a shower of meteors may be seen. For Schiaparelli
(the Italian astronomer who first started the ideas which led when
properly followed up to the discovery of the relations existing
between meteors and comets) asserts that it has happened before now
that the November meteors have appeared in great numbers in years
lying midway between the times of _maximum_ display. These times are
separated on the average by about 33-1/4 years. Thus, in 1799, there
was a great display of November meteors, a shower rendered specially
celebrated by Humboldt's description. In 1833 there was another, the
display which so terrified the negroes of South Carolina, but more
interesting scientifically as described by Arago. In 1866 the shower
again attained its _maximum_ splendour, though the display of 1867 was
little inferior. It will not be till 1899 that another great shower of
November meteors may be confidently looked for. But if Schiaparelli
be right, it is quite possible that there may be a shower this year,
due to some scattered flight of the November meteors which, delayed
accidentally (through some special perturbation) many hundreds of years
ago, has come in the course of ages to travel nearly half a circuit
behind the richest part of the system, the 'gem of the meteor-ring,'
as it has been poetically called. Even, however, though no display of
November meteors should be seen, yet the recognition of even a few
scattered stragglers would be exceedingly interesting to astronomers.
A single meteor seen to-night which could be regarded as certainly
belonging to the November system would suffice to show the possibility
that a whole flight of the November meteors might travel at a similar
distance behind the main body. It would be more easy, however, to
identify two such meteors than one, six than two, and a score than
half-a-dozen. The only way in which a meteor can be questioned, so to
speak, respecting the family it belongs to, is by noting its path
across the sky. If this path tends directly from the constellation
Leo (however remote Leo may be from the part of the heavens traversed
by the meteor), the chances are that the meteor is a Leonid, or one
of the November family. If the path tends from that particular part
of the constellation Leo (near the end of the curved blade of the
so-called Sickle in Leo), the probability of the meteor being a Leonid
is increased. If two or more meteors are seen to-morrow morning (after
12.30) which both tend from the Sickle in Leo, even though they seem to
tend in opposite directions, the chances are yet greater that they are
travelling in parallel paths along the track of the November meteors,
but some 2,000 million miles behind the main body. Should the number
mount up to a score or so, the conclusion would be, to all intents and
purposes, certain; and the possible occurrence of even a shower of
Leonids at a time midway between the customary _maxima_ of the meteoric
displays would be placed beyond question.
We must, however, admit that it seems less likely there will be
anything like a display of Leonids to-night than that patient observers
may be able to identify a few of these bodies, and thus--though by
observations of a less attractive kind--to advance our knowledge of
this interesting system. Far more likely is it that towards the end
of the month there will be a display of meteors belonging to another
and an entirely distinct family, a family scarcely less worthy to be
called November meteors _par excellence_, but actually rejoicing in the
classically unsatisfactory name of Andromeds.
_EXPECTED METEOR SHOWER._
(From the _Times_ of November 25, 1878.)
It is probable that during the next three nights some light may be
thrown on one of the most perplexing yet most interesting of all
the problems recently suggested to the study of astronomers. It is
confidently expected that many of those November meteors called
Andromeds will be seen on one or other of those nights, if not on all
three. No meteor systems, not even the famous systems of August and
November, are more remarkable than this singular family. To explain
why astronomers regard the Andromeds with so much interest, it will
be necessary to speak of an object which at first sight seems in no
way connected with them--an object, in fact, which, so long as it was
actually known to astronomers, was never supposed to be connected with
any family of meteors--the celebrated lost comet called Biela's (or, by
Frenchmen, Gambart's comet). In February, 1826, Biela discovered in the
constellation Aries a comet which was found to be travelling in an oval
path round the sun, in a period of about six years seven and a half
months. Tracing its course backwards, astronomers found that it had
been seen in 1772 by Montaigne, and observed for two or three weeks in
that year by Messier, the great comet hunter. Nothing very remarkable
was recognised regarding this comet in 1826, except the fact that its
path nearly intersects that of our own earth; so that if ever the earth
is to encounter a comet, here seemed to be the comet she had to fear.
Great terror was, indeed, excited by the announcement that in 1832 the
comet would cross the earth's track only four or five weeks before the
earth came to the place of danger. But no harm happened. In that year,
and again in 1839, the comet returned quietly enough, though in 1839
it was not observed, being so placed that it was lost in the splendour
of the solar rays. In February, 1846, the comet was again seen, this
being the third return since its discovery in 1826, or rather, since
its recognition as a member of the solar system, the eleventh since it
was first seen by Montaigne. At this time everything seemed to suggest
that this comet, unless our earth at some future time should absorb it,
would remain for a long time a steady member of the sun's comet family.
But only a few days after its detection in February, 1846, the comet
was found to have divided into two, which travelled side by side until
both vanished from view with increasing distance. In 1852 the companion
comets reappeared, and again both continued in view till their motion
carried them beyond telescopic range. As the distance between the
coupled comets had increased from about 160,000 miles in 1846 to about
1,250,000 miles in 1852, astronomers anticipated a most interesting
series of observations at the successive returns of the double comet to
the earth's neighbourhood. Unfortunately, in 1859 the comet's course
carried it athwart a part of the sky illuminated by the sun's rays, so
that astronomers could not then expect to see it. But in 1866 it was
looked for hopefully. Its orbit had now been most carefully computed,
and many observers, armed with excellent telescopes, were on the watch
for it, with very accurate knowledge of the course along which it
might be expected to travel, and even of its position from day to day
and from hour to hour. But it was not seen. Nor, again, was it seen
in 1872, when fresh computations had been made, and observations were
extended over a wider range, to make sure, as was hopefully thought,
that this time it should not escape recognition. Could it have come,
asked Herschel in 1866--and in 1872 the same question might still
more pertinently be asked--into contact or exceedingly close approach
to some asteroid as yet undiscovered? or, peradventure, had it
plunged into and got bewildered among the rings of meteorolites, which
astronomers more than suspected?