about some dim and mysterious objects to which we have not yet
alluded. They are what are called nebulæ, or little clouds; and in
one sense they are justly called little, for each of them occupies but
a very small spot in the sky as compared with that which would be
filled by an ordinary cloud in our air. The nebulæ are, however,
objects of the most stupendous proportions. Were our earth and
thousands of millions of bodies quite as big all put together, they
would not be nearly so great as one of these nebulæ. Astronomers
reckon up the various nebulæ by thousands, but I must add that most of
them are apparently faint and uninteresting. A nebula is sometimes
liable to be mistaken for a comet. The comet is, as I have already
explained, at once distinguished by the fact that it is moving and
changing its appearance from hour to hour, while scores of years
elapse without changes in the aspect or position of a nebula. The most
powerful telescopes are employed in observing these faint objects. I
take this opportunity of showing a picture of an instrument suitable
for such observations. It is the great reflector of the Paris
Observatory (Fig. 5).
[Illustration: FIG. 5. A GREAT REFLECTING TELESCOPE.]
[Illustration: FIG. 6. THE RING NEBULA IN LYRA, UNDER DIFFERENT
TELESCOPIC POWERS.]
There are such multitudes of nebulæ that I can only show a few of the
more remarkable kinds. In Fig. 6 will be seen pictures of a curious
object in the constellation of Lyra seen under different telescopic
powers. This is a gigantic ring of luminous gas. To judge of the size
of this ring let us suppose that a railway were laid across it, and
the train you entered at one side was not to stop until it reached the
other side, how long do you think this journey would require? I
recollect some time ago a picture in _Punch_ which showed a train
about to start from London to Brighton, and the guard walking up and
down announcing to the passengers the alarming fact that "this train
stops nowhere." An old gentleman was seen vainly gesticulating out of
the window and imploring to be let out ere the frightful journey was
commenced. In the nebular railway the passengers would almost require
such a warning.
Let the train start at a speed of a mile a minute, you would think,
surely, that it must soon cross the ring. But the minutes pass, an
hour has elapsed; so the distance must be sixty miles at all events.
The hours creep on into days, the days advance into years, and still
the train goes on. The years would lengthen out into centuries, and
even when the train had been rushing on for a thousand years with an
unabated speed of a mile a minute, the journey would certainly not
have been completed. Nor do I venture to say what ages must elapse ere
the terminus at the other side of the ring nebula would be reached.
A cluster of stars viewed in a small telescope will often seem like a
nebula, for the rays of the stars become blended. A powerful telescope
will, however, dispel the illusion and reveal the separate stars. It
was, therefore, thought that all the nebulæ might be merely clusters
so exceedingly remote that our mightiest instruments failed to resolve
them into stars. But this is now known not to be the case. Many of
these objects are really masses of glowing gas; such are, for
instance, the ring nebulæ, of which I have just spoken, and the form
of which I can simulate by a pretty experiment.
We take a large box with a round hole cut in one face, and a canvas
back at the opposite side. I first fill this box with smoke, and there
are different ways of doing so. Burning brown paper does not answer
well, because the supply of smoke is too irregular and the paper
itself is apt to blaze. A little bit of phosphorus set on fire yields
copious smoke, but it would be apt to make people cough, and, besides,
phosphorus is a dangerous thing to handle incautiously, and I do not
want to suggest anything which might be productive of disaster if the
experiment was repeated at home. A little wisp of hay, slightly damped
and lighted, will safely yield a sufficient supply, and you need not
have an elaborate box like this; any kind of old packing-case, or even
a bandbox with a duster stretched across its open top and a round hole
cut in the bottom, will answer capitally. While I have been speaking,
my assistant has kindly filled this box with smoke, and in order to
have a sufficient supply, and one which shall be as little
disagreeable as possible, he has mixed together the fumes of
hydrochloric acid and ammonia from two retorts shown in Fig. 7. A
still simpler way of doing the same thing is to put a little common
salt in a saucer and pour over it a little oil of vitriol; this is put
into the box, and over the floor of the box common smelling-salts is
to be scattered. You see there are dense volumes of white smoke
escaping from every corner of the box. I uncover the opening and give
a push to the canvas, and you see a beautiful ring flying across the
room; another ring and another follows. If you were near enough to
feel the ring, you would experience a little puff of wind; I can show
this by blowing out a candle which is at the other end of the table.
These rings are made by the air which goes into a sort of eddy as it
passes through the hole. All the smoke does is to render the air
visible. The smoke-ring is indeed quite elastic. If we send a second
ring hurriedly after the first, we can produce a collision, and you
see each of the two rings remains unbroken, though both are quivering
from the effects of the blow. They are beautifully shown along the
beam of the electric lamp, or, better still, along a sunbeam.
[Illustration: FIG. 7. HOW TO MAKE THE SMOKE RINGS.]
We can make many experiments with smoke-rings. Here, for instance, I
take an empty box, so far as smoke is concerned, but air-rings can be
driven forth from it, though you cannot see them, but you can feel
them even at the other side of the room, and they will, as you see,
blow out a candle. I can also shoot invisible air-rings at a column of
smoke, and when the missile strikes the smoke it produces a little
commotion and emerges on the other side, carrying with it enough of
the smoke to render itself visible, while the solid black looking ring
of air is seen in the interior. Still more striking is another way of
producing these rings, for I charge this box with ammonia, and the
rings from it you cannot see. There is a column of the vapor of
hydrochloric acid, that also you cannot see; but when the visible ring
enters the invisible column, then a sudden union takes place between
the vapor of the ammonia and the vapor of the hydrochloric acid; the
result is a solid white substance in extremely fine dust which renders
the ring instantly visible.
What the Nebulæ are made of.
There is a fundamental difference between the illumination of these
little rings that I have shown you and the great rings in the heavens.
I had to illuminate our smoke with the help of the electric light,
for, unless I had done so, you would not have been able to see them.
This white substance formed by the union of ammonia and hydrochloric
acid has, of course, no more light of its own than a piece of chalk;
it requires other light falling upon it to make it visible. Were the
ring nebula in Lyra composed of this material, we could not see it.
The sunlight which illuminates the planets might, of course, light up
such an object as the ring, if it wrere comparatively near us; but
Lyra is at such a stupendous distance that any light which the sun
could send out there would be just as feeble as the light we receive
from a fixed star. Should we be able to show our smoke-rings, for
instance, if, instead of having the electric light, I merely cut a
hole in the ceiling and allowed the feeble twinkle of a star in the
Great Bear to shine through? In a similar way the sunbeams would be
utterly powerless to effect any illumination of objects in these
stellar distances. If the sun were to be extinguished altogether, the
calamity would no doubt be a very dire one so far as we are concerned,
but the effect on the other celestial bodies (moon and planets
excepted) would be of the slightest possible description. All the
stars of heaven would continue to shine as before. Not a point in one
of the constellations wrould be altered, not a variation in the
brightness, not a change in the hue of any star could be noticed. The
thousands of nebulæ and clusters would be absolutely unaltered; in
fact, the total extinction of the sun would be hardly remarked in the
newspapers published in the Pleiades or in Orion. There might possibly
be a little line somewhere in an odd corner to the effect "Mr.
So-and-So, our well-known astronomer, has noticed that a tiny star,
inconspicuous to the eye, and absolutely of no importance whatever,
has now become invisible."
If, therefore, it be not the sun which lights up this nebula, where
else can be the source of its illumination? There can be no other star
in the neighborhood adequate to the purpose, for, of course, such an
object would be brilliant to us if it were large enough and bright
enough to impart sufficient illumination to the nebula. It would be
absurd to say that you could see a man's face by the light of a candle
while the candle itself was too faint or too distant to be visible.
The actual facts are, of course, the other way; the candle might be
visible, when it was impossible to discern the face which it lighted.
Hence we learn that the ring nebula must shine by some light of its
own, and now we have to consider how it can be possible for such
material to be self-luminous. The light of a nebula does not seem to
be like flame; it can, perhaps, be better represented by the pretty
electrical experiment with Geissler's tubes. These are glass vessels
of various shapes, and they are all very nearly empty, as you will
understand when I tell you the way in which they have been prepared. A
little gas was allowed into each tube, and then almost all the gas was
taken out again, so that only a mere trace was left. I pass a current
of electricity through these tubes, and now you see they are glowing
with beautiful colors. The different gases give out lights of
different hues, and the optician has exerted his skill so as to make
the effect as beautiful as possible. The electricity, in passing
through these tubes, heats the gas which they contain, and makes it
glow; and just as this gas can, when heated sufficiently, give out
light, so does the great nebula, which is a mass of gas poised in
space, become visible in virtue of the heat which it contains.
We are not left quite in doubt as to the constitution of these gaseous
nebulæ, for we can submit their light to the prism in the way I
explained when we were speaking of the stars. Distant though that ring
in Lyra may be, it is interesting to learn that the ingredients from
which it is made are not entirely different from substances we know on
our earth. The water in this glass, and every drop of water, is formed
by the union of two gases, of which one is hydrogen. This is an
extremely light material, as you see by a little balloon which ascends
so prettily when filled with it. Hydrogen also burns very readily,
though the flame is almost invisible. When I blow a jet of oxygen
through the hydrogen, I produce a little flame with a very intense
heat. For instance, I hold a steel pen in the flame, and it glows and
sputters, and falls down in white-hot drops. It is needless to say
that, as a constituent of water, hydrogen is one of the most important
elements on this earth. It is, therefore, of interest to learn that
hydrogen in some form or other is a constituent of the most distant
objects in space that the telescope has revealed.
Photographing the Nebulæ.
[Illustration: FIG. 8. THE PLEIADES.]
Of late years we have learned a great deal about nebulæ, by the help
which photography has given to us. Look at this group of stars which
constitutes that beautiful little configuration known as the Pleiades
(Fig. 8). It looks like a miniature representation of the Great Bear;
in fact, it would be far more appropriate to call the Pleiades the
Little Bear than to apply that title to another quite different
constellation, as has unfortunately been done. The Pleiades form a
group containing six or seven stars visible to the ordinary eye,
though persons endowed with exceptionally good vision can usually see
a few more. In an opera-glass the Pleiades becomes a beautiful
spectacle, though in a large telescope the stars appear too far apart
to make a really effective cluster. When Mr. Roberts took a photograph
of the Pleiades he placed a highly sensitive plate in his telescope,
and on that plate the Pleiades engraved their picture with their own
light. He left the plate exposed for hours, and on developing it not
only were the stars seen, but there were also patches of faint light
due to the presence of nebulæ. It could not be said that the objects
on the plate were fallacious, for another photograph was taken, when
the same appearances were reproduced.
When we look at that pretty group of stars which has attracted
admiration during all time, we are to think that some of those stars
are merely the bright points in a vast nebula, invisible to our
unaided eyes or even to our mighty telescopes, though capable of
recording its trace on the photographic plate. Does not this give us a
greatly increased notion of the extent of the universe, when we
reflect that by photography we are enabled to see much which the
mightiest of telescopes had previously failed to disclose?
Of all the nebulæ, numbering some thousands, there is but a single one
which can be seen without a telescope. It is in the constellation of
Andromeda, and on a clear dark night can just be seen with the unaided
eye as a faint stain of light on the sky. It has happened before now
that persons noticing this nebula for the first time have thought they
had discovered a comet. I would like you to try and find out this
object for yourselves.
If you look at it with an opera-glass it appears to be distinctly
elongated. You can see more of its structure when you view it in
larger instruments, but its nature was never made clear until some
beautiful photographs were taken by Mr. Roberts (Fig. 9).
Unfortunately, the nebula in Andromeda has not been placed in the best
position for its portrait from our point of view. It seems as if it
were a rather flat-shaped object, turned nearly edgewise towards us.
To look at the pattern on a plate, you would naturally hold the plate
so as to be able to look at it squarely. The pattern would not be seen
well if the plate were so tilted that its edge was turned towards you.
That seems to be nearly the way in which we are forced to view the
nebula in Andromeda. We can trace in the photograph some divisions
extending entirely round the nebula, showing that it seems to be
formed of a series of rings; and there are some outlying portions
which form part of the same system. Truly this is a marvellous object.
It is impossible for us to form any conception of the true dimensions
of this gigantic nebula; it is so far off that we have never yet been
able to determine its distance. Indeed, I may take this opportunity of
remarking that no astronomer has yet succeeded in ascertaining the
distance of any nebula. Everything, however, points to the conclusion
that they are at least as far as the stars.
[Illustration: FIG. 9. THE GREAT NEBULA IN ANDROMEDA.]
It is almost impossible to apply the methods which we use in finding
the distance of a star to the discovery of the distance of the
nebulæ. These flimsy bodies are usually too ill-defined to admit of
being measured with the precision and delicacy required for the
determination of distance. The measurements necessary for this purpose
can only be made from one star-like point to another similar point. If
we could choose a star in the nebula and determine its distance, then
of course, we have the distance of the nebula itself; but the
difficulty is that we have, in general, no means of knowing whether
the star does actually lie in the object. It may, for anything we can
tell, lie billions of miles nearer to us, or billions of miles further
off, and by merely happening to lie in the line of sight, appear to
glimmer in the nebula itself.
If we have any assurance that the star is surrounded by a mass of this
glowing vapor, then it may be possible to measure that nebula's
distance. It will occasionally happen that grounds can be found for
believing that a star which appears to be in the glowing gas does
veritably lie therein, and is not merely seen in the same direction.
There are hundreds of stars visible in a good drawing or a good
photograph of the famous object in Andromeda, and doubtless large
numbers of these are merely stars which happen to lie in the same line
of sight. The peculiar circumstances attending the history of one star
seem, however, to warrant us in making the assumption that it was
certainly in the nebula. The history of this star is a remarkable one.
It suddenly kindled from invisibility into brilliancy. How is a change
so rapid in the lustre of a star to be accounted for? In a few days
its brightness had undergone an extraordinary increase. Of course,
this does not tell us for certain that the star lay in the glowing
gas; but the most rational explanation that I have heard offered of
this occurrence is that due, I believe, to my friend Mr. Monck. He has
suggested that the sudden outbreak in brilliancy might be accounted
for on the same principles as those by which we explain the ignition
of meteors in our atmosphere. If a dark star, moving along with
terrific speed through space, were suddenly to plunge into a dense
region of the nebula, heat and light must be evolved in sufficient
abundance to transform the star into a brilliant object. If,
therefore, we knew the distance of this star at the time it was in
Andromeda, we should, of course, learn the distance of that
interesting object. This has been attempted, and it has thus been
proved that the Great Nebula must be very much further from us than is
that star of whose distance I attempted some time ago to give you a
notion.
We thus realize the enormous size of the Great Nebula. It appears that
if, on a map of this object, we were to lay down, accurately to scale,
a map of the solar system, putting the sun in the centre and all the
planets around their true proportions out to the boundary traced by
Neptune, this area, vast though it is, would be a mere speck on the
drawing of the object. Our system would have to be enormously bigger
before it sufficed to cover anything like the area of the sky included
in one of these great objects. Here is a sketch of a nebula, Fig. 10,
and near I have marked a dot, which is to indicate our solar system.
We may feel confident that the Great Nebula is at the very least as
mighty as this proportion would indicate.
[Illustration: FIG. 10. THE SOLAR SYSTEM AS COMPARED WITH A GREAT
NEBULA.]
RAIN AND SNOW
(FROM THE FORMS OF WATER.)
BY JOHN TYNDALL.
Oceanic Distillation.
[Illustration: SNOW CRYSTALS.]
At the equator, and within certain limits north and south of it, the
sun at certain periods of the year is directly overhead at noon. These
limits are called the Tropics of Cancer and of Capricorn. Upon the
belt comprised between these two circles the sun's rays fall with
their mightiest power; for here they shoot directly downwards, and
heat both earth and sea more than when they strike slantingly.
When the vertical sunbeams strike the land they heat it, and the air
in contact with the hot soil becomes heated in turn. But when heated
the air expands, and when it expands it becomes lighter. This lighter
air rises, like wood plunged into water, through the heavier air
overhead.
When the sunbeams fall upon the sea the water is warmed, though not so
much as the land. The warmed water expands, becomes thereby lighter,
and therefore continues to float upon the top. This upper layer of
water warms to some extent the air in contact with it, but it also
sends up a quantity of aqueous vapor, which being far lighter than
air, helps the latter to rise. Thus both from the land and from the
sea we have ascending currents established by the action of the sun.
When they reach a certain elevation in the atmosphere, these currents
divide and flow, part towards the north and part towards the south;
while from the north and the south a flow of heavier and colder air
sets in to supply the place of the ascending warm air.
Incessant circulation is thus established in the atmosphere. The
equatorial air and vapor flow above towards the north and south poles,
while the polar air flows below towards the equator. The two currents
of air thus established are called the upper and the lower trade
winds.
But before the air returns from the poles great changes have occurred.
For the air as it quitted the equatorial regions was laden with
aqueous vapor, which could not subsist in the cold polar regions. It
is there precipitated, falling sometimes as rain, or more commonly as
snow. The land near the pole is covered with this snow, which gives
birth to vast glaciers.
It is necessary that you should have a perfectly clear view of this
process, for great mistakes have been made regarding the manner in
which glaciers are related to the heat of the sun.
It was supposed that if the sun's heat were diminished, greater
glaciers than those now existing would be produced. But the lessening
of the sun's heat would infallibly diminish the quantity of aqueous
vapor, and thus cut off the glaciers at their source. A brief
illustration will complete your knowledge here.
In the process of ordinary distillation, the liquid to be distilled is
heated and converted into vapor in one vessel, and chilled and
reconverted into liquid in another. What has just been stated renders
it plain that the earth and its atmosphere constitute a vast
distilling apparatus in which the equatorial ocean plays the part of
the boiler, and the chill regions of the poles the part of the
condenser. In this process of distillation _heat_ plays quite as
necessary a part as _cold_, and before Bishop Heber could speak of
"Greenland's icy mountains," the equatorial ocean had to be warmed by
the sun. We shall have more to say upon this question afterwards.
The heating of the tropical air by the sun is _indirect_. The solar
beams have scarcely any power to heat the air through which they pass;
but they heat the land and ocean, and these communicate their heat to
the air in contact with them. The air and vapor start upwards charged
with the heat thus communicated.
Tropical Rains.
But long before the air and vapor from the equator reach the poles,
precipitation occurs. Wherever a humid warm wind mixes with a cold dry
one, rain falls. Indeed the heaviest rains occur at those places where
the sun is vertically overhead. We must enquire a little more closely
into their origin.
Fill a bladder about two-thirds full of air at the sea level, and take
it to the summit of Mount Blanc. As you ascend, the bladder becomes
more and more distended; at the top of the mountain it is fully
distended, and has evidently to bear a pressure from within. Returning
to the sea level you find that the tightness disappears, the bladder
finally appearing as flaccid as at first.
The reason is plain. At the sea level the air within the bladder has
to bear the pressure of the whole atmosphere, being thereby squeezed
into a comparatively small volume. In ascending the mountain, you
leave more and more of the atmosphere behind; the pressure becomes
less and less, and by its expansive force the air within the bladder
swells as the outside pressure is diminished. At the top of the
mountain the expansion is quite sufficient to render the bladder
tight, the pressure within being then actually greater than the
pressure without. By means of an air-pump we can show the expansion of
a balloon partly filled with air, when the external pressure has been
in part removed.
But why do I dwell upon this? Simply to make plain to you that the
_unconfined air_, heated at the earth's surface, and ascending by its
lightness, must expand more and more the higher it rises in the
atmosphere.