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HH TURNER
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A VOYAGE IN SPACE
{Frontispiece
ICAROMENIPPUS
(See Notes to Illustrations)
A
VOYAGE IN SPACE
A JUVENILE AUDITORY DELIVERED
AT THE ROYAL INSTITUTION
AT XMAS 1913
BY
H. H. TURNER, D.Sc., D.C.L., F.R.S.
SAVII.IAN PROFESSOR OF ASTRONOMY IN THE
UNIVERSITY OF OXFORD
WITH OFER I JO ILLUSTRATIONS
SOCIETY FOR PROMOTING CHRISTIAN KNOWLEDGE
LONDON: NORTHUMBERLAND AVENUE, W.C.
1915
BY THE SAME AUTHOR
MODERN ASTRONOMY
Constable & Co., 1901. Price 2s. bd.
ASTRONOMICAL DISCOVERY
Edward ^ArnjoM, 1904. Price los. 6d.
T.HE .GREAT' 'STAR MAP
Price zs. 6d.
TO
MY MOTHER'S GRANDCHILDREN
342311
PREFACE
THESE lectures, delivered at the Royal Insti-
tution at Christmas 1913, as the eighty-eighth
course of Juvenile Lectures, were taken down at
the time by a shorthand writer. When I entered
on the revision for publication, my first intention
was to abandon the language of the lecture-room,
substituting a narrative form. But I found the
translation attended by all sorts of difficulties, so
that the task assumed somewhat alarming dimen-
sions. At this point I happened to look again at
Faraday's Chemical History of a Candle, and saw
that he had not thought it necessary to depart
from the lecturing mode : with great relief I there-
fore ventured to follow him in this matter.
There are passages where I will ask the indulgent
reader to remember that many of my audience
were of very tender years : there are others where
he will perhaps kindly allow his thoughts to dwell
on the parents who came with them. If the alter-
nation of view-point is somewhat erratic at times, I
trust he will make some allowance for the difficulties.
Very few alterations of importance have been
made, although the two years which have slipped
away between the giving of the lectures and the
passing of the final proof-sheets have added their
due share to astronomical history. It seemed de-
sirable to note the discovery of a ninth Satellite to
viii PREFACE
Jupiter, by Mr. S. B. Nicholson, in the table on
p. 180 and on p. 181 ; but footnotes have been used
for this addition, in order to keep the text nearer
the original date. Possibly I have overlooked
something in dealing with similar recent discoveries :
Astronomy moves fast in these days, and it is not
easy to keep the pace she sets.
The hypothesis of a Sunspot-swarm of meteors,
given on pp. 200-206, has been added intentionally.
At the time of the lectures it had only just been
formulated, and although the picture on p. 205
was shown, little was said on the matter. Two
years' consideration has strengthened my con-
fidence in this interpretation of the facts known
to us, without producing any objections of a fatal
character; and it seemed to me therefore that, as
I was writing a book, I ought to put it in; to
omit it might be interpreted as a lack of confidence
on my part. At the same time I do not wish to
ignore the attitude of other astronomers, which is
duly acknowledged on p. 205.
I have made every effort to acknowledge the
source of the illustrations and to obtain permission
for their publication where needful. But there
may be some oversights. If so, I would beg the
same kind consideration for the lecturer turned
author as is extended to him from all quarters
during his preparation of the lectures, immediately
he mentions the magic words " Royal Institution "
and " Children's Lectures."
H. H. T.
University Observatory,
Oxford :
November 10, 1915.
CONTENTS
PAGE
PREFACE ........ Vii
NOTES TO ILLUSTRATIONS . . . .XI
LECTURE I
THE STARTING-POINT, OUR EARTH ... I
LECTURE II
THE LENGTH OF OUR VOYAGE AND THE START
THROUGH THE AIR . . . .47
LECTURE III
JOURNEYING BY TELESCOPE ..... 87
LECTURE IV
VISITS TO THE MOON AND PLANETS . . .138
LECTURE V
VISIT TO THE SUN ...... IQ2
LECTURE VI
VISITS TO THE STARS . . . . . .249
INDEX . . . . . . . 300
NOTES TO ILLUSTRATIONS
COVER OF BOOK
ON the back is the great Tower Telescope of the Mount
Wilson Observatory, California, to which reference is made
on p. 122.
On the panel is the representation of the billiard-ball
experiment on p. 286. The experiment was made by
some of the children in the audience, and the author asked
one of them to make a picture of it for the book. With-
out any suggestion from him, she chose to idealize it,
introducing some of the zodiacal signs as agents; and
seeing that the notion to be illustrated is that of the falling
together of stars from different quarters of the heavens,
the picturesque change is specially appropriate. Alto-
gether it was felt that no other illustration was so repre-
sentative of the voyages of the heavenly bodies; and to
choose it for the cover was therefore natural.
ICAROMENIPPUS (Frontispiece]
WHILE these lectures were being prepared for publica-
tion, the author was privileged to admire various draw-
ings by " Alice (aged 13) " contributed to a magazine
of original drawings which makes periodical but rather
mysterious appearances in Oxford. It seemed to him
that a picture of Icaromenippus, designed by an artist
of an age representative of the important part of his
audience, would be a welcome addition to this volume.
The application was kindly received and in a few weeks'
time no less than nine quite different designs were sent,
any one of which would meet requirements. The choice
was difficult, but skilled assistance indicated the one here
reproduced.
A comment on the eternal divergence between Art
and Science is suggested. The mere astronomer will no
doubt point out that the crescent moon is not limited
by its illumination, but is part of a large globe which
xii NOTES TO ILLUSTRATIONS
would make the position of Icaromenippus impossible.
But artists have so often and so persistently claimed the
right to ignore this fact that it would be discourteous to
maintain this objection.
PAGES 15 and 16. The pictures of Tycho Brahe's obser-
vatory and sextant are from the beautiful collections made
by Professor Weinek of Prague, and published in the
observatory volumes.
PAGE 19. The Earth and Halley's Comet. This draw-
ing originally appeared in the Illustrated London News
for Sept. 25, 1909, and is reproduced by kind permission.
PAGE 36. The Great Comet of 1858 is from Telescope
Teachings by the Hon. Mrs. Ward (Groombridge & Sons,
1859), a book with numerous beautiful illustrations much
prized by our fathers and grandfathers.
PAGE 39. The Return of Halley to see his Comet is, as
stated in the text, from a picture in the possession of
Mr. H. P. Hollis, of the Royal Observatory, Greenwich,
and is reproduced by his kind permission.
PAGE 42. Changes in a Comet's tail by E. E. Barnard.
This is not the pair of pictures shown on the screen at the
lectures, but is a triumph of Professor Barnard's skill
given in the A strophysical Journal, Vol. XXII., Plate VIII.
(facing p. 251).
PAGE 48. Wreck of the Searchlight apparatus. This
illustration is from p. 380 of Pearson's Magazine, in the
volume July-December, 1907; and is reproduced by kind
permission of the Editorial Manager. The author is
fortunate enough to possess the original pair of drawings
for this excellent story.
PAGES 60 and 61. Transit of Venus. These illustra-
tions are reproduced (by permission of the Delegates
of the Clarendon Press) from Chambers 's Handbook of
Astronomy.
PAGE 63. Horrox observing the Transit of Venus.
This is reproduced, by kind permission of Mr. Napier
Clark, formerly of Southport and now of 16 Hallhead
Road, Edinburgh, from a picture in his possession painted
by W. R. Lavender. There is no description or portrait
of Horrox in existence, and the representation is therefore
purely imaginary. The observation was made at the
village of Much Hoole, near Southport, on Nov. 24 [o.s.],
1639. Horrox was at that time about twenty. The
NOTES TO ILLUSTRATIONS xiii
picture has been painted with careful attention to all that
is known, or can be conjectured, of the circumstances.
PAGE 84. Appley Bridge Aerolite. Reproduced by kind
permission of the Royal Astronomical Society from Monthly
Notices, Vol. LXXV. p. 94. The aerolite fell on Oct. 13,
1914 (after the lectures were given). Its length was
9.65 in., depth 9.13, and width 6.62 ins. It ranks as the
second largest recorded fall in Great Britain.
PAGE 93. W. Herschel's 2o-foot telescope. My thanks
are due to the Herschel family for permission to reproduce
this illustration. It appears as Plate B in Vol. I. of the
recently published Collected Papers, and is described as
" from a Drawing made either at Datchet or at Clay
Hall."
PAGE 97. " The Great Five-foot being taken up Mount
Wilson," and p. 98, " An Accident to the Traffic up Mount
Wilson," are due to the courtesy of Professor Hale. The
latter represents the only serious accident attending the
whole of the extended series of transport operations. Two
of the men jumped from the car as it was slipping over
the track, but the driver stuck to his car and went down
with it. Mr. Adams, dashing down the steep incline,
found him badly cut and bruised ; but with ropes he was
hauled up and taken to hospital.
PAGE 114. Great Nebula in Andromeda. This picture
is taken by kind permission of the Royal Astronomical
Society from No. 98 of their series. It is from a photo-
graph taken by G. W. Ritchey and F. G. Pease at the
Yerkes Observatory on Sept. 18, 1901, with a colour
screen on the 4O-in. refractor.
PAGE 116. Victoria Telescope. This record of an inci-
dent of the erection of the telescope I owe to the kindness
of the late Sir David Gill.
PAGE 119. A coelostat at Oxford. The coelostat was
invented many years ago by a Frenchman named August.
It was, however, not much used. In 1896 Dr. Johnstone
Stoney directed attention to a paper by the great French
physicist Lippmann, in which he recalled the principle. It
was at once seen that the instrument was specially suitable
for eclipse work, and a pair of coelostats were constructed
under the direction of Dr. Common for the 1896 eclipse.
The picture shows one of these being mounted in the garden
of the University Observatory at Oxford, in order to test
xiv NOTES TO ILLUSTRATIONS
its performance before starting for Japan to observe the
eclipse.
PAGE 120. The Snow Horizontal Telescope, and p. 122,
The High " Tower " Telescope, are from Professor Hale's
Annual Reports. One sometimes feels that more than a
perfunctory word of thanks is due to the astronomers of
the Harvard, Lick, Yerkes, Mount Wilson and other great
American observatories for the plentiful supply of beautiful
pictures to the world at large.
PAGE 124. The ascent in the bucket is from a snap-
shot taken during the visit of the Solar Union to Mount
Wilson in 1910, and kindly sent to me by one of the occupants
of the " elevator."
PAGE 127. The cities of Pasadena and Los Angeles,
from a beautiful print kindly sent me from Mount Wilson
the particulars of which have unfortunately been mislaid.
PAGE 133. Algol, "An Astronomical Reprobate."
This illustration is reproduced by the special permission
of the Proprietors of Punch. It is from the number for
Jan. 1 8, 1899. The note under the drawing is as follows :
" The star Algol behaved in a most ill-bred manner.
He would advance, wink, and then retire. For years his
motion and behaviour puzzled astronomers, until at last
the mystery was solved by Professor Vogel, who showed
that Algol had associated with him a dark star, which was
invisible, and that the latter sometimes obscured the
former. Algol and his invisible playmate revolved round
each other, and this accounted for the fact that Algol
seemed to us to wink." Sir Robert Ball's Lecture at the
Royal Institution.
PAGE 138. Mars, as drawn by N. E. Green, is taken by
kind permission of the Royal Astronomical Society from
Plate II., Fig. n, of their Memoirs, Vol. XLIV. It was
made by use of a 13-in. reflector by Geo. With, and is
dated Sept. 10, 1877, n hrs. 20 min; longitude 297. The
original is beautifully tinted, but it has not been possible
to reproduce the tinting here.
PAGE 145. Mars, with the 4O-in. from a print kindly
sent me by Professor Barnard. His note on the back is
" 40-in. direct enlargement, Sept. 28, 1909."
PAGE 147. Jupiter, by Mr. Scriven Bolton. This pair
of drawings was kindly made specially for this book. The
original is a chalk drawing in which Jupiter's equatorial
NOTES TO ILLUSTRATIONS xv
diameter is 6 in. The date of the observations is Dec. 4,
1906, and the interval between the sketches is ij hours.
PAGE 151. Saturn. The three photographs are from
a longer series kindly sent me by Professor E. E. Barnard,
taken by him with the 6o-in. reflector on Mount Wilson
(Cal.), Nov. 19, 1911.
PAGE 163. Leverrier Statue. From the memorial
volume to Leverrier, published by the Institut de France,
on the centenary of his birth (March 12, 1811). The
statue by Chapu was erected in 1889 in the Cour du Nord
of the Paris Observatory.
PAGE 164. Plaque of Adams is reproduced from a
photograph taken in Westminster Abbey by Mr. Wright.
PAGE 185. By kind permission of the publisher, Mr.
John Murray.
PAGE 189. Liquid Air Experiment. Reproduced from
the Illustrated London News of Jan. 10, 1914, by kind
permission of the Editor.
PAGE 195. Sun as taken at Greenwich. One of a
series kindly supplied for lecture purposes by the Astronomer
Royal.
PAGE 205. Collision of Saturn and Leonids. Repro-
duced from the Illustrated London News of Dec. 20, 1913, by
kind permission of the Editor.
PAGE 212. Drawing by Huggins. Reproduced by kind
permission of the Royal Astronomical Society from Monthly
Notices, Vol. XXVI. , p. 263.
PAGE 213. Drawing by Nasmyth. The original draw-
ing was presented by the artist to the Oxford University
Observatory, where it now hangs.
PAGE 21 5. Hansky's photographs, from Mitteilungen der
Nikolai-Hauptsternwarte zu Pulkowo, Band, I. No. 6.
PAGE 218 and 220. Figs. 62, 63 and 65-70 are again
due to the courtesy of Professor Hale. They may be
found in the Astrophysical Journal, Vol. XIX. and Vol.
XXVIII.
PAGE 237. The Japanese eclipse. The 1896 expeditions,
for which the coelostats were prepared (see note above
to p. 119), went one to Norway and one to Japan; but
both had cloudy weather at the critical moment. A
Japanese artist accompanied the latter expedition from
Tokio to its observing station, and was to have painted
xvi NOTES TO ILLUSTRATIONS
the Corona, which never appeared. During the time of
preparation, however, he made several sketches of the
instruments.
PAGE 251. The Southern Cross. From a photograph
taken with a 6-in. lens at the Sydney Observatory, on
August 13, 1890. Exposure three hours.
PAGE 255. ---The split nebula in Andromeda is from a
photograph taken by W. S. Franks.
PAGE 256. The Nebula in Cygnus is from a photograph
kindly presented to the Observatory by the late Dr. Isaac
Roberts. Exposed Oct. 27, 1896, for 2 hrs. 18 min.
PAGE 259. The star cluster in Hercules (M 13), from
G. W. Ritchey's beautiful photograph taken with the 4O-in.
Yerkes refractor, with a colour screen.
PAGE 273. The coloured plate of spectra is the plate
arranged by Professor Newall for his book on the Spectro-
scope. At the suggestion of the publishers, and with
Professor Newall's kind permission, it is adopted here, to
save the preparation of a special plate.
PAGE 281. The flight of ducks. From the close re-
semblance of this picture to Fig. 91, representing star
movements, it might be supposed that it was designed
to fit. But the resemblance is purely accidental. Wander-
ing one day among Mr. Newton's stacks of beautiful lantern
slides, my eye fell on this picture, and I purchased it to
illustrate stellar migrations, without realizing at the
moment how closely it fitted the Taurus cluster. It is
from a series which appeared originally in the Illustrated
London News, and is reproduced by kind permission.
PAGE 286. The billiard ball experiment. See note
above on the illustrations for the cover.
PAGE 297. The expanding nebula round Nova Persei
is from the photographs by G. W. Ritchey, taken with
the two-foot reflector of the Yerkes Observatory. See
Astrophysical Journal, Vol. XIV. p. 293.
A VOYAGE IN SPACE
LECTURE I
THE STARTING-POINT, OUR EARTH
THE idea of a " Voyage in Space " is not new by any
means. Two thousand years ago it occurred to the
Greek writer Lucian to suppose a man flying up to
the heavens. He called him Icaro-menippus, be-
cause Icarus was supposed to have been the first
man to fly. In one of the old stories which were
told to Greek children (just as stories are still told
to children even in our twentieth century), it was
related how Daedalus, the father of Icarus, made him
some beautiful wings ; but they were only fastened
on with wax, and when Icarus flew near the Sun,
the wax melted and he fell. Lucian's Icaromenippus
was wiser. He caught a large eagle and a large
vulture ; and then he relates how
[I] cut off very carefully the right wing of the
eagle and the left of the vulture, then tied them
on, securing them over the shoulder by strong
straps, and at the ends of the quill feathers I
put in things like loops for my hands to grip. 1
1 Six Dialogues of Lucian, by S. T. Irwin. Methuen & Co.,
1894.
' 'A' VOYAGE IN SPACE
: -Me practised* 'flying, gently at first, and then more
boldly, until he flew up to the Moon, and looked down
on the Earth (see Frontispiece), which he declared
was much smaller than the Moon. One of the strik-
ing points in Lucian's tale is the suggestion that
the eagle's wing endowed Icaromenippus with the
eagle's sight.
An eagle has the keenest sight of any living
thing, and the eagle alone looks straight at the
Sun.
And so he was able to see things happening on the
distant Earth.
Nowadays, of course, we should bring a telescope
into the story; but the telescope was not invented
until more than a thousand years after Lucian's
time, and the best he could think of was this vague
notion of the eagle's " strong sight," which in some
mysterious way helped Icaro-menippus to see very
distant things. But since he had not the magnifying
power of a telescope the distant things looked very
small. He tells how our earthly cities, " including
the inhabitants, were not at all unlike ant-hills." It
is good for us sometimes to remember how small we
are compared with the universe, or even with the
solar system of which our Earth is a rather unim-
portant member : and it is noteworthy that in this
early example of a pretended " Voyage in Space "
Lucian does not forget to make this use of it.
Since his day many others have had the same
notion of pretending to get away from the Earth,
and to visit one or other of the planets.
In my boyhood we read a book by Jules Verne,
THE STARTING-POINT, OUR EARTH 3
in which a projectile carrying three men is supposed
to be shot from an enormous cannon towards the
Moon. The pictures were fascinating, and it is
distressing to find that the edition with the pictures
seems to be out of print ; at any rate, I have not
been able to recover a copy, though I much wanted
to reproduce some of the pictures for you on the
screen. There was a terrible explosion when the
cannon was fired, and how those poor men ever
survived the shock is a mystery which only great
writers like Jules Verne can understand. But they
did survive, and had most wonderful adventures.
They even had the complacency to go over their
calculations again, while inside their projectile,
travelling at a furious speed towards the Moon;
and found that they had made a mistake ! It was
rather late to find it out, wasn't it ? after they had
started all wrong. It is better to go over such
calculations twice before one commits one's safety
to them; but if these particular people had done
so, we should perhaps not have had such a good
story. The result of the mistake was that they
never reached the Moon at all, but circled round it ;
and by an ingenious device they ultimately came
back to Earth. I hope you will read all their adven-
tures for yourselves.
In recent times, Mr. H. G. Wells has written about
a supposed visit to the Moon, called The First Men
in the Moon. He had not quite the courage to ex-
plode his travellers out of a huge cannon like Jules
Verne, and so he invents a curious material called
Cavorite, which screens off gravity (we shall have
a good deal to say about gravity in a moment).
4 A VOYAGE IN SPACE
Mr. Wells relates how a car, covered with this strange
material, can rise quite smoothly from the Earth
without the fearful explosion : and the occupants
of the car are thus able to reach the Moon, where
they find that the inhabitants are like insects rather
than like us men and women : and though they are
smaller and weaker than we are, their cattle their
" moon-calves "are immensely greater than ours.
There are plenty of exciting adventures in this book
too, and Mr. Wells has tried to keep as nearly as he
can to what is possible, according to such informa-
tion as our telescopes give us about the Moon. Of
course, his " Cavorite " material is as yet quite
unknown we have not even an inkling of an idea
how to screen off gravity ; but if we agree to make
him a present of this rather startling notion, he
uses it very skilfully to pilot his readers through
varied experiences. Such books as those of Verne
and Wells are worth reading, not only by a " juvenile
audience " such as this, but by astronomers too :
it is a most useful lesson to them to try and specify
exactly where the author is keeping within the pos-
sibilities, and where he is making a real mistake.
Let me mention one more of such books which I
have read with keen pleasure A Honeymoon in
Space, by Mr. George Griffith. I think you can buy
it for sixpence, and it teaches us a lot of astronomy
in very pleasant fashion, because we have not the
idea of lessons in our minds when reading it ; and it
is much nicer to avoid lessons if we can. I am afraid
I shall not be able to avoid the appearance of giving
lessons so skilfully as Mr. Griffith or Mr. Wells, be-
cause for one thing I am not going to attempt to tell
THE STARTING-POINT, OUR EARTH 5
a connected story as they have done. What I shall
endeavour to do is to bring before you some of the
difficulties of making such voyages, so that you may
understand as clearly as possible how they arise :
which of them we may hope to get rid of in time, and
which seem beyond our powers. And we shall begin
to-day with the first difficulty of getting away from
this Earth at all.
Why is it so difficult to get away? If we jump
up we come down again. We call the reason
" gravity," and sometimes we are tempted to let the
matter rest when we have given a name to the diffi-
culty. But I want us to think a little more about it
than this : to think about what the nature of it is,
and also about the great men who found out for us
what gravity is. Because it took a great deal of
finding. About 2000 years ago, in Greek times,
they had some curious ideas about it. The great
Greek philosopher Aristotle said that if you took
a light thing and a heavy thing and dropped them
together, the heavy thing fell much faster. Here
are his actual words; some of you (very few, I
hope) may not be able to read Greek, and so we will
translate them; but it is better to show you the
actual words that Aristotle used, because what he
said seems so strange to us now.
ev ra juel^co Qoni]V e%ovra r) fiaQovt; fj Kovcporriro^,
ear rd.Ua o^o/cog lyr\ role; o%r}[jiaoi, ddrrov (pegojusva
TO 'loov %a>Qiov } nal xara 2.6 yov 6v e%ovoi TO.
:c We see that bodies which have a greater ten-
dency of heaviness or lightness, if they are otherwise
6 A VOYAGE IN SPACE
similar in shape, pass more quickly through the
same space, and in the ratio which the magnitudes
have to each other."
What it comes to is this, that if you have a ten-
pound weight and a one-pound weight and you drop
them together, the ten-pound weight will fall ten
times as quickly. The magnitude of the weight
settles how quickly it shall fall. That seems to us
so astonishing that we can hardly imagine that any
one believed it. Yet people went on believing that
for 2000 years just because Aristotle had said it.
Now is it better to believe things people tell you,
or is it better to try and find out for yourselves?
That is about the hardest question to answer that
can be set. At first sight it seems better to believe
what people tell you. They may tell you, for in-
stance, that fire burns ; and so you need not put
your hand in the flame to find out. Or they may
tell you that something is poisonous; and if you
believe what you are told, you avoid eating it and
continue to live. If on the other hand you persist
in trying for yourself, you may die; and there is
the end of trying things. In this way we seem to
have answered our question pretty easily : it seems
far better to believe what you are told, because you
get all the advantages of other people's experience :
and perhaps if they would confine themselves to
telling us what they had actually tried, we might be
satisfied with this answer.
Unfortunately, however, we cannot always be sure
that people are speaking from experience : some-
times we are quite sure that they are not; for
instance, we are sure that Aristotle had not tried
THE STARTING-POINT, OUR EARTH 7
the experiment of dropping two different weights;
if he had, he would have written something very dif-
ferent. Yet he was believed because he wrote so
confidently, and because people had a great respect
for him in other ways, and perhaps most of all
because we are taught in childhood to believe what
we are told. The boy who likes to try things for
himself is regarded as a troublesome and naughty
boy, and is apt to get a whipping, even if he does
not get burnt or poisoned. Yet the world has good
reason to be very thankful to some of these
naughty boys who have persisted in trying things
for themselves.
Galileo was one such, and came in for punishment
at the time : here on the screen you see a copy of a
fine picture in the gallery at Cologne showing his pun-
ishment in prison. His troubles began when he first
questioned this statement of Aristotle that a ten-
pound weight would fall ten times as quickly as a
one-pound weight, and by his trying the experiment
for himself he first threw some light on this problem
of gravity the force which prevents our getting
away from the Earth. Galileo was born at Pisa, a
little town in the north of Italy, famous for its lean-
ing tower. He went to school at Florence, not far
away; and was determined to become a professor
of mathematics. Several times he seemed to have
a good chance of such a position, and was disap-
pointed ; but ultimately, to his great delight, he was
elected Professor at his native city of Pisa, and he
made use of the leaning tower to try his famous
experiment with a ten-pound weight and a one-pound
weight. In the presence of many witnesses, who
8
A VOYAGE IN SPACE
had all pinned their faith on Aristotle, he dropped
the two weights together; and they struck the
ground together, showing that Aristotle's statement
was quite wrong. Even then such is the reluctance
to abandon an old idea people would not believe
Galileo dropping the two weights from the
Leaning Tower of Pisa.
the thing they saw with their eyes. Such was their
profound respect for Aristotle that they declared
that there must be some mistake in the experiment,
since Aristotle could not be wrong.
In honour of Galileo's memory, I want us to try
this experiment to-day, as well as we can. We have
THE STARTING-POINT, OUR EARTH 9
no leaning tower, but the roof of this building is
pretty high : we have no Galileo, but some one has
kindly offered to represent him, and he will drop these
two balls, a wooden one and a leaden one, from the
roof overhead into this box of sand, so that we may
see for ourselves whether they fall together or not.
There ! you see they fall together just as they did
300 years ago from Pisa's leaning tower. Perhaps
you may think that Galileo's representative did not
drop them quite fairly together? That is quite
possible, though I feel sure he tried to do so ; but
to make quite sure of being fair we have arranged
a kind of trap for the balls up aloft which can be
worked from below by means of this long string,
which perhaps one of the audience will pull for
me.
There again ! they have fallen, you see, quite
together; and do you see that the wooden one is
lying on the top of the sand while the leaden one
has quite disappeared? It is buried so deep in
the sand that it is quite difficult to find it. This
fact does not concern us very much at the moment,
though it is interesting as showing the different
weights of the two balls; but when we come to
speak of meteorites in the next lecture, we shall find
it useful to remember how a body striking the Earth
may bury itself deeply. A very great man, Charles
Darwin, used to say that we ought to learn all we
can from an experiment, whether it is of imme-
diate importance or not : and this tendency of heavy
falling bodies to bury themselves in the earth is
just such an extra consequence as he would have
delighted to note.
ID A VOYAGE IN SPACE
But to return to the main point ; there may have
been, perhaps, a little difference between the times
of the two balls : indeed there certainly was, and we
know the cause of it. It is the resistance of the air,
which acts more effectively on light bodies than on
heavy. For two solid balls such as we have used
the difference is not great : but if we took a feather
or other very light body we know that it would fall
very slowly. This does not mean that gravity acts
less on a feather than on a piece of lead, but that
the air resists its fall more effectively. If there were
no air, the feather would fall just as quickly as the
balls of wood and lead. I cannot show you this by
exhausting all the air from this room, because we
want some of it to breathe, but if you will kindly
be content with a much smaller drop, we can do the
experiment with this tall jar. It is an experiment
which has been done many times before and is called
the " coin and feather " experiment. The coin used
to be a golden guinea, but I fear none of my audience
is likely to have such a coin with them, and perhaps
they might not even like to lend me a sovereign, so
I must use one of my own. We put a feather along-
side it in a little trap, rather like that which released
the balls, and then we close the tall jar over them
and exhaust all the air. In old days this was done
with a hand-pump, and required some exertion, but
with the beautiful resources of this Institution we
have merely to attach a tube and turn a tap ; in a
few seconds we have a nearly perfect vacuum. And
now, if we release the trap, you see that the coin and
feather fall together. Directly we turn the tap again
and let the air in, the feather blows about while the
THE STARTING-POINT, OUR EARTH n
coin rests stolidly at the bottom. Here, again, is a
thing worth noting that is not immediately before
us this rapid inrush of the air from outside into the
exhausted space. If the jar had been full of air and
the outside empty the rush would of course have been
the other way and would soon have left very little in
the j ar. Now in Jules Verne's book, of which I spoke
early in the lecture, the brave men who went in the
projectile took with them a dog, and the poor dog
died. They thereupon opened a trap-door and put
it outside; but I fancy this would have been much
more difficult and dangerous than Jules Verne
thought, because though the travellers had taken
the precaution to carry plenty of air with them inside
their projectile, there would be very little outside it
when once they had got some distance from the
Earth. We shall see in the next lecture how very
shallow our atmosphere is the projectile would pass
right through it in a few seconds. Hence, when the
trap-door was opened to put out the body of the poor
dog, I fear all the air would have rushed out almost
at once, and the men would have died too, and so
would the story.
If we want the story to go on, we must let Jules
Verne tell it in his own way : but there is no harm
in making mental reservations, and we will take the
opportunity to make one more, because it brings to
our notice another fact about Gravity, in which we
are specially interested to-day.
We have already said that when we jump up we
come down again because of Gravity because the
Earth pulls us to itself. Now when Jules Verne's
travellers opened the trap-door and put the poor dog
12 A VOYAGE IN SPACE
out probably with a fairly good push, because they
would be in a hurry not to let more air escape than
they could help when they pushed the dog away
from the projectile, would it fall back again on to the
projectile as we fall back on to the Earth when we
push ourselves away by jumping? We know that
the Earth pulls things to itself, but do other bodies
also pull other things to themselves in the same way ?
We can scarcely believe that they do because we
never feel the pull of anything but the Earth. If I
stand near an object such as this desk in front of me
I do not feel any pull towards it, and yet, as a matter
of fact, it is pulling me and so is everything else in
the room, though the pulls are too slight for me to
feel. That is one of the laws of Gravity, that every-
thing is pulling or attracting everything else; but
the amount of pull or attraction depends firstly on
the size of the body that pulls, and secondly on its
distance away. The great Sir Isaac Newton first
proved these laws some 250 years ago, and we will
presently recall how he did it, just as we have re-
called Galileo's experiment to memory. But for
the moment I only want you to notice one point
that a large body like our Earth exerts a greater pull
than a small one like this desk or like the projectile
from which Jules Verne's travellers pushed the dog.
Undoubtedly the projectile would try to pull the dog
backwards, but owing to its small size it might not
succeed, and then the dog would go farther and
farther away. We cannot tell what would happen
without knowing how hard the dog was pushed out.
If a thing is pushed hard enough it can get away even
from the Earth indeed, that is exactly the point of
THE STARTING-POINT, OUR EARTH 13
Jules Verne's story, how a very hard push, from a huge
cannon, sent the projectile right away from the Earth.
The push had to be very hard indeed because the
Earth is so big that it pulls very strongly : the pull
of the little projectile would be very feeble indeed
so that even a very slight push would send the dog
right away, never to fall back. This, however, is
not what happens in the story : the dog neither goes
right away nor falls back, but goes round and round
the projectile as a " satellite," that is, as our Moon
goes round us : and here I am sorry to say Jules
Verne makes another mistake, though it would take
too long to explain why. Let us be satisfied for the
moment to notice this great fact that a large body
pulls harder than a small one. Perhaps you think
that is not surprising, perhaps it seems natural to
expect more from a large body. Let me remind
you that it was just by following what seemed
natural that Aristotle made his big mistake : it
seemed to him natural to expect a large body to fall
quicker than a small one, and he was wrong, as
Galileo proved by trying it. If I had no better
reason than that it seemed natural, I should not
ask you to believe that a large body pulls harder
than a small one ; but there is a much better reason,
namely that the experiment has been tried many
times.
One famous experiment was made in 1798 by
Henry Cavendish (of the same family as the Dukes
of Devonshire) , but it is by no means the only one :
the experiment is really being made by astronomers
every day, for if the fact were not true none of their
calculations would come right as we know they do.
14 A VOYAGE IN SPACE
Well, then ! we may accept this fact that all bodies,
even the tiniest little particles, are pulling as hard
as they can, though particles cannot do much. It is
only when a great crowd of them are massed together
to make the Earth that the pull becomes easy to feel.
And when we get an enormously greater crowd still,
such as the Sun, the pull is enormously greater. Why,
then, does not the Sun pull us away from the Earth ?
Well, there is more than one reason; but one at
least is that he is so much farther away, and this
brings us to the next fact we have to notice about
Gravity that it depends on the distance.
A body close to you pulls hard : take it farther
away and the pull becomes less. Mathematicians
say that it diminishes as the " inverse square of the
distance " ; but we can put the law into simpler
language in this way. Take a body twice as far
away, and the pull is one quarter of what it was;
bring it twice as near, and the pull is four times what
it was. And now let us go back to the history of
the discovery and see how Newton found this out
for us.
About the time when Galileo lived in Italy, there
was another wonderful man called Tycho Brahe,
living in Denmark, who also mistrusted some of the
things told him on good authority. For instance,
there were books giving the positions in which the
planets ought to be seen at any time, which were still
trusted although the planets could be seen not to
follow the books. Tycho determined to go to work
for himself, observing carefully the movements of
the planets with the idea of making better books or
tables than those in use. A beautiful observatory
THE STARTING-POINT, OUR EARTH 15
was built for him on the little island of Hveen, and
he called it Uraniborg Heavenly City because it
was devoted to the study of the heavens. Kings
and nobles visited him there out of respect for the
Tycho Brahe's Observatory on the Island of Hveen.
great work he was doing : and he himself had such
reverence for it that whenever he observed the stars
and planets he put on his richest robes. The tele-
scope had not yet been invented, so that his observa-
tions were made by the use of " sights " such as are
used on rifles a "back "-sight near the eye, and a
i6
A VOYAGE IN SPACE
" fore "-sight at the further end of a straight bar
pointed towards the star. Compared with what we
can now do with a telescope the observations were
rough indeed ; and yet they were the means by
which we learnt the laws of Gravity. This, however,
was not until many years after Tycho Brahe himself
was dead : he made thousands of observations, but
did not live to find out
all they could tell him.
Fortunately he had a de-
voted pupil, Kepler, a Ger-
man, who went on working
at them, and found out
the very important things
they showed. Kepler is
represented with a pair
of compasses in his hand,
showing that he was fond
of measuring things, which
is really the chief business
of an astronomer not
merely looking at the stars
and planets, but measur-
ing their distances apart,
or something else that can
be measured. Tycho Brahe was also animated by
this devotion to measurement, otherwise Kepler
would not have been able to use his work as he did.
By use of it he found three great laws of movement
for the planets. No one suspected the simple laws
of Gravity as yet, but Kepler made it possible to find
them by establishing the three great laws of move-
ment which have ever since been called by his name
A Sextant used by
Tycho Brahe.
THE STARTING-POINT, OUR EARTH 17
- Kepler's Laws. The first is that the planets
move round the Sun in ellipses. You can easily
draw an ellipse for yourself with two drawing-pins
and a loop of string. Fix the pins in a drawing-
board and put the loop of string round them, keeping
it stretched into a triangle with a pencil point. You
can still move the pencil about and it will draw you
an ellipse. The drawing-pins are called the foci of
the ellipse, and when a planet moves in an ellipse
the Sun is at one of the foci. Once when the late
i8 A VOYAGE IN SPACE
Sir Robert Ball (who several times gave these
Christmas lectures) had mentioned this fact that
the Sun occupies one of the foci of the ellipse, he
was earnestly implored by a lady after the lecture
to tell her which of the foci was the one. I am afraid
I should have tried to explain that it did not really
matter, and the explanation might have taken a long
time. But Sir Robert was cleverer than that :
raising his right hand, he said, " Madam ! the right
DRAWING AN ELLIPSE (FIG /.)
one " (which of course was right in any case), and
the lady thanked him warmly and went away quite
satisfied. But whichever focus we choose it will be
nearer one end of the ellipse than the other. Now
Kepler's second law tells us that the planet moves
quicker when it is near the Sun than when far away.
He put it into more precise form, but that will be
enough for us to remember that when a planet or
comet is near the Sun, it bustles along at a great
pace, when it is far away it moves more soberly,
THE STARTING-POINT, OUR EARTH 19
and when very far away it loiters terribly. That is
really because the gravity or attraction of the Sun
is feeble at a distance and strong when close; but
Kepler did not quite realize about this attraction,
he contented himself with noting that the planet
moved more slowly or more quickly. And finally
he found a third law, which is rather tiresomely
C
How the Earth and Halley's Comet move round the Sun.
(See Notes to Illustrations.)
mathematical at first sight, but was the most im-
portant of all for taking the next step, and therefore
we should pay it some attention.
It states that if a planet be placed at a distance
from the Sun 4 times as great as that of our Earth,
it will take 8 times as long as we do to go round the
Sun completely, that is to say, 8 years ; and if it is
9 times as far away, then 27 years ; if 100 times as
far away, then 1000 years. Can you see how these
20 A VOYAGE IN SPACE
numbers are related? Please look carefully at
Simple
Number
Square for the
Distance
Cube for the Time
I
2
3
4-
10
1X1= I
2X2= 4
3X3= 9
4 x 4 = 16
10 X 10 = 100
ixixi= i
2X2X2= 8
3X3X3= 27
4X4X4= 64
10 X 10 X TO = TOGO
this little table. Four is called the square of two,
because if one square is twice as long and twice
as broad as another, it is four times the first in size.
We are almost reminded of the Red Queen's argu-
ment with Alice, " Five times as warm and five
times as cold just as I'm five times as rich as you
are and five times as clever." Poor Alice gave it up
like a riddle with no answer : but we must not give
it up here, we must go even a step further. When
one thing is twice as long and twice as broad and
twice as high, we go beyond the square to the cube.
If we multiply 2 X2 X 2, we get 8, which is the
cube of 2 : and when the distance is the square of
a number, the time is the cube of the same number.
None of the planets fits these numbers exactly :
there is no planet at 9 times the Earth's distance,
but Saturn is at 10 times just a little bigger than
9 and we can therefore see from the table that it
will take rather more than 27 years to go round the
Sun (29! years as a matter of fact). But we need
not trouble about the exact figures if we understand
the principle of this great Third Law of Kepler.
On that law Sir Isaac Newton and others went to
work, and soon saw that it told them the Law of
THE STARTING-POINT, OUR EARTH 21
Gravity. You have heard the story of Newton and
the apple, which must surely be true, because here
is a bit of the actual apple tree from Newton's gar-
den at home : it now belongs to the Royal Astron-
omical Society and they have kindly lent it to us for
this occasion. In the entrance hall you will find a
beautiful picture of Newton under the apple tree,
thinking about Gravity. But though the story of
the apple may be quite true, it may suggest to you
what is not true, namely, that he thought out the
law of Gravity very quickly after seeing the apple
fall. This was not the case : it took him 20 years
to work out the great law fully in all its bearings.
When he saw the apple fall, he began to think; he
wondered if the pull of the Earth which made the
apple fall, and which brings us down again when we
jump up, would still be the same if we could go far-
ther away, or how it would alter ; and his thoughts
took him farther and farther away till he thought of
the Moon, and whether it was feeling the pull of the
Earth. And then he must have thought of the move-
ment of the Moon and of other bodies in the heavens,
and naturally he would think of Kepler's three laws :
and then the third of them, which we have just been
talking about, led him to see that if the movements
of the planets were in any way controlled by a pull
from the Sun, the pull must be weaker at a distance,
and stronger close to the Sun : and he calculated
how much stronger it would be. Instead of stating
the law in the language he used, let us illustrate it
with this piece of apparatus, because it is easier to
understand a thing when we have something to look
at. You can easily make the apparatus for your-
22
A VOYAGE IN SPACE
selves. Take two equal wooden bars SA and SB
(Fig. 2) and put a screw through them at S, into the
side of a ladder. The holes in the bars should be
larger than the screw shaft so that they will turn
freely. At A and B tie the ends of two exactly equal
cords ACB and AKB. To the middle (K) of one of
them hang the weight W, and hold up the other
yourself at its middle point C.
You may put something at C
to represent a comet say a
bundle of horsehair ; and some-
thing at S to represent the Sun
say a card circle. For the
Sun at S will attract the comet
with a force like that of gravity
" the inverse square of the
distance ' ' SC. [For those who
like a mathematical proof, I
put one at the end of the
chapter, but the great thing
is to try the experiment for
yourselves.]
Now when you go up the
ladder and hold C high up,
as in Fig. 2, you will find that it is quite easy
to support the weight W perhaps with one finger.
You may have to ask some one to lift up the weight
for you in the first instance, but once lifted, it is very
easy to hold it up. If a pair of reins were made on
this principle, a child could hold back a runaway
horse.
But now drop C down to halfway so that the bars
open out. You will find that the pull is much
Fig. 2.
SPRING
BALANCE
THE STARTING-POINT, OUR EARTH 23
greater four times as great, as Newton tells us.
It is not very strong even yet, but it is beginning to
be felt. Here is a spring-balance with which we can
measure the pull at C, and you will see that the
spring is scarcely moved. If again I let C (the
comet) come twice as near to the Sun the pull is four
times as great again, or sixteen times as great as at
first. But now that we
are getting nearer to the
Sun, which we represent by
a cardboard circle, one very
important matter forces
itself on our attention.
Where are we measuring
our distances from? Is it
from the top of the card,
which is the nearest point
of the Sun ? or is it to be
from the centre where the
screw is? In a sense we
are twice as near the card
when we are twice as near
the nearest point of it ; and
there is a good deal to be
said for measuring in this
way, since the very point we are illustrating is
that gravity is so immensely greater when we are
close to the attracting body than when we are far
away ; and therefore we might expect that the part
of the Sun nearest would be the important part.
At any rate this seems to have been what first
occurred to Newton, and he could not get the idea
out of his head for twenty years : and the complete
Fig. 3-
24 A VOYAGE IN SPACE
establishment of the Law of Gravity had to wait all
that time in consequence, just as I have been keeping
you waiting to see the end of the experiment. It
was only after twenty years' thinking, off and on,
that Newton discovered that we must measure all
our distances from the centre of the Sun, where the
screw is. And now, measuring in this way, we will
go again twice as near, and the pull is getting very
strong (Fig. 3) : twice as near again and it is almost
too much for me to hold : nearer still and it is too
much the weight wins the battle, as you see.
Perhaps you will let me tell you the history of
Newton and the Law of Gravity. We shall see from
it how much human nature enters into such dis-
coveries at times; and it will have the further
advantage of introducing us to another experiment
which I want to show you, proving the rotation of
the Earth. Newton was born in the year 1642,
the same in which Galileo died. (Perhaps we may
like to remember that Galileo was born in Shake-
speare's birth-year and died in Newton's.) He began
thinking about Gravity when the apple fell in the
autumn of 1665, when only twenty-three years old;
but having got so far as the idea of a pull which
altered with the distance, as our apparatus illustrated,
Newton was beset by the difficulty we have noticed.
It is all very well, he reflected, to talk of the pull
altering with the distance, but how are we to measure
distance from a large body like the Earth or the Sun ?
Are we to measure it from the nearest point of the
Earth, or from its centre, or from some point in
between? When the whole Earth is far away, it
does not much matter : but when we are dropping
TOWER
THE STARTING-POINT, OUR EARTH 25
a weight as Galileo did, from the top of a tower, as at
T in Fig. 4, then part of the Earth near F, the
foot of the tower, is pretty close to us, and we have
just seen how enormous the pull is when we come
close to the attracting body. The other side of the
Earth at K is 8000 miles away, and the pull will be
much less. Suppose, for instance, the tower were
a mile high, which is far higher than any tower we
are r likely to ascend. Even
then, the point K is 8000
times as far away as F, and
its pull would be 8000 x
8000, or 64 million times
weaker. So that when he
wanted to add up the pulls
from all parts of the Earth
together, Newton naturally
thought that the far side
of the Earth near K would
count for very little, and the
part near F would be all-
important. Even Newton,
therefore, did not at first
suspect the truth which he found out twenty years
later, and which is, that we must measure from the
centre C, which is just as near to K as to F. He
thought that the correct starting-point would be at
some point like R, nearer to F than to K; and he
could not make any such point fit in with what was
known about gravity at the time when he saw the
apple fall, which was in the autumn of 1665. Hence,
after puzzling over it for some time, he put the
matter aside. Fourteen years later, Mr. Hooke, Secre-
Fig. 4.
26 A VOYAGE IN SPACE
tary of the Royal Society, wrote to Newton asking
him whether he had any interesting suggestion for
one of the meetings. Newton replied in a famous
letter, which became even more famous because it
disappeared, and was only quoted from memory.
Fortunately it was recovered by Trinity College
(Newton's College at Cambridge) about thirty years
ago, and is now in their library. They have kindly
allowed me to show you a picture of part of it, in
which Newton makes the suggestion Hooke asked
for. He suggested a method of proving that the
Earth is turning round on its axis. Nowadays we
are all so well convinced of this fact that it seems to
require no proof, but the case was very different in
Galileo's time, when to state the fact got him into
serious trouble. Newton came after Galileo, as we
have noticed, and in his day the truth was better
known, but even then it was not so firmly estab-
lished but that he thought it worth proving. And
so he suggested that a weight should be dropped
from a tower T, much as Galileo had dropped weights
from the leaning tower of Pisa ; but with a different
object in view. This time it was to be carefully
noted, not how quickly the weight fell, but exactly
where it fell. If the Earth were not turning round
then it would fall at F, the foot of the tower : but if
the Earth is turning round, then the weight would
be flicked forward a little to G, just as a splash of
mud is flicked forward by a rotating wheel (Fig. 4).
The rotation of the Earth can indeed be shown in
this way, as Newton thought and as we will presently
mention. But unfortunately Hooke misunderstood
the point. His mind was full of a different problem,
THE STARTING-POINT, OUR EARTH 27
which had gradually come to the front as the most
important scientific problem there was at the time,
and which no one could solve. It was this. If
gravity really changed with the distance as New-
ton had suspected fourteen years before, and as
many others had since suspected from study of
Kepler's great Third Law, then would Kepler's
Second Law follow as a consequence, that is to say
would the planets all move in ellipses round the Sun ?
People felt that probably this was a necessary con-
sequence, but no one could prove it. Hooke claimed
to have proved it, but no one now believes his claim.
And he accused Newton of not knowing the fact,
much less being able to prove it. This made New-
ton very angry. He first sat down and proved the
proposition for his own satisfaction did what every
one was trying to do and could not and then he
tossed the matter aside, as he could not bring him-
self to reply to the disagreeable man who had wrongly
accused him. And so things might have remained
had it not been for Edmund Halley, a man of a very
different kind, who came in the nick of time to rescue
this great achievement of Newton's. Halley was an
Oxford man, Newton was at Cambridge : Oxford
and Cambridge often meet in friendly rivalry, but
this time they met in co-operation, and took each a
share in the great discovery of Gravity, though the
share of Cambridge is of course much the greater.
Halley had tried without success to get an answer
to the difficult question whether planets moved in
ellipses because the Sun attracted them in the way
suspected ; and as a forlorn hope he travelled all the
way to Cambridge to ask Newton about it. To his
28 A VOYAGE IN SPACE
great delight, Newton replied that he had proved the
proposition completely; though oddly enough he
could not find the paper on which he had written it
out. Newton was not careful and tidy, as we all are,
I hope. But he was very, very able : and soon
wrote out a new proof and sent it to Halley, who had
already hastened to London to tell the Royal Society
the good news that the great problem was solved.
This was, however, rather premature exultation,
though no one suspected it but Newton. He knew
that there was still the formidable difficulty we have
noticed : he had not yet found out where to measure
the distance from, whether from the nearest point of
the Earth (or other attracting body) , or its centre, or
some other point. But so much had he been stimu-
lated by Halley's visit and genial influence, that he
forthwith attacked this difficulty again, this time
with success : he found the astonishing result that
you must measure always from the Earth's centre.
We may therefore summarize the history of this
great discovery in three steps
(i) In 1665 Newton saw the apple fall, and was led
to think of the Law of Gravity : " the attraction is
inversely as the square of the distance." But he
could not see how to measure the distance accurately.
(ii) In 1679 he made the suggestion of proving the
Earth's rotation; and Hooke's reply irritated him
into solving " the problem of the ellipse " ; the solu-
tion, however, he kept to himself.
(hi) In 1685 Halley's visit elicited the solution and
stimulated Newton into finding that the distance
must be measured from the centre of the attracting
object.
THE STARTING-POINT, OUR EARTH 29
You see what an important crisis it was when
Newton suggested his experiment for proving that
the Earth rotates : and in honour of him we will now
make such an experiment, though not the one he
suggested. I might perhaps have shown you his
very experiment, or one founded upon it : for during
the last few years the rotation of the Earth has been
proved and measured in this way, using, however, the
apparatus known as At wood's machine, to drop the
weight. Here is a fine At wood's machine, which has
often been used in this Institution : you see two
weights, one heavier than the other, connected by
a string over a pulley. The heavier weight falls, pull-
ing the other up ; but the fall is comparatively slow,
and can be made as slow as we please by making the
weights nearly equal. Now this slow fall gives the
sideways " flick " due to the Earth's rotation plenty
of time to develop. When the experiment is made
just as Newton described it, the fall is so rapid that
the " flick " has scarcely time to work : with At-
wood's machine, it gets a much better opportunity.
And where do you think this ingenious development
of Newton's idea has been put into practice and the
rotation of the Earth demonstrated and measured ?
Why ! at the observatory of the Vatican, where 300
years ago things were made so unpleasant for Galileo
merely because he asserted this rotation. Truly
Time brings changes !
But this beautiful method of Father Hagen is per-
haps not the easiest way for us to see the rotation of
the Earth within a few minutes. We had better use
the method of an ingenious Frenchman called Fou-
cault, who used for the purpose a long pendulum like
30 A VOYAGE IN SPACE
this. If you swing a pendulum truly backwards and
forwards, without any wobble, it will go on swinging
in the same way even though the Earth rotates
underneath it. Here is a little pendulum mounted
on a model earth : I set it swinging and turn the
model earth round the pendulum still goes on
swinging in the same way : it takes no notice of the
rotating Earth underneath, though I have turned
the model a quarter turn (so that if the pendulum
went with it, the swing would be at right angles to
its former direction) . If, however, there were people
living on this model earth, they would not be con-
scious that I had turned it round : to them the pen-
dulum would seem to change : to them it would
seem to be swinging now at right angles to its former
direction. And so our long pendulum will not really
change, but will seem to us to be changing. But to
turn our actual Earth a quarter round takes six
hours : we should have to go home to tea long before
that : we must be satisfied to watch the real pendu-
lum for a few minutes only. Nevertheless we can
see its apparent change if we magnify it in the in-
genious way devised by Sir James Dewar.
Here is the pendulum tied back. Presently I am
going to swing it by burning this string which ties it
up. When the string breaks in burning, the pen-
dulum will swing backwards and forwards. At one
end of the swing it comes very close to this lamp
which is throwing the shadow of the pendulum-wire
on the screen, and you will then see the wire and the
motion much magnified; so that we can easily tell
whether the swing changes even a little bit. But it
is not the pendulum that is changing, it is the Earth
THE STARTING-POINT, OUR EARTH 31
rotating underneath; and in a minute or two we
shall see the effect of the earth moving.
[In Fig. 5 the pendulum bob was at first swinging
from D to C ; the shadow being thrown on the screen
from the point X was near the middle of the screen
most of the time, but as the pendulum bob reached
the point C, the shadow fell at c, away to the right.
Presently the earth's rotation altered the path of the
bob to BA, and at A the shadow fell at a, away to
the left.]
F/G. 5. FOUCAULT'S PENDULUM.
Magnification of the Effect.
You will see that as the pendulum comes up to the
lamp the shadow will flash to the right, and it would
go on doing that always if the earth did not move;
but as the earth moves it will presently go to the left.
There will be a moment when it will spread out on
both sides ; and after that it will go to the left. I
hope we can manage the experiment without keep-
ing you too long. Now I will burn the thread.
There, you see it goes to the right, and presently it
will go to the left. You remember when Peter Pan
32 A VOYAGE IN SPACE
asks those who believe in fairies to clap their hands ?
Well, I don't want you to believe in things you do
not see, but when you see the Earth rotating, that is,
when the shadow goes to the left instead of to the
right, then you clap your hands ; for then you will
have seen the rotation of the Earth. Surely it is
beginning to go over now, it is more to the left
than to the right ! (The audience here testified as
requested.) In this way you can soon see the Earth
rotating, if the experiment is carefully prepared
beforehand.
There is another way in which it has been shown
by M. Foucault that the Earth is rotating. He
showed it by means of a gyrostat. He spun a top
quickly and set it up in a certain position. Now a
thing that is spinning has got a great tendency to
remain in the same position. For instance, we
know that a spinning top does not fall down, though
it may turn round. I cannot show you M. Fou-
cault's exact experiment, but I can show you some
pretty things of the same kind which tell us as much
as we want to know. Perhaps you have played with
gyrostats already : but probably you have wound
them up with string; Mr. Gray, of Glasgow, has
invented some beautiful gyrostats which are wound
up by means of a small motor. Here is one now
spinning very quickly, and if I put it horizontally
with one end of the axis on a stand, it will not fall
down, as it would if it were not spinning, but simply
goes round and round. Its weight tries to make it
fall, but can only make it turn round. And if we
try to lift the end of the axis, we only make it turn
round the other way.
THE STARTING-POINT, OUR EARTH 33
That principle has been used recently for steering
ships. A large gyrostat has its axis floated hori-
zontally in mercury. If the axis points East and
West, then the rotating Earth is always trying to lift
the West end, but it only makes the axis turn round.
And this is also more or less true for any other posi-
tion except that when the axis is due North and
South. Hence the axis will be always seeking that
due North and South position, as the magnetic needle
seeks the Magnetic Poles. This has been known a
long time, but only recently has the tendency .been
used to make a good practical compass. This has
at last been done, and the most wonderfully con-
vincing proof of the rotation of the Earth is thus
provided we are steering ships by means of it, and
entrusting thousands of lives to its efficacy.
Perhaps it will interest you to know what was the
nature of the invention which has made a practical
success of this compass. You have seen a workman
use a plumb-line to get buildings vertical, and you
know that a plumb-line is very like a pendulum.
If you set it swinging it will go on swinging for some
time, and be no use as a plumb-line. To bring it
to rest the workman sometimes puts the bob in a
bucket of water, which quickly stops the swinging.
This kind of stopping is called " damping " the
swing, though it has nothing to do with the water
wetting the bob; it merely means that the incon-
venient swinging is stopped. Now when a gyro-
compass is set up, unless it is pointed exactly North
from the first (which is unlikely) , it will swing to and
fro to find the North, just as a plumb-bob swings to
and fro to find the vertical. But the swinging of the
34 A VOYAGE IN SPACE
gyrostat will be much slower and more persistent
than that of the pendulum so slow and so persistent
that most people thought it impossible ever to bring
it to rest. But some ingenious Germans have ac-
tually found a way, and their invention (which cor-
responds to the bucket of water with the plumb-bob)
has made a practical success of the compass. The
gyrostat in spinning round at a great rate makes air
currents; and the inventors have led these air
currents past a delicately adjusted metal tongue,
so that any tendency to swing is resisted. Perhaps
it will also interest you to hear how they came to pay
attention to the matter at all. They had the notion
of going to the North Pole by submarine. They
thought that a boat could be sunk so as to travel
under the ice to the North Pole and back. But the
question arose how they were to steer. The mag-
netic compass would only take them to the Magnetic
Pole, which is several hundred miles away from the
true North Pole. The stars would not be visible
from under the ice. Hence they had to think of
some other plan, and it occurred to them to enquire
whether they could not make use of this long-known
but never-used principle of the gyrostat setting its
axis North and South. Their success has been so
complete that their compass is actually superior to
the magnetic compass.
But we must now return to Newton and the law
of Gravity. Newton proved, then, that the law of
Gravity made bodies move in ellipses. An ellipse
is called a conic section, because if you take a cone
and cut it across you may get an ellipse. But it is
not the only possible curve ; you may get a parabola
THE STARTING-POINT, OUR EARTH 35
curve which does not go round and come back
again, but goes on for ever. I can show you how to
make a parabola, which is just as easy as to make an
ellipse, I think almost easier. Take two straight
lines, and set off on each a series of equal distances
let us say inches, or half inches, or whatever you like
and then join the corresponding points in the way
shown in the diagram, outwards on one line and in-
wards on the other. I believe in some schools this
is done as a needlework exercise, joining the points
DRAWING A PARABOLA (FIG. 6.)
by threads, which shape out a parabola. You need
not stop at the point 8 ; you can go on past it, pro-
vided the lines are suitably extended, and there is
no stopping-place : so you see a parabola is a curve
that goes on without end. That is important to
remember : for suppose we started on a journey
from the Earth and moved under gravity we might
get into one of these curves that goes on without end,
and that would be inconvenient, for we should never
get back again to Earth.
At one time it was thought that comets moved in
parabolas and never came back ; that they came to
36 A VOYAGE IN SPACE
us once, went away and never came back at all. But
soon after Newton's discovery about the law of
Gravity, Halley thought he would calculate the
orbits of these comets. Perhaps you have not seen
a big comet, such as that which appeared in 1858.
The Great Comet of 1858.
A " comet " means a hairy star, and in old days
comets were represented as stars with hair. Now-
adays we get photographs of them. Anybody who
has got a Kodak and can strap it to a telescope with
clockwork can take a photograph of a comet.
Well, it was supposed that comets moved in para-
THE STARTING-POINT, OUR EARTH 37
bolas, as we should if we started off with the proper
velocity ; and it was thought they never came back.
Halley found out that they did come back, and that
comets move in ellipses as other planets do, only that
they are very long ellipses. It was a well-deserved
reward for Halley, who took such pains to get the
proof of the law of gravitation from Newton, that
he should be led to this great discovery. I may add
to what I have already told you, that even when
Halley took Newton's great work to the Royal
Society in triumph, he found to his astonishment
that they were at the moment too poor to publish it.
Thereupon Halley paid for it out of his own pocket,
although he was not a rich man. He got his reward
when he made this great discovery about comets
coming back to us, and that was soon after he
went back to Oxford as Savilian Professor of
Geometry.
He set out to examine as many orbits of comets
as he could find observations of assuming that they
went in parabolas and never came back and he
calculated the orbits of twenty-four of them. Now
the calculation of an orbit is a complicated matter
that I am not going to trouble you with, at present
at any rate; but we can all of us understand that
when figures come out the same, whatever the figures
may mean, they must refer to the same thing. Look
now at these figures which Halley obtained for three
comets which were thought at that time to be quite
different. One of them had appeared in 1531, the
second in 1607, an d the third in 1682; and when
Halley calculated the particulars of their orbits
quite separately and independently he found
A VOYAGE IN SPACE
Year of
Comet
Node
Perihelion
Inclination
Distance
1531
1607
1682
49
50
5i
301
302
302
1 8
17
1 8
0'57
o-59
0-58
The figures are not quite the same, but they are so
nearly the same that if we tried to draw the three
orbits we should find the pencil going over the same
line as nearly as possible : and Halley immediately
said that instead of three separate comets, the same
one must have come three times, in the same path,
which it took about seventy-six years to go round.
The path must, therefore, not be a parabola, which
does not admit of return, but a long ellipse. And the
comet would continue to pay us visits every seventy-
six years, the next visit being due in 1758. He
knew that he would be dead then, but he hoped that
when his words came true, the world would remember
to credit an Englishman with the prophecy. Here
are his actual Latin words in those days scientific
papers, intended to be read by the whole world,
were written in Latin a language all nations knew,
though they might not know the language of their
next neighbours.
Quocirca si secundum predicta nostra redierit
iterum circa annum 1758, hoc primum ab
homine Anglo inventum fuisse non inficiabitur
aequa posteritas.
He was proud of his achievement, not for himself
but for his nation ; and that national pride is appre-
ciated not only by 'us, but by other countries. My
THE STARTING-POINT, OUR EARTH 39
friend, Mr. Hollis, when in Paris one day, saw a pic-
ture of the supposed return of Halley's comet ; he
bought it, and has allowed me to reproduce it here.
An angel is calling Halley from the grave to see the
The Return of Halley to see his Comet
(By a French Artist.)
return of the comet, the fulfilment of the prediction
he ventured to make.
When Halley had once shown that the comet
comes back every seventy-five years, it was easy to
calculate when it had appeared before, and they
found that it was the comet mentioned by Josephus
40 A VOYAGE IN SPACE
as having appeared at the siege of Jerusalem.
Josephus recorded " a fiery sword hanging over
Jerusalem," which foretold its destruction. People
were terrified of comets in those days, and generally
associated them with disaster : and it was only when
Newton and Halley showed that comets obey regular
laws that this fear was dissipated. It is seldom
very difficult to .find some disaster which could be
Halley's Comet on the Bayeux Tapestry.
associated with a comet, and we shall find that on
several occasions Halley's comet has been associated
with some event in our own national history. For
instance, it appeared in 1066, at the time of the
Norman Conquest, and it is represented on the
tapestry that Queen Matilda made the Bayeux
tapestry to celebrate her husband's victory. The
English were frightened, and the Normans inspirited,
by this wonderful sign in the heavens, and these two
different effects may perhaps have decided the battle.
There is one other^ thing about comets that is in-
THE STARTING-POINT,. OUR EARTH 41
teresting because it shows there is another force
besides gravity which affects them. The head of
the comet feels only the pull of the Sun, which is very
strong when it is near the Sun and very slight when
far away; but the tail of the comet feels another
force altogether, and behaves in a totally different
way. Instead of following the comet all the way
like the smoke of an engine, it points directly away
Various Positions of a Comet's Tail.
from the Sun. That. is due to the fact that besides
gravity that pulls the head towards the Sun, there
is another force away from the Sun ; it is as though
the Sun was blowing the tail of the comet away from
the head. Of course there is no actual blowing as
one blows with one's breath; it may be electrical
repulsion; or it may be light pressure the fierce
light of the Sun acting as a blast would act and
blowing the tail out.
We can see this action of the*Sun in blowing away
42 A VOYAGE IN SPACE
the tail by comparing two photographs taken one
after the other. Here is a pair taken by that skilful
photographer, Professor Barnard, of the Yerkes
Observatory in America. He was the first man to
show these beautiful effects. Early in the night he
Changes in a Comet's Tail
(By E. E. Barnard.)
got one picture of the tail; then, by waiting an
hour or two, he got another, showing how the tail
had changed in the interval. Finally he has very
skilfully combined the two pictures, so that the
stars on one fall exactly on the stars in the other.
You see how the head of the comet has moved, and
that the tail is certainly not following it like smoke
following an engine. * To trace the changes in the
THE STARTING-POINT, OUR EARTH 43
tail requires scrutiny, but with care we see that
each feature is being driven away from the head.
We thus realize that though gravity is the principal
force with which we have to deal when we are travel-
ling out into space, there are, nevertheless, other
forces such as that which acts on the lighter particles
of a comet's tail. I hope none of us on the journey
will be blown away like the lighter particles.
The facts that the Earth rotates and that it pulls
us towards itself by gravity are so familiar to us that
we scarcely realize their immense importance, and it
may seem strange to you to have spent so much time
talking about them. But you remember the old
proverb that a cow never knows the value of her tail
till she has lost it. If we leave the Earth to take our
voyage in space we shall lose both these things. We
shall lose the Earth's rotation, and with it the
changes from day to night. If the Earth stood fast
as our forefathers believed, the Sun and the stars re-
volving round it, we should retain something of the
daily change in our journey : but now we know that
it is the heavens which remain still, while the Earth
rotates, we must make up our minds to lose the peace-
ful night with its invitation to sleep. If we sleep at
all, we must sleep by broad daylight, for the Sun will
shine continuously when once we lose the Earth's
shelter. Our watches will tell us the time, but
clocks, which are driven by weights, and regulated
by pendulums, will be useless when we lose the
gravity that pulls the weights down and causes the
pendulum to swing. It was Galileo who first found
out that a pendulum kept time, and he made the
discovery in the cathedral at Pisa when the lamps
44 A VOYAGE IN SPACE
were lit. There was a lamp hung like a long pen-
dulum from the roof : and the verger pulled it toward
him to light it and then let it swing back. Galileo
thought that as the swinging died down it would
become either slower or quicker, and to test this he
timed the swings with his pulse : to his amazement
they were all made in the same time. He naturally
tried other pendulums for himself, and found that
they all had this property of keeping time though
the swing may alter in size it does not alter in time.
One way of realizing this property of the pendulum
Fig. 7.
is to take something else which swings to and fro,
rather like a pendulum, and yet does not keep time,
such as this pair of inclined planes AC and CD (Fig.
7), in which a groove is cut so that a ball B can run
down AC and up the other side CD, the join at C
being rounded so as to avoid having a bump which
would stop the roll. The ball swings back and for-
ward rather like a pendulum, as you see, but it does
not keep time. The short swings at the end are
made much more quickly than the long ones at the
beginning. But when we swing a pendulum and let
its swings die down there is no such quickening, and
hence the usefulness of a pendulum in regulating
clocks, and the usefulness of a clock is to tell people
like lecturers to stop talking after a reasonable time,
before you are thoroughly sick of them and their
gravity and levity both.
THE STARTING-POINT, OUR EARTH 45
Note on the Gravity Experiment
The arrangement described on p. 22 is called a
" Peaucellier Cell," and can be used to draw a
straight line. It was Sir G. H. Darwin who first
pointed out that it could be used to illustrate gravity.
Notice that (Fig. 8)
CS = CM + MS = KM + MS
SK == KM - MS
.-. CS.SK = KM 2 -MS 2
= KA 2 - AS 2
Since KA and AS are a string and a rod of constant
46 A VOYAGE IN SPACE
length, the product CS . SK is therefore constant,
and it is this property which is used in drawing the
straight line ; for if K be made to move on a certain
circle, C will move on a straight line.
As regards the use for gravity : the pull at P is to
the weight W in the same ratio as the tensions in AC
and AK.
Now AC is parallel to KB; and thus ALK is the
triangle of forces for the point A. The ratio of the
tensions is therefore that of KL to KA : or SL to SA,
since KS bisects the vertical angle of the triangle :
or finally SK to SC, since AC and KL are parallels.
Thus P/W = SK . SC/SC 2
= (KA 2 - AS 2 )/SC 2 .
In practice it is convenient to use, instead of the
lower string AKB, one just half the length, attached
to the middle points of the bars SA, SB : or reduced
in any other ratio. The advantage gained is that
S need not be so high above the ground.
LECTURE II
THE LENGTH OF OUR VOYAGE AND THE START
THROUGH THE AIR
WE have talked of the primary great difficulty of
getting away from the Earth at all. But it is not
quite fatal, for balloons and aeroplanes do allow us
to leave the solid Earth in some measure. We have
just read in the newspapers that an aeroplane has
ascended to a height of nearly four miles, and the
question arises whether this is as far away as we
wish to go. If we have any idea of visiting the Moon,
then it is not nearly far enough, for the Moon is not
four, but 240,000 miles away; an aeroplane must
beat the present record by a considerable margin if
it is to take us to the Moon. It is not altogether
easy to realize what such a distance means, even
when we remember that it is about ten times round
the Earth. A little time ago there was a story in
Pearson s Magazine of an American who used a huge
searchlight apparatus to throw an advertisement
on to the Moon, " USE MOON SOAP." The picture
does not suggest any great difficulty in this achieve-
ment until we remember that owing to the Moon
being 240,000 miles away, each of the letters would
be hundreds of miles high; and then we begin to
wonder whether even the most powerful searchlight
47
4 8
A VOYAGE IN SPACE
possible would send light enough to illuminate
brightly such an enormous area. It seems probable
The Wreck of the imaginary Searchlight Apparatus
for advertising on the Moon
(See Notes to Illustrations.)
that it would not. But you can never tell what they
may do next in America.
But now I do not want you to take this statement
THE LENGTH OF OUR VOYAGE 49
about the distance of the Moon on my simple word ;
I want you to know for yourselves how it is measured.
We saw what an advantage it was to Galileo to try
things for himself rather than take them on trust;
and though we cannot exactly measure the Moon's
distance in this room, we can imitate the method
with something else. We cannot perhaps make so
good an imitation as we did of Galileo's experiment,
by dropping balls from the roof, but we will make
as good an imitation as we can. When astronomers
measure the distance of the Moon or of any other
object in the sky, they use precisely the same method
as surveyors use on Earth, and indeed the method
which every one of us uses nearly every moment of
our wakeful lives. With our two eyes we are con-
tinually trying to estimate how far away men and
things are from us. At this moment, for instance,
I am trying to see whether the important people are
in the front rows * and the unimportant people at
the back; and I do it really by squinting. I turn
inwards my two eyes to converge on some one in the
front row, and I can feel how much squint it takes :
then I converge them similarly on some one in the
back row and I feel that it does not take so much.
This process is quite unconscious because we have
all done it so many millions of times that we do it
without thinking, but our muscles and nerves tell
us the result just as well. From what they tell me
I conclude that the people are pretty much in their
right places.
Not only is this measurement of distance with our
1 These lectures being "adapted to a juvenile audience,"
the front rows were " reserved for juveniles."
E
50 A VOYAGE IN SPACE
eyes generally unconscious, but it is also vague :
that is to say, we seldom express the distances in
feet and inches : we usually judge whether one thing
is nearer than another, or farther away, or at the
same distance. Thus, when I try to light a cigar,
I have to use my eyes to tell me when the match is
at the same distance as the end of the cigar : for if
I put the match nearer I shall spoil the cigar by
lighting it in the middle ; and if I put the match
farther away than the end I shall not get a light at
all. This equality of distance is gauged by squinting
with the eyes ; and if you look at your father's eyes
when he is lighting a short cigar, or a short stump of
one, you will see him squint horribly. But if he has
a very long cigar like this, 1 he will squint far less.
For that reason it is not quite so easy to light a big
cigar. If I could get one as long as this room, even
if my arms were long enough to put the match near
the end, I might not hit the end without some
trouble : for the amount of squint would be so small
that I could scarcely feel it, and it would be very
nearly the same even if I put the match a foot away
from the cigar end.
Now astronomers use the same kind of method,
and meet with the same difficulties, when they
measure the distance to the Moon. Instead of two
eyes, they use two telescopes, one at a place we
will call A, the other at a place B ; and they measure
the distance AB, which is called the " base," and is
of great importance (we do not need to measure the
distance between our two eyes, which is the " base ''
for ordinary life, because we are thoroughly familiar
1 A curiosity called a "Giant," purchased at Winchester.
THE LENGTH OF OUR VOYAGE
OBJECT AT
DISTANCE
REQUIRED
with it, which does nearly as well). The base may
be a few feet long, or some miles, or many millions
of miles, according to the distance to be measured :
but we must know its length before we can measure
the distance required. Then the two telescopes at
the ends are pointed at the Moon (or other object
at the distance to be measured), and we see how
much they " squint." Some
people may tell you that the
proper word for this is the
Greek word " parallax " : but
we will be content to use
the good old English word
" squint."
Let us try an actual experi-
ment to see how a distance is
determined by this squint. To
save time we will put all the
squint in one eye ; that is to
say, we will have one telescope
(Fig. 9) not squinting at all,
but pointing straightforwards
from the base AB to the point
C. We must measure the length
of the base AB with this tape measure, and we will
make it just 12 feet, putting the other telescope at B
just 12 feet from A. Instead of telescopes, however,
through which only one person can look at a time,
we will have small searchlights which will make the
direction visible to every one. You can all see when
the searchlight B is pointed so as to throw its light
on to C. To save us time in measuring the different
distances I have had various positions of C marked
BASE
Fig. 9.
5
52 A VOYAGE IN SPACE
off at 6 feet, 12 feet, 18 feet, and so on, from A : and
you will be able to see on which of these the light falls.
But I want one of my young friends to turn his back
on the screen so that he cannot see where the light
falls : all he can now see is the amount of " squint "
marked on this card, and yet you will find that by
reading the " squint " he can tell you where the light
is falling. Thus I point the telescope and from the
" squint " he tells you that the squint is marked i,
meaning that the distance is 12 feet : we change it
and he tells you that the " squint " is ij, that means
i J times the base, or 18 feet ; change it again and he
says it is between | and i ; or between 6 feet and
12 feet, as we see it is. He can tell just as well what
the distance is from the " squint," as we can by see-
ing it on the screen. And this is no conjuring trick :
it is the simplest and most straightforward process :
and it is the same process which the astronomer uses
to find the distance of the Moon.
But before going on to the Moon, let us consider
this card of " squint" a little (Fig. 10). We put i
for a " squint " on an object just as far away as the
base. In our case the base was 12 feet ; but the
same card would serve if the base were a mile long :
when the " squint " was I, the distance would then
be a mile. The mark 2 tells us the " squint " corre-
sponding to twice the base : 3 to 3 times the base,
and so on. We can also mark J for half the base ; or
other fractions. But what I want you to note is that
all the marks for distances greater than i fall in one
half of the arc : they all fall between E and Z : in the
part AE the marks are all for distances less than i.
Moreover, the distance 2 uses up a considerable part
THE LENGTH OF OUR VOYAGE
53
of EZ, leaving only FZ for all distances greater than
2 ; 3 uses up still more, leaving only GZ : and yet
we have a great many still to come, 4, 5, 6, up to hun-
dreds and millions. All these have to be crowded
into an arc which gets continually smaller and
smaller, so that we find it more and more difficult to
make the marks or to distinguish between them.
When you get home, take a card and mark it for
yourself and see how
many " squints " you
can put accurately
upon it. You may
get up to 10, or 20
even; but you will
soon find how difficult
it becomes. Probably
you have a watch
with the 60 minute
spaces marked. Well,
one minute space is
the " squint " mark
for 10 : divide it into
halves and you get
5COPE
CARD OF "SQUINT"
Fig. 10.
the "squint'' mark for 20. One half must have
all the marks between 10 and 20, the other must
have everything above 20 ! When we use our eyes
we have no card, but we attend to the feelings of our
nerves and muscles, which can scarcely distinguish
between these various " squints " at long distances.
That is why it is harder to put the match to the end
of a very long cigar than of a short one.
And when the astronomer tries to determine dis-
tances of objects in the sky, such as the Moon, he
54 A VOYAGE IN SPACE
comes across this same difficulty, since the " squint "
is always small, owing to the object being very
far away. Now there is one method of getting over
this difficulty which I have not yet mentioned : if
we could put our eyes at the end of long horns, not
like those of a snail, which project forward, but horns
projecting sideways a foot or two, then for both eyes
to look at the same object they would have to
" squint " much more than at present (Fig. n). This
is because we have made the base much bigger : in-
stead of being about 2 inches we have made it several
feet. We have not heard of any actual men with
eyes set on horns in this way : Sir John Maundevile
related some wonderful " traveller's tales " in old
days, when there was less chance of finding him out
than there is now : he wrote
" of anthropophagi and men whose heads
Do grow beneath their shoulders/'
but I don't think he ever mentioned any race whose
eyes grew on horns. Yet by means of an apparatus
THE LENGTH OF OUR VOYAGE
55
called a " range-finder " which soldiers and sailors
use to find the distance of the enemy, they manage
to use their eyes just as though they were on long
horns.
In Fig. 12 A and B represent the real eyes of
RANGE FWDER
A B
\
OBJECT
Fig. 12.
the observer, close together; C and D his artificial
eyes wide apart. They are two lenses C and D, with
mirrors behind them G and H : and these artificial
eyes can be turned on the object by screws, just as
our eyes can be turned by our muscles. Rays of
light EC from the distant object falling on C are re-
flected by the little mirror behind it so as to travel
56 A VOYAGE IN SPACE
along G J : and another little mirror at J sends them
into the eye A. Rays which start along FD are
similarly reflected along HKB into the eye B. Hence
without moving his own eyes at all, the observer
measures the " squint " of the wide artificial eyes
C and D by turning the screw (or both screws, if
there are two). He then knows the distance of
the enemy and tells his comrades for what " range "
to sight their rifles. If they did not find out the
" range " in this way, the bullets would either go
over the heads of the enemy, or hit the ground in
front of them. 1
Now the astronomer is fully alive to this method of
getting over the great difficulty. What it comes to
is that we make the base as long as possible. Nature
has put our eyes so close together that the base is
very short : and though it suffices for estimating the
distance of a cigar tip, or of people in the same room,
it is too short for telling us the distance of the enemy
half-a-mile away. So the range-finder is made with
a much longer base and the difficulty is largely re-
duced. But for greater distances still, such as that
of the Moon, the range-finder becomes as useless as
our eyes : the base is not nearly large enough.
Instead of having our artificial eyes a few feet apart,
we must put them miles apart ; and we may as well
put them as far apart as we can, let us say, on opposite
sides of the Earth (Fig. 13) ; and then we find that
the " squint " required to set both eyes (that is to
1 Since these lectures were delivered, the use of the peri-
scope has become familiar to most of us. The eyes on
horns could be represented by using a periscope for each
eye.
THE LENGTH OF OUR VOYAGE 57
say, two telescopes at the end of the base) on the
Moon is quite considerable, and by measuring it we
find that the distance of the Moon is 240,000 miles,
as I told you. And it was to find out this fact that
our Government established the Royal observatories
at Greenwich and at the Cape of Good Hope, one in
the northern hemisphere and one in the southern.
They are not quite as far apart as possible, because
one might have been at the North Pole and one at
the South Pole : but you will probably agree with
MOON
Fig. 13-
me that the astronomers would not have been quite
so comfortable in that case, and we have to sacrifice
something for comfort.
This old difficulty, however, is not yet done with :
it crops up again in a most tiresome way when we
want to find the distance of the Sun instead of that
of the Moon, because the Sun is still further away
nearly a hundred million miles : and even when our
base is the biggest we can get on our Earth, the
" squint " required for the Sun is very small, and
the difficulty of measuring it very great. The only
thing then left for the astronomer is to take advan-
tage of special occasions when the difficulty happens
58 A VOYAGE IN SPACE
to be less than usual. Such an occasion was the
Transit of Venus. Perhaps my present audience
has not heard so much about the Transit of Venus as
their parents and grandparents did, because the last
Transits occurred in 1874 and 1882, long before you
were born : and the next will happen in 2004 and
2012, when all of you will be a good deal older. But
the echo of the great sensations may not yet have
died down, and so you may have heard of a Transit
of Venus and wondered what it was.
B VENUS
EARTH
TRANSIT OF VENUS FROM OPPOSITE SIDES OF OUR EARTH.
Fig. 14.
What concerns us is that it is simply an occasion
when the difficulties of measuring the Sun's distance
are less than usual. The base remains the same,
because we cannot get away from the Earth : the
best we can do is to put our eyes or telescopes on
opposite sides of it, and indeed in all parts of it.
Astronomers were scattered for the Transits of Venus
to such places as the Sandwich Islands, New Zealand,
and Australia, Mexico, Kerguelen Island (a very
desolate spot), Egypt, and so on : and they watched
Venus cross (or " transit ") the Sun, and noticed the
exact moment when the transit began and ended.
You will scarcely want me to explain fully how this
told them the Sun's distance ; but I think you can
see in a general way how they worked it out if you
THE LENGTH OF OUR VOYAGE
59
look at Fig. 14. You can see that for a telescope at A
en one side of the Earth, Venus would cross the Sun's
disc along the path CD ; while a telescope B on the
other side would see Venus travel in a different path
EF ; and you can see also that if the Sun were brought
nearer, the two paths would not be so different : if
the Sun were further
away the paths would
be more different. In
other words, the
amount of difference
tells us just how far
off the Sun is, if we
can measure the differ-
ence accurately.
Unfortunately
astronomers found,
when the great events
had taken place, that
they could not
measure the differ-
ence as accurately as
they had hoped.
They hoped to deter- F ig. 15. An Ideal Transit of Venus.
mine the exact second
when the transit began or ended. It would have been
best for them if there could have been a moment such
as that labelled II in Fig. 15, when the black disc of
Venus just touched the bright edge of the Sun.
Before that moment, as in I, it would have been only
partly on the Sun, and after it, as in III, the black
disc is completely within the Sun. It would have
been nice for the astronomers if between I and III
60 A VOYAGE IN SPACE
there had been just a single instant like II, which
they could note accurately. Then they would have
found out the Sun's distance very exactly. Un-
fortunately it was not so, and they knew from pre-
vious Transits that it would not be so. Instead of
Fig. 16. The "Black Drop."
an appearance like II, they knew they would see
something like Fig. 16, the black disc being not at
all round, but pear-shaped. This does not mean
that Venus is really of that shape, for when it is fully
on the Sun we can see that it is properly round, as in
Fig. 17 ; and we also see that it has an atmosphere
like ours, through which the Sun's rays are bent.
THE LENGTH OF OUR VOYAGE
61
The pear-shape is, however, not caused by this atmo-
sphere being illuminated (though this illumination
causes trouble of another kind), but by the peculiar
behaviour of light when a bright light is screened off.
Near the edge of the screen we get peculiar appear-
ances which go by the name of diffraction. You can
see them if you look (with one eye only) at the sky
through two finger-tips nearly closed together : when
they are very close,
one seems to go out
to meet the other,
though you can feel
that they are not
yet touching. As-
tronomers knew that
there would be
these " diffraction "
appearances, but
they hoped to make
good allowance for
them by practising
beforehand with
Fig. 17. Venus and its Atmosphere.
models of the expected Transit which were ingeni-
ously constructed. But they were disappointed.
They knew they had been disappointed long before
they collected and compared the observations made
at different stations; because two people side by
side using similar telescopes gave quite different
times instead of the same time. Hence they knew
that one of them, and perhaps both, must be wrong :
and you cannot get a right result from wrong
observations.
Transits of Venus only rarely occur. We have
62 A VOYAGE IN SPACE
said that the next pair (they always occur in pairs
like twins) will be in 2004 and 2012, and the last pair
were in 1874 and 1882. The pair before that were
in 1761 and 1769, and one of the people who observed
the Transit of 1769 was the famous Captain Cook,
who was killed by the natives of the island of Hawaii
ten years afterwards. A more famous Transit still
was that of 1639, because, it was the first that any one
ever observed. It occurred to a young clergyman
named Horrox that there might be such a thing,
though no one had thought of it before ; and he cal-
culated the day on which it would fall, which turned
out to be Sunday. He got out his telescope at sun-
rise, and set it to look at the sun ; for he did not
quite know at what time the Transit would come.
The hours passed on, and it began to get near church
time. Then came rather a struggle between duty
and inclination. What was he to do ? Transits o
Venus do not come every Sunday. Was he to go to
church and take the service, or to watch Venus?
Well, really he had no doubt what he should do. He
lived long before Nelson, but he knew what " England
expects of every man." So he went to his duty first,
and took the service. Should you think that he
thought of Venus sometimes ? At any rate, when it
was over, he flung off his surplice, and hurried back
to his study, and, to his great delight, he saw Venus
just coming on to the Sun. So Horrox and his
friend, Crabtree, a weaver, whom he had told of the
great event, were the first to see a Transit of Venus.
Horrox was quite a young fellow, and I am sorry
to say that he died at the age of only 24. He had
done so much before 24 that some people think that
THE LENGTH OF OUR VOYAGE 63
if he had lived he might, perhaps, have been an even
greater man than Newton.
We see that our Voyage in Space will take us over
big distances ; 240,000 miles to the Moon, and 93
million miles to the Sun. It hardly seems worth
while in starting to bother about the first few miles.
But perhaps you have noticed, when you have taken
Horrox observing a Transit of Venus for the first time.
See Notes to Illustrations.)
a railway journey, what a long time it takes to get
out of the station ; or if you take a voyage on a ship,
what a business it is getting out of dock. Afterwards
when the ship has started the voyage becomes rather
uneventful ; but at any rate the first bit is not un-
eventful. So I think for a few moments we might
think about the air through which we must first
ascend, and which corresponds to the dock from
which we start for a sea voyage.
There is one property of the air that seems very
64 A VOYAGE IN SPACE
remarkable to those who have never thought about
it before. Here it is all around us. It does not
seem to be exerting any pressure and yet it is press-
ing on us all with tremendous force 15 pounds to
every square inch. We are all supporting that great
pressure, both inside and outside. If we had not the
pressure outside, I suppose we should burst ; and if
we had not the pressure inside we should go flat.
We can easily prove there is that kind of pressure by
one of those old experiments which I think every
generation ought to see, though it has been per-
formed many times, and was first done many years
ago : viz. the experiment of the " Magdeburg
Hemispheres." We take two cups or hemispheres
that are easily pulled apart when there is air both
inside and outside. But if we fit them together,
and if Mr. Heath kindly connects up the exhaust and
takes all the air from inside, they do not, indeed, go
flat, because they are made of good solid brass, but
we shall find it very difficult to pull them apart.
Now the air is sufficiently exhausted, and I want
two very strong men from the front row. Mr. Heath
and I will stand behind them in case by some acci-
dent the air is not all out and they come apart too
easily. But you see it is all out, and though these
strong giants are pulling as hard as they can, they
cannot separate them. Even when Mr. Heath and
I help them we cannot do it. We will now let
the air in; and you will see it is as easy to pull
them apart as it was at first.
We need not always use an air-pump to get rid
of air. Sometimes one can get the air out from
between one's hands by squeezing them tightly
THE LENGTH OF OUR VOYAGE 65
together, and there are people who can make a
fascinating noise in that way. I have here two
beautiful planes, that Mr. Whitworth told us how
to make; they fit one another so closely that we
can exclude the air without a pump. We must
not merely put them together in the ordinary way ;
we must slide one on to the other in order to
squeeze out the air : and then you find them stick
so that they are as hard to pull apart as the
Magdeburg hemispheres.
TRYING TO SEPARATE THE TWO
HEMISPHERES*
These two experiments show the enormous pres-
sure air has. But that is only near the Earth;
as we go up it gets less and less; or, perhaps, a
better way to look at it is that as we come down
in the air the pressure gets greater, because
every layer has to support all the layers above it.
Fig. 1 8 is a picture of a totem-post in the Oxford
Museum, kindly made for me by a member of the
audience and her mother. It used to be outside
the tent of an Indian chief. There is a raven at
the bottom, and on that a bear holding a hunter;
then another bird ; above that another bear hugging
66
A VOYAGE IN SPACE
Fig. 18.
Fig. 19.
a hunter. I believe the
idea is that when a tribe
conquers another tribe they
take their totem and put
their own on the top : and
then again, if that tribe is
conquered, the totem of the
conquering tribe is again
put on top. Now obviously
these animals are strong and
solid, so they can bear all
that weight upon them with-
out getting squashed. But
the air is not like that. The
air beneath gets squashed
down by the air on top. So
it would be like the second
picture (Fig. 19), when the
raven gets squashed flat by
all the weight it supports :
and even the bear above
it gets very much flattened.
That is how our air gets
treated near the Earth.
But high up it gets thin like
the chief on the top of the
second picture, because
there is nothing to squash
it. The air is not of course
divided into separate layers,
as the totem-post is divided
into separate animals, but
it will do no harm to
THE LENGTH OF OUR VOYAGE
67
Record
''Ballon Sonofe"
(8 MILES
treat it as though it were, in thinking out what
happens.
Let us make believe that there are different
layers, as in Fig. 20, copied from a beautiful picture
at the Meteorological Office at South Kensington
(which you ought to
visit some time, be-
cause you will find all
sorts of interesting
things there) . The
first layer takes us
about six miles up,
and may be called the
layer of mountains.
The highest mountain
in the world (Mount
Everest) reaches just
about to the top, and
you see how small the
Eiffel Tower looks
beside it. The highest
aeroplane record is
about four miles, so
that the first layer is
also that of aeroplanes.
A kite has been up a
little higher than that. And a man in a balloon
has actually been just into the second layer,
but only just. So that our human experience of
the air is practically confined to the first layer,
and we knew nothing about the layers above it
till quite recently. All our knowledge of them is
due to the use of balloons, rather like toy balloons,
Q Average "Ballon Sonde"
Record Manned Balloon.
IMAGINARY LAYERS OF ATMOSPHERE
Fig. 20.
68 A VOYAGE IN SPACE
though larger. There is one hung up near the roof ;
perhaps we can get it down at the end of the lecture.
These balloons do not carry men, they only carry
apparatus; but this apparatus is so skilfully made
that it brings down for us information about these
upper layers. We learn that the pressure gets less
and less, as we expected; but we also learn some-
thing that we did not at all expect. We thought
that it would get colder and colder as we went higher,
but these " sounding-balloons " (ballons sondcs) tell
us that soon after we leave the first layer it ceases to
get any colder. The figures for the temperature in
Fig. 20 show that it does not get very much colder.
They are measured from what we call absolute zero.
You know well the ordinary thermometer which
your mother puts into your mouth when she thinks
you " have a temperature." If we used that we
should have to alter these figures a good deal : and
if your mother used the one the Meteorological Office
uses, I fear she would get a great fright, for she
always hopes to find your temperature below 100,
doesn't she ? Yet with this thermometer she would
find it over 300, even if there was nothing the
matter with you at all. But, of course, if she were
warned beforehand that that was the proper tem-
perature for you to have with this kind of thermo-
meter, she would not be alarmed : there are such
various kinds of thermometers, that before using
any of them it is well to know beforehand what to
expect.
Well, the great fact we have recently learnt about
these upper layers of the air is that the temperature
ceases to fall in a way that has been a great surprise
THE LENGTH OF OUR VOYAGE 69
to us. The sounding balloons have gone up higher
and higher until they burst, and then the apparatus
carried by them falls down to the ground. You
might think it would get hopelessly smashed by such
a fall (from a height of 20 miles, say), but the burst
balloon case acts as a kind of parachute, or drag, so
that the blow on the ground is not really so very hard,
and also the apparatus is attached to an ingenious
light framework called a " spider," which eases
the blow still further : so that usually it is not
damaged, and not only does it tell us what happened
to it in the upper layers, where man has never
reached, but it can be used again and again to get
more news. The news has been of intense interest
to scientific men, but would scarcely interest us
enough to justify dwelling longer on it. Rather, let
us return to the bottom layer and recall a few things
which have happened in it.
Let us begin with the mountains. Many people
have ascended mountains and got above the clouds.
In order to get above the clouds they have put a
magnificent observatory on Mt. Wilson in California,
6000 feet above the dust, which can also be seen
lying below them. Astronomers have begun to use
mountains not only to get above dust and clouds, but
because they also get above a considerable amount
of air. Professor Campbell went up Mount Whitney,
sometimes called the " top of the United States,"
to make observations upon Mars, especially to see
whether it has an atmosphere containing water
vapour. He made his arduous expedition in order
that he might not mistake for water vapour in Mars
what really belonged to our own Earth. Having
70 A VOYAGE IN SPACE
much less of our own air between him and Mars, he
was less likely to make that mistake. He concluded
that there could not be very much, perhaps not more
than on the Moon, which we know to be very dry
indeed. He lived in a desolate little hut on the top
of Mount Whitney for a considerable time to make
these observations, but usually he lives on the top of
a more comfortable mountain (4000 feet high) at the
Lick Observatory. I got a pretty Christmas card
from him, showing the Lick Observatory in winter
with the snow on it, which I am sure he would like
me to share with you. The snow is, of course, only
there in winter, and rarely even then ; generally the
Lick Observatory has a beautiful summer climate.
But there are mountains, as you all know, on which
THE LENGTH OF OUR VOYAGE 71
there is perpetual snow, and yet climbers of moun-
tains have gone up even higher than aeroplanes have
gone. But I am sure you are much more interested
in aeroplanes, because to go in one seems more like
leaving the Earth behind ; whilst we are on a moun-
tain we are still touching the Earth. Aeroplanes are
becoming so common that we have nearly forgotten
how hard it was to invent them, and who did the
early pioneer work which led to the invention. Who
first thought of the name " aeroplane " ? When you
want to find out a thing like that, one good way is to
look in the dictionary : and I hope you all look in
the dictionary when you want to find out things,
instead of bothering other people with questions
which they cannot answer. But if one wants to be
sure of getting the right answer, an even better way
is to bother the man who makes the dictionary :
and we are fortunate enough to have in Oxford Sir
James Murray, 1 who is making a very big dictionary,
and who was able to tell me a curious thing about
aeroplanes. First of all, the word means a plane
used for experiments on air. That is the way he has
defined it in his dictionary, of which the letter A was
published in 1888 (though they have not yet got
to Z !). He says the word was then used in England
to mean " a plane placed in the air for aerostatistical
experiments." But he also says that " aeroplane "
was used in France in quite a different way ; plane
meaning there not a plane surface at all, but a thing
which soars. Those who know French will know
that they use the word " planer " to mean soaring
1 Sir James Murray died while this book was in the
press.
72 A VOYAGE IN SPACE
like birds. I am sure you have all heard of a
" vol plane," which means a soaring flight. And
these two different ways of using the word may be
illustrated in this way. A piece of paper like this is
a plane : and we might put it at the end of a mechan-
ical arm, and find the amount of pressure on it when
it is moved about. Several such arms or sails put
together would make a toy windmill : and, although
that is scarcely a piece of scientific apparatus as it is
usually made, a very little alteration would make it
into one. In the toy, all the sails are set at the same
angle, and they blow round merrily. But suppose
we set some one way and some another, so that they
want to go round in opposite directions, we could
balance them so that the machine would turn neither
way, however strong the wind was, and now we have
a piece of scientific apparatus. Lord Rayleigh made
many experiments with a simple machine of this
kind, like a toy windmill, of which he could put the
vanes at different angles. I think there were six
vanes, and he set two of them in one direction and
four in the other, but at a different angle, which he
chose, so that the two would balance the four, and
then the machine would not go round at all. By
varying these angles he learnt a great deal : and the
plane sails of his apparatus were aeroplanes. But I
can take the same piece of paper and make it into
another kind of aeroplane a thing which soars ; you
probably know how to make this kind of dart, which
soars about. That is a different kind of aeroplane,
like those in which people fly. Here is a beautiful
little model of an aeroplane that has been kindly lent
from the Grahame- White Company. It was the
THE LENGTH OF OUR VOYAGE 73
prize at one of the Hendon Race Meetings, I believe.
And the same company have kindly lent a few
slides, which I am sure you will like to see. You
are regularly spoilt children, because every one seems
so ready to lend things to be shown to you.
When we went over to America in 1910 to have
a meeting of astronomers who study the Sun the
modern Sun worshippers I was lucky enough to
see a great aeroplane race meeting : and it was speci-
ally interesting to see Mr. Grahame White dropping
chalk balls to try and hit the funnels of an imaginary
ship marked out on the ground. It was quite difficult
to hit the funnel from 100 feet up, when travelling at
full speed, and of course an aeroplane would have to
fly much higher up in time of war to avoid being shot
at. So at the end of the meeting Mr. Grahame White
kindly volunteered to try from a much greater height
1700 feet, I think. A hard chalk ball falling from
this height might by accident hurt some one badly,
so in this instance he dropped eggs, and presently a
man came out with a megaphone to say what the
result was ; and he said something like this
MISTER GRAHAME WHITE HAS DROPPED
ELEVEN EGGS FROM SEVENTEEN HUNDRED
FEET BUT THE UMPIRES HAVE NOT BEEN
ABLE TO SAY WHERE ANY OF THEM FELL.
You can well believe that the crowd laughed at
this odd result, and it was suggested that the eggs
hatched out on the way down and flew away !
Here you see that wonderful modern achievement
called looping the loop. But, with all our admiration
for aeroplanes, we must not forget that in the old days
74
A VOYAGE IN SPACE
balloonist s did some extraordinarily plucky things.
There is a fascinating book about ballooning called
Travels in the Air, by James Glaisher, 1 but I fear it
is now out of print, so that your only chance is to find
it in some library. It has beautiful pictures of the
SEVEN
MILES
HIGH
balloon travelling above the clouds, with wonderful
sunrise effects. Sometimes the balloonists saw their
own shadow thrown on the clouds : and on one occa-
sion, in 1866, they saw a great shower of meteors.
But the most exciting occasion was when they got
1 Bentley, London, 1871. New edition, 1880. Price 255.
THE LENGTH OF OUR VOYAGE 75
nearly seven miles high and nearly died from the cold.
Mr. Glaisher himself fainted : his companion, Mr.
Coxwell, wished to open the valve so that the balloon
might descend, but found that his hands were frost-
bitten : however, he managed to climb up with his
elbows and knees and open it with his teeth, and then
they dropped quickly to a warmer region and revived.
Mr. Glaisher's object in making these ascents was
to find out the temperature and pressure of the upper
air, for which purpose he carried up a whole trayful
of instruments : and he did find out a very great
deal practically all we knew until recent times.
But since seven miles was the highest he ever went,
we knew nothing at all about the air above that
point. We could only guess, and we guessed quite
wrong.
And now we come to the balloons that have really
taught us about the upper air, about which the peo-
ple who study the weather have been trying to learn
for years, and have at last succeeded. First of all
they tried by sending up kites of a peculiar shape :
you see a fine specimen up there. They are not like
ordinary kites, but are in the shape of a box almost ;
but they have been copied in the form of toys, and
perhaps this one does not seem so strange to you as
it would have to us in our childhood. These kites
can be got up to great heights by attaching one to
another in a series. The top kite has been up above
the aeroplane record, but not so high as the man-
balloon record. They do not, of course, take up men
with them, but they carry a recording apparatus,
including a barometer and thermometer. The ther-
mometer is that curious spiral spring at the back-
;6 A VOYAGE IN SPACE
All these things write their own story on this piece
of paper in front. I wonder if you can guess what
is the use of that ping-pong ball tied to a thread?
It tells the force of the wind, because the wind blows
the ball away from the kite, so that it pulls the string,
and the more it pulls the more it writes on this dia-
gram. Then presently the kite is hauled down, and
this diagram is read ; and from this we find out what
has been going on in the upper air. When these
kites are up miles high, the pull on the wire (they use
wire and not string) is tremendous, and special
machines are necessary for winding the wire.
But lately it has been found better to use balloons
instead of kites ; balloons filled with hydrogen, which
is so light that it takes them up for miles and miles
before they burst. They always burst at last, be-
cause the pressure of the air outside keeps getting
less and less as we go up (you remember how we
illustrated J:hat with the totem-post pictures) , but
the hydrogen inside does not get less, and so swells
the balloon out more and more. I told you that
if there were no air outside us, we might burst ; and
that is actually what happens to the balloon when
it gets so high that the air outside it scarcely presses
it at all. And then down it comes like a parachute,
bringing the precious records, like those attached
to the kite. But of course the kite is pulled down
gently and smoothly by means of its wire, whereas
the balloon-case drops from a great height, and
it is important to prevent a jar on reaching the
ground which might damage the instruments which
make the records. They are costly, and must be
used again and again if possible. Accordingly the
THE LENGTH OF OUR VOYAGE 77
instruments are made of very light materials; and,
as already mentioned, they are attached to a light
framework called a " spider," consisting of three
bamboo rods tied together, at their middle points,
and set each one at right angles to the other two.
To keep them in this position their ends are joined
up by thin strings, the whole forming a framework
which is very strong for its weight, and which may
fall on any part of itself without much damage. The
precious apparatus is securely fixed to the very centre
of this framework, so that when the balloon drops,
it falls always on its feet so to speak. You know
how a cat prevents its body being hurt when it falls
by twisting in a remarkable way so as to fall on its
springy feet ? Well, we cannot provide the appara-
tus with the agility of a cat, so as to make its feet
twist under it; but the "spider" has feet in all
directions, which acts just as well. Make a " spider "
for yourselves when you get home, and see how
strong and light it is. Here is a miniature " spider "
which Mr. Bellamy has kindly made out of three bits
of straw and some thread ; and it is so light that even
a toy balloon will carry it up if properly filled with
hydrogen. We will make the experiment. There !
you see ; up it goes ! It cannot rise high enough to
burst in the proper way because the roof stops it;
but fortunately Galileo is still up there in the roof,
or has gone back again, and he can arrange the burst-
ing for us ; and down come the case and spider and
all ! One very important thing I had nearly for-
gotten. The spider may fall anywhere perhaps
far away from any town or people. Who is to find
it, and send it home? It stands well above the
78 A VOYAGE IN SPACE
ground and can be seen from some distance, so that
it is not long before some wanderer finds it ; and he
also finds a label attached to it, saying that if he will
send the apparatus and records to the Meteorological
Office he will get five shillings reward. I am sure
five shillings would be very useful to many of you,
so I advise you to keep a sharp look-out whenever
you are in the country to see whether you can find
a " spider," and get the five shillings reward.
There is one thing about the upper atmosphere
which these sounding-balloons do not tell us, though
if they could bring down some of it they might ; and
perhaps it may be arranged at some future time that
they shall bring samples down with them, and then
we should be able to verify what we believe to be
true, viz., that certain gases, which are present in
our lowest layer in very minute quantities, become
much more common up there. When I was a boy,
we were taught that the air consisted of oxygen,
nitrogen, carbon dioxide (or carbonic acid gas, if you
like that name better), and water vapour; nothing
else but these four. But in the year 1894 Lord Ray-
leigh announced to the British Association, which
was then meeting at Oxford, that he had found
something else in the air a gas so like nitrogen that
he had had the utmost difficulty in separating the
two, but clearly different from nitrogen when suffi-
cient care was taken to divide them. It was a most
exciting announcement, because we all thought that
we knew practically all there was to be known about
the nature of air, and it was a great shock to find that
we were all wrong. The new gas was called argon ;
and soon other new gases were found, especially by
THE LENGTH OF OUR VOYAGE 79
Sir William Ramsay helium, and neon, and xenon,
and a heap of others; so that unfortunate school-
boys, instead of having to learn only four things as
making up air, as we did, have now to learn a large
number of names. Life has a terrible way of getting
more and more difficult !
Well, now, one of the reasons why these gases were
not found before is that down here in the lowest layers
they are very scarce, like rare butterflies, such as the
Camberwell Beauty, which is very hard indeed to find
in England. But if you go abroad, you may find Cam-
berwell Beauties by no means uncommon. So also,
if we could go " abroad " into the upper air, we might
find these rare gases much more easily. And there
is one thing about them that will interest you, I hope,
though their names may be tiresome to learn ; they
give very pretty colours when we electrify them.
We have a series of tubes here filled with specimens
of various gases, and when we pass a current through
them you will see their different beautiful colours.
There is one other way in which we learn about
the upper air, and that is through those meteors,
which you saw on one of the balloon slides. A
meteor, or, as it is often called, a shooting star, is
a bright light like a star that darts across the sky;
but it has nothing to do with stars ; it is only a piece
of stone or metal. Here are some meteors and frag-
ments of meteors; many of the shooting stars you
see would be much less than any of these; tiny
specks so small that they get burnt up entirely.
What makes them shine and burn is the friction of
the air as they rush into it. You have heard of pro-
ducing fire by rubbing two pieces of wood together ?
8o
A VOYAGE IN SPACE
The faster you rub the sooner you get the fire. It is
not at all easy to make the fire because it is not easy
to rub fast enough ; but a meteor rushes into the air
at a great speed, fifty miles a second at least. It
seems to us a great speed when our motor-car goes
at 50 miles an hour ; think what it must be to go at
50 miles a second, nearly 4000 times as fast as a
motor-car ! I am afraid the police would not allow us
to try it ; but if we could increase the speed more and
more, we should
find that the air
which at first
gently blows us
cool, was becoming
a fierce wind; and
presently, instead
of being at all cool,
it would seem hot;
and long before we
reached the speed
of a meteor t
would be unbear-
able.
It may seem surprising that anything so soft and
thin as air can produce the same effect by friction
as we get from hard wood, but we can actually prove
it by experiment. Sir James Dewar devised this in-
genious apparatus to do so (Fig. 21). A wooden arm
AB can be spun about the end A. The end B thus
rushes at a considerable speed through the air not
the speed of a meteor, or anything like it, but still a
sufficient speed to be heated by friction. The heating
will not be very great, but I think we can detect
Fig. 21.
THE LENGTH OF OUR VOYAGE 81
it and even show it to you on the screen; and the
method is this : we join two wires of different metals
at both ends, and put one junction at the end A of
the spinning bar and the other at the end B. Now
it is a well-known fact that if one of these junctions
be heated more than the other, an electric current
will flow in the wires, which we can detect by using
this galvanometer. You see that spot of light on
the screen? Well, when an electric current flows
it will move to right or left ; and we want to know
which way it moves when the end B is heated. Per-
haps one of the ladies in the front row will kindly
put her cheek against the end B for a moment.
There goes the spot to the right, showing quite a
warm cheek ! (Perhaps that is better than a cool
cheek ? ) Accordingly we know that if the end B gets
warm, the spot will move to the right. Now we will
buzz the arm round as quickly as we can, and there
goes the spot to the right again, showing that the
junction B is heated by its rushing through the air.
The end A, you see, is so near the centre that it
remains practically stationary.
If we could whirl the arm round ever so much
more quickly we might heat the wire so much that it
would shine like a meteor, and perhaps be burnt up ;
but you will easily understand why I cannot do this.
Some meteors, however, are too big to get burnt
up. They come right through the air and bury
themselves in the ground, as you remember the
leaden ball buried itself when Galileo dropped it
from the roof. I hope you will never have the ill-
luck to be hit by a meteor, for it would hurt much
worse than any bullet. Once a monk was sleeping
G
82
A VOYAGE IN SPACE
in a tent, and a meteor went right through tent and
monk and bed below him, and buried itself deep in
the ground. But that is the only case I ever heard
of where any one has been struck in this way, because
meteors are very rare. Mr. Gregory has been kind
enough to lend us his wonderful collection for you
to look at, and I hope you will look carefully at them
after the lecture.
Before a meteor enters our air it is travelling
The Gross Divina Meteorite.
through the terribly cold regions of space, and is
chilled to its marrow. When it rushes into our air
it becomes heated as we have said ; but it travels so
quickly that it comes right through the air in a few
seconds, and though the outside gets very hot, there
is not time for the heat to get inside. The inside
remains just as cold as it was before, and only a thin
coating near the surface is heated. When the
meteor comes to rest in the ground, the heat of the
crust is soon overcome by the bitter cold of the in-
side ; and in a very few minutes the crust which was
so hot as to be shining brightly is so cold that hoar
THE LENGTH OF OUR VOYAGE 83
frost settles on it ; and in this state meteors have often
been found by those who have seen them flash down,
have marked the spot where they fell, and hastened
towards them as quickly as possible. Mr. Gregory
has kindly cut some of his meteors so that you can
see where this thin outer crust stops ; you see how
very little had time to get heated ; all the inside
remained cool.
Here is a specimen which should be of particular
interest to the ladies present, because it has diamonds
in it : not very large ones, but still unmistakable
diamonds. Mr. Parsons (whom you all know, I hope,
as the inventor of the turbine) tells me that he thinks
these diamonds are made when two meteors strike
one another. I have told you at what fearful speeds
they travel, 50 or 100 miles a second, say ; and if two
of them hit, the pressure developed must be enor-
mous, far greater than anything we can imitate on
this Earth. And Mr. Parsons further tells me that
he believes all diamonds are made in this way. He
has himself tried many times to make diamonds
without success, and he thinks it is because he cannot
get pressures nearly big enough ; and so he inclines
to the view that all the diamonds we find have been
brought to the Earth by meteors which fell on it
in ages past. This makes us look with increased
respect at Mr. Gregory's collection, doesn't it ?
But diamonds are not the only substance meteors
have brought us ; shut up inside them various gases
have been found. Where were these gases collected ?
Probably in the upper layers of our own atmosphere.
You remember how we regretted that the sounding-
balloons were not able to bring us down samples of
8 4
A VOYAGE IN SPACE
the air up aloft ? Well, fortunately, meteors are able
to do this very thing to a limited extent. In their
fierce rush they catch the gases and shut them up
B
r*
B
Centre 1
The Appley Bridge Aerolite, Oct. 13, 1914.
(See Notes to Illustrations.)
Scale
inside themselves so that chemists have been able to
get them out afterwards and find their nature. They
have often found hydrogen in this way inside me-
teors, and so we learn, what can be shown to be
probable in other ways, that there is a good deal of
THE LENGTH OF OUR VOYAGE 85
hydrogen in the uppermost layers. Unfortunately
these gases often escape before the chemists can get
at them; they burst through the outer covering.
In some of Mr. Gregory's specimens you see the holes
through which the gases have burst out before we
could examine them.
In one way and another, therefore, we have learnt
a good deal about the upper air lately, so that we
know what we should have to pass through in leav-
ing port when we start on our voyage. The air will
get thinner and colder and change ultimately into
hydrogen, and then what after that? We shall
get out into the cold and silence of space the great
Silence. Perhaps it had not occurred to you that
we should lose all sensation of sound? Our ances-
tors thought that the heavenly bodies were making
beautiful music the " music of the spheres "; but
there is nothing to carry music in space. Most
sounds we hear are carried by air; though they
can be carried by other things as well. Here is a
wooden rod which goes down through the floor into
the basement where there is a musical-box. The
sound of the music is coming up this rod into this
room, though only those close to the rod can hear
it, even faintly. But if two of my audience will
kindly lift this wooden tray on to the top of the
rod, the tray will act as a resonator and the whole
room will hear the musical-box through the instru-
mentality of the rod. (Experiment as indicated.)
So that materials other than air can carry sound.
But in outer space there is no material at all, not
even air; so that no sounds can travel. We can
realize this by another experiment with this bell,
86 A VOYAGE IN SPACE
which is ringing plainly enough even when I cover it
with a glass jar. But now, if we exhaust the air
from the jar the sound will die away until you can-
not hear it at all, though you can see that the ham-
mer is still striking the bell as vigorously as ever.
(Experiment.) And if we gradually let the air in
again the sound comes back. All the apparatus
for making the sound was the same throughout ;
only the air which carries it was removed, and the
other materials round (such as the table) did not
carry it sufficiently for you to hear.
Hence we must be prepared for a great silence on
our voyage. When Jules Verne's travellers were shot
out of the enormous cannon to the Moon, they were
puzzled because they did not hear even the sound of
the explosion which started them ; and they reasoned
it out that they travelled more quickly than sound,
so that the sound was unable to catch them up. Of
course, they talked to one another inside the pro-
jectile because they took air with them; but they
were cut off from all outside sounds. Other pro-
jectiles might have blown foghorns louder than any
steamer that ever passed us on the sea, but the
warning would not be of the slightest use because
it could not be heard.
It seems almost better for us not to venture into
this great silence, doesn't it ? Especially if I am to
continue these lectures. You will not be able to
hear me if we have no air, for several reasons. Con-
sequently I shall next time propose a method of
making our voyage which will avoid this difficulty,
and allow us to stay comfortably in this room all the
time.
LECTURE III
JOURNEYING BY TELESCOPE
WE may now consider that we have properly said
good-bye to our old friend the Earth, who attracts us
in more ways than one ; we have also looked at the
station or port, and it is really time to be starting.
We must seriously choose our carriage and get into
it. What kind of a carriage is it to be ?
We saw last time that an airship or aeroplane can
only take us a very little way six or seven miles at
the most and we want to go much farther than that.
Jules Verne chose to shoot a great projectile holding
three people, which went as far as the Moon ; but then
he had to start it with great velocity, and it was only
his enthusiastic imagination which enabled the travel-
lers to survive the shock. Mr. H. G. Wells had the
happy idea of screening out gravity by a newly
discovered substance called Cavorite (in honour of
its inventor). For that purpose blinds were fitted
to the windows of the " sphere " that he mentions
in his fascinating book; and so this " sphere " was
enabled to float, or move in some indescribable way
(because we are so accustomed to gravity we do
not quite know what to call movement without it)
as far as the Moon. It might have gone even much
further, without difficulty or inconvenience, except
87
88 , A VOYAGE IN SPACE
that it would almost certainly have taken a long
time; and we want something to go much quicker
than that sphere. It accordingly occurred to me
that we could not do better than adopt a hint from
our old friend Mark Twain in his Tramp Abroad (a
delightful book which can be bought for sevenpence
nowadays). Perhaps you have read it, and you
remember that after he had braved the perils of the
ascent of the Riffelberg (which had taken him seven
days !), he was contemplating going up Mont Blanc.
But the perils of the actual ascent seemed so great
that he decided to make it by telescope. He found
in the village street at Chamonix a man with a tele-
scope directed on a party going up the mountain ; he
followed them up by means of that telescope, feeling
all the time just as though he were along with them ;
so that when they had got to the top, he quite felt
that he had got there himself, and cheered so loudly
that he disturbed the people round and they made
remarks which called him back to Chamonix.
The advantage of going by telescope is, first of all,
as Mark Twain hints, that it is very much safer. I am
afraid I have not got a licence to drive an aeroplane,
and if I had I might even then have an accident.
But besides being safer, the telescope takes us much
more quickly. Supposing you wanted to get to the
Sun, even in two years, how fast do you think you
would have to go? We said the Sun is 93 mil-
lion miles away; and there are about a million
minutes in two years; so that you would have to
go about 93 miles a minute which is pretty quick !
But we can go much quicker than that by telescope.
We can get to the stars, I was going to say, in no time ;
JOURNEYING BY TELESCOPE 89
but it is really less than no time because we shall
get there before to-day ; we shall see things happen-
ing not to-day, but many years ago; just as when
abroad you get a newspaper not of to-day, but of
yesterday or perhaps a week before; and thus you
learn what your friends were doing yesterday or
a week before. Now, although light travels very
quickly, it takes time to travel; and the light that
comes to us from the distant stars tells us what hap-
pened years ago, perhaps thousands of years ago;
and this gives one a very curious feeling when we
reflect upon it. When we were little babies and went
out for our first walk, and saw the sky for the first
time, some of the light which fell upon us went back
towards the sky and part of it enabled our nurse to
see us. Unless the light did come back from the baby
in that way, the nurse would not see it. But she
did not use up all the light from the baby ; another
part of it went on past the nurse, perhaps into the
telescope of an angel (if angels have telescopes), and
then the angel saw the baby ; but not till some time
after the nurse had seen it, because the light would
take time to get from nurse to angel. And another
part of the light which missed the nurse and missed
the angel would perhaps go on to a star ; but it would
only get there after some years. Now if you could
be put in one of those stars immediately, and if you
had a powerful enough telescope, you might be able
to see yourself as a baby going out for the first time ;
because the light would not have got there immedi-
ately, but would have taken all the years you have
been alive to get there. If you went on to more
distant stars you might (if you had a strong enough
A VOYAGE IN SPACE
telescope ; it is a large IF), you might see your grand-
mother going out as a baby for the first time.
Well, as I say, that means that if we use the tele-
scope, we shall get to the heavenly bodies in really
less than no time, which would be very convenient ;
Fig. 22.
and so I propose to say a word or two about this
wonderful car we are going in the Telescope; for
it is just as well to understand your car. Any one
who gets a motor-car looks carefully at all the parts ;
and to-day I want you to learn about the telescope,
and what has been its history. Perhaps I ought not
JOURNEYING BY TELESCOPE 91
to say telescope, but camera, for most of the telescopes
astronomers use nowadays are just photographic
cameras on a large scale. So we might say that we
will journey by camera. But in its early history the
telescope was not a camera, and so we will for the
present use the name telescope.
The earliest telescopes of any importance were
very long and thin. Perhaps you will hardly recog-
nize that object in Fig. 22 as a telescope; one end
is high up in the air and the other is down on the
ground. Sometimes these telescopes were 200 feet
long, and they must have been extraordinarily dim-
cult to manage. The astronomers who used them,
and really got very accurate results with them, must
have been men of great skill. Why did they make
them so terribly long when we in modern days have
been satisfied with much shorter telescopes? The
reason was that they were bothered by colour. We
shall see how colour comes into the question in a few
moments when we do some experiments ; and we
shall also see how colour is now a positive advantage
to astronomers, when properly used; but in those
early days it was only recognized as a disadvantage
which drove astronomers to make their telescopes
so long and thin that they became difficult to manage.
In Fig. 23 is a way of making a long telescope, sug-
gested by the great astronomer Hevelius ; he was very
proud of his invention and wrote a book about it;
and the whole invention was that you need not have
an actual tube for the telescope (made of four planks,
like a long thin box ; that was the way some of them
were made), but that a single plank would suffice,
if only a lot of circular diaphragms were attached to
A VOYAGE IN SPACE
the plank at intervals. His whole invention was to
save three of the four planks and use one only !
Perhaps I am not quite fair, for he was also rather
proud of his invention of the lifting gear and of the
floor under which the telescope could be stowed when
out of use.
You will readily see the great difficulties of using
these machines, so that astronomers cast about for
some other way of getting over the difficulty of
colour. It occurred to several people independently
that if a mirror could be used instead of a lens to
bring the light to focus there would then be no
colour and therefore no difficulty. We will explain
this fact and illustrate it by experiment in a few
minutes; but first let me say a word or two about
the consequences of this new move in the making of
JOURNEYING BY TELESCOPE
93
telescopes. They were made with mirrors reflect-
ing telescopes, as they are called and they ceased
to be so very thin. Indeed, on the contrary, they
became quite fat, for there is a great advantage in
having the mirror as wide as possible : you can see
W. Herschel at his great Telescope.
much more clearly with a wide telescope than with
a narrow one, always assuming that the wide one is
just as well made. But it is much harder to make
a big wide mirror than a little one, just as it is harder
to get 100 runs at cricket than to get ten ; the bats-
man may play several overs successfully, but he has
to be terribly careful not to make a mistake if he
94 A VOYAGE IN SPACE
wants to make 100. So with mirrors; it may be
easy to make a little one without any mistake, but
it is awfully hard to make a large one without some
flaw in it which prevents it being a good mirror. The
first man to get a century, so to speak, by making
a really large mirror without flaw, was William Her-
schel, who did not begin to try until he was 37 years
old. As a boy he was in the band of a Hanoverian
regiment, like his father and several of his brothers.
It is sad that we cannot claim Herschel as a born
Englishman, but he settled in England and got a
position as organist at Bath. He showed some
cleverness in getting that position, but scarcely of
a kind to suggest the wonderful work he was to do
in astronomy afterwards. The story is that when
the different candidates for the post of organist were
being tried, one of them played so skilfully that a
friend remarked to Herschel that it was scarcely
possible to play any better. " I don't think fingers
can do it," agreed Herschel. Nevertheless he went
up into the organ loft and produced such astonishing
effects that the judges awarded him the post. The
same friend, who had shared in the general astonish-
ment, could not help asking how in the world he
had managed to play as he did ; whereupon, with a
twinkle in his eye, Herschel produced from his pocket
two leaden discs, and said, " You remember I told
you fingers alone could not beat the playing we had
heard ; but I put these discs, one on a low note and
the other on a high note, leaving my hands free for
the middle, and in that way I managed to surprise
you." After all, though the trick may seem a simple
one, it must have required great skill to use it to full
JOURNEYING BY TELESCOPE
95
advantage ; I expect only a remarkable man would
have thought of it. And presently Herschel showed
much more clearly how remarkable he was. He
became interested in the making of mirrors almost
by accident ; but once started, he went on from one
success to another. I have compared making a big
mirror to getting 100 runs ; and it required quite as
much watchful care on Herschel' s
part. His innings at polishing
went on for long hours some-
times twelve hours at a stretch,
with no intervals for meals. All
the food he got was put into his
mouth by his sister Caroline,
while he still went on polishing.
She was a splendid sister to him
took down notes of what he
saw through his telescopes when
he had made them, and wrote
them out neatly for publication ;
and when he was not using his
telescopes, Caroline would use
them herself, and she found
several comets in that way.
William Herschel worked very
hard, but Caroline worked just as hard in helping
him ; and whenever the brother is mentioned with
honour, I think the sister ought to be mentioned
also. The greatest thing William Herschel did was
to discover the planet which we now call Uranus,
though he himself wished it called Georgium Sidus,
after King George III, who had given him much
encouragement. Other people wanted it called
Caroline Herschel.
96 A VOYAGE IN SPACE
Herschel, in honour of the discoverer, and both
these names were used for many years. But still
other people said that for anybody to have his
name attached to a planet was far too great an
honour, whether he were discoverer or king; and
ultimately these got their way by establishing the
name Uranus.
Herschel's largest mirror was four feet wide ; but
afterwards Lord Rosse made one six feet wide, which
is, up to the present, the largest telescope ever made.
A man could stand in the mouth of it. The making
of the mirror itself was a great achievement, but
that was by no means all ; to fit it into a telescope
that could be easily handled was a triumph of en-
gineering skill and could only have been accom-
plished by a great engineer such as Lord Rosse was.
One of his sons is also a great engineer; he is the
Mr. Parsons who invented the steam turbine, and
made a success of it in spite of enormous discourage-
ments; and he told us, you remember, about the
diamonds in meteorites.
Herschel's four-foot mirror and Lord Rosse's six-
foot are still in existence, and the latter is actually in
use in the telescope, though the former has been dis-
mounted. But the surfaces of both have become
tarnished, so that they no longer show things clearly.
They are made of a peculiar kind of metal called
speculum metal, which remains bright for some
time, but gradually tarnishes in the course of years
in a manner difficult to remedy. To avoid this
gradual deterioration, mirrors have been made re-
cently of glass which is then silvered with a thin film
on the front surface. This film also tarnishes after
,
JOURNEYING BY TELESCOPE
97
a time, but it can easily be removed with a little acid,
and then a new bright film can be put on so that the
mirror is as good as new. A good many mirrors two
or three feet wide have been made in this way ; but
the first really big one was made by Dr. Common at
Ealing. It was five feet wide, not so wide as Lord
The great Five-foot Telescope being taken up Mount Wilson.
Rosse's (six feet), but still a great advance on any
previous " silver-on-glass " mirror. With it he took
some wonderful pictures of nebulae and other objects
of the heavens, but he did not live long after he had
made it, and the telescope was sold at his death to
the Harvard Observatory in the United States.
Since then a five-foot mirror even better than
Dr. Common's has been made and set up at Mount
98 A VOYAGE IN SPACE
Wilson in California; and it has been so successful
that they are attempting a much bigger one still,
100 inches wide, or 8 feet 4 inches. If they succeed
in making it, they will have the first telescope to
surpass in size that of Lord Rosse. In this case,
besides the great difficulties of making the telescopes,
An accident to the traffic up Mount Wilson.
they had the extra ones of getting them to the top of
a mountain 6000 feet high, which are by no means
inconsiderable. When I first visited Mount Wilson
in 1904, the path up the mountain was only three feet
wide, with a steep face on one side and a sudden
drop on the other; the path was just about wide
enough for one mule ; but the mules did not think
so; they kept trying to crowd past one another,
JOURNEYING BY TELESCOPE 99
which is not pleasant for the rider, especially as they
try to pass on the outside, where a slip means tumb-
ling down the precipice. Professor Hale had this
path widened to five feet all the way before he could
venture to send up the big five-foot mirror and its
mounting; and I believe it is now being widened
further still to twelve feet, so that they can carry
up the loo-inch mirror in comfort when it is made.
The five-foot has been at work on the top of the moun-
tain for several years, and has taken some most
beautiful photographs of objects far away in the
depths of space. We will presently put some of
them on the screen, and in that way we shall be prac-
tically allowing this magnificent telescope to carry
us in imagination for a long " voyage in space/'
such as was not possible a few years ago. But it
is a curious thing that when you get to the top of
Mount Wilson, the Earth seems almost more striking
than the heavens. Down below you in the plains
are the two cities of Los Angeles and Pasadena ; and
when these are lit up at night they make glittering
constellations of which the stars are brighter than
those in the sky above. (See illustration facing
p. 126.)
Having told you something about the history of.
these big reflecting telescopes the biggest telescopes
in the world I want to go back and talk a little
about the reason for making mirrors instead of lenses :
so that the trouble about colour is avoided.
When we make a telescope with a lens we use a
property of light called refraction, a word from the
Latin which means simply breaking back. You know
how a cricket ball can be bowled so as to go straight
ioo A VOYAGE IN SPACE
for some distance, and then when it hits the surface
of the ground it " breaks back " owing to the spin
the bowler has given it ? Well, a beam of light be-
haves in much the same way when it is refracted ;
after going straight for some distance, it strikes a
surface and turns off at an angle.
In Fig. 24 the beam of light AB goes along
straight from A to B, and then strikes SF, the surface
of glass or water; thereupon instead of continuing
straight along BC it is " broken back," or is refracted
along BD. Now when we use a lens, we have two
AIR
S ^
WATER
OR
GLASS
Fig. 24.
surfaces to deal with, one where the light goes in and
one where it comes out, and both of these are curved,
which is a new complication. To keep matters as
simple as possible I will first use a prism ; that is, a
piece of glass with two flat surfaces. When the ray
AB strikes the first surface SF (Fig. 25) , it is broken
back as we have said along BD ; and when it comes
to the second surface to get out of the glass again it
is broken back again along DE. You may think
that I have drawn DE the wrong way and that it
should have been in some such direction as DX ; but
JOURNEYING BY TELESCOPE
101
when you try with a prism you will find the ray be-
haves as drawn; it gets bent in the same direction
AIR
AIR
AIR
Fig. 25.
both times, and comes out in a line quite turned
aside from its original direction AB. But at the
Fig. 26.
same time it becomes coloured, with all the colours
of the rainbow. Whenever you get this refraction
or " breaking back " you also get colour. When I
'''** i>
102
A VOYAGE IN SPACE
PARALLEL
RAYS OF LIGHT
put this prism in the path of a straight beam of
light it is bent aside so that it no longer falls on the
screen in front, but on this other screen to the side ;
and at the same time it becomes beautifully coloured
(Fig. 26). The fact is that blue colours are more
bendable than yellow, and yellow than red ; so that
though all the colours start to-
gether in the same ray out of the
lantern, when the prism bends
them, some bend more than others
and we get them separated. Now
those colours have become lately
of the greatest value to astrono-
mers, but at first they were merely
a nuisance; they interfered with
the use of lenses in telescopes. A
lens is a piece of glass with curved
surfaces ; when rays of light strike
those surfaces they are bent to a
focus (Fig. 27), as is wanted for
making a telescope. But the bend-
ing introduces colour which is not
wanted; and yet they could not
in early days get rid of the colours
without also getting rid of the
bending to focus. All they could
do was to keep the colour effects as small as possible
by making the bending also small, which means that
the rays have to travel a long way before they come
to focus. That is why the early telescopes made
with lenses were so long and thin ; if they had been
shorter and wider the colour effects would have been
so great that the observations would have been use-
' FOCUS
Fig. 27.
JOURNEYING BY TELESCOPE
103
PARALLEL
HAYS OF LIGHT
less. Newton and Gregory and others therefore
turned aside from the making of lenses and proposed
to use mirrors, which can also collect rays of light to
a focus if they are hollowed out to a particular shape.
You can try this for yourselves with the reflector of
a common oil-lamp ; it will bring
rays to a focus just as a burning
glass will (Fig. 28).
But now let me show you
another prism. When this is
put in the path of the rays you
see they are not broken back at
all; they still pass straight to
the screen ; but at the same time
they are wonderfully coloured.
How is this? We learnt that
colour was a result of refraction,
and yet here is colour without
refraction. The fact is, this
prism is really made up of two ;
one bends the rays aside and
colours them, the other bends
them back again without undoing
the colour entirely. That is a new
and valuable idea, that one
bending can balance another and
yet the colour effects do not balance, because it
suggests to us at once that we ought to be able to do
just the opposite, that is to balance the colour effects
and leave the bendings unbalanced ; and in this way
we can make a lens which will not show colour. When
this was once realized astronomers went back to
lenses for their large telescopes, and they have suc-
MIRROR
Fig. 28.
104 A VOYAGE IN SPACE
ceeded in making lenses larger and larger until the
largest is now 40 inches wide the great lens at the
Yerkes Observatory near Chicago. That is a long
way short of the largest mirror telescope (you re-
member that Lord Rosse's was six feet wide) ; but
it is far easier to make a mirror than a lens. You
see the mirror has only one surface to be made ; you
must be careful to get that surface as nearly perfect
as possible, but when that is done your work is over.
But a lens has at least four surfaces; for to get rid
of colour we must put two lenses together, balancing
their colour effects, but leaving the focussing effects
unbalanced. Each one of these four surfaces must
be as carefully made as the single surface of the
mirror ; and besides, we have to be very careful that
the glass inside is as perfect as possible, since a slight
flaw in it will make trouble and confusion at the focus
of the telescope. You may ask why, then, do astro-
nomers bother to make lenses at all, since mirrors
are so much easier ? Well, there are several reasons,
but I will mention only one : a lens can be made to
show more of the sky at once. When we look through
a mirror-telescope (or, let us say, when we photo-
graph with it, for that makes the explanation simpler) ,
then we see that the objects in the middle of the
picture are in good focus, but not those at the edges.
Perhaps many of you have got cameras of your own,
kodaks or brownies, or some other sort, and you may
know the difference between good and bad lenses
with a good lens all the picture is in focus, with a poor
one the edges show up fuzzy. Well, with a mirror
we cannot help the edges being fuzzy, because we
have nothing we can alter. If we try to alter the
JOURNEYING BY TELESCOPE
105
surface, we shall put the centre of the picture out of
focus, which leaves us no better off than before.
But with a lens we have four surfaces to deal with
and can set one against another, much in the same
Photograph showing very bad edges, though good in centre.
way that has been already done for colour ; a little
alteration in one can be accompanied by such a corre-
sponding alteration in another that the edges of
the field are improved while the centre is no worse
off. This way of setting one thing against another
becomes even more easy when we have three or four
io6 A VOYAGE IN SPACE
lenses instead of two only; we can then arrange to
get quite a large picture with no fuzziness anywhere.
Let me now tell you a little about some of the
largest lenses in the world. They have nearly all
been made by an American firm called Alvan Clark.
It would have been nicer for us to say that they
were made in England : we have, however, the
satisfaction of knowing that an Englishman, the
Rev. W. R. Dawes, was chiefly instrumental in
drawing attention to the excellent workmanship
of the Clarks : so that when a big telescope was
to be made, the order was placed with them. One
of their first successes was a 26-inch lens built for
the Washington Observatory; and almost directly
it was pointed to the heavens, two tiny moons of
the planet Mars were discovered with it. Up to
that time Mars was believed to have no satellite :
When in 1832 Tennyson was writing the " Palace of
Art," he put in some astronomical verses, including
the line
" She saw the snowy poles of moonless Mars,"
but these verses were left out of the poem, and not
published until 1898, when two moons had been
discovered, so that the line was altered to
" She saw the snowy poles and moons of Mars."
Another interesting thing about these moons is
that Dean Swift jokingly predicted them in his
Gulliver's Travels. He speaks of the astronomers
of Laputa as having
" discovered 2 lesser stars or satellites which
revolve about Mars."
JOURNEYING BY TELESCOPE 107
Of course he was only writing in fun, but an old
proverb says that " There's many a true word
spoken in jest," and so it proved in this instance.
Soon after this the Czar of all the Russias wished
to have the largest lens in the world for his ob-
servatory at Pulkovo, near Petrograd. The Alvan
Clarks were given the order for a 3o-inch lens, and
at the same time the machinery for the telescope
was to be made by the brothers Repsold at Hamburg.
As a young man beginning astronomy, I had the
privilege of seeing this great telescope in the work-
shop at Hamburg in 1884, and I well remember
my astonishment, not only at the size of the telescope,
but at the number of beautiful devices for working
it, especially at the eye end. When an astronomer
is looking through a telescope, it is a great con-
venience to him to be able to make notes and meas-
ures of what he sees without taking his eye away :
William Herschel had his sister Caroline to make the
notes for him, but other astronomers have not had
such devoted sisters. The Repsolds had accord-
ingly arranged for the observer to do all manner
of things by pressing this button or turning that
screw, while he was still looking continuously at
the star or planet under observation. But no
arrangements are perfect, and an eminent Ger-
man astronomer who was inspecting the telescope
when I was there pointed out that this complicated
apparatus would infallibly collect much cigar ash !
The Czar was not allowed to remain long in pos-
session of the largest lens : one of 36 inches was
ordered for the Lick Observatory and again the
Alvan Clarks succeeded in making it. James Lick
io8 A VOYAGE IN SPACE
was a rather eccentric millionaire in San Francisco,
who would probably have spent his money in a
very different way but for the persuasiveness of
an astronomer, Mr. George Davidson. There are
many stories about James Lick : perhaps you
would like to hear one of them. A great many
people applied to him for work, and he had a curious
way of deciding whether to give them employment
or not. He attached great importance to their
obeying orders, however stupid the orders might
seem. So when a man came asking for work, he
would set him to plant trees upside down, with
their roots in the air and their branches in the
ground ! Those who set to work without protest
he kept in his service; but if a man objected or
asked questions, he was sent away. You can well
imagine that a man like this had queer ideas about
what to do with his money, and he was chiefly
anxious to have some great memorial to himself.
Mr. Davidson persuaded him that a large telescope
would be a very good form of memorial; and the
bones of James Lick now rest under the great
36-inch telescope on the top of Mount Hamilton.
He died, indeed, before it was completed, and was
temporarily buried elsewhere : but when the great
telescope was at last in place, his bones were dug
up and deposited under his chosen memorial. Only
one possibility seems to threaten their peaceful
rest : California is rather a region for earthquakes,
and already the Observatory has been twice seri-
ously shaken. San Francisco was, as you know,
wrecked by an earthquake some few years ago.
The Lick Observatory is some distance away from
JOURNEYING BY TELESCOPE 109
San Francisco and only suffered slightly, but a
telescope is a delicate instrument that cannot stand
much shaking without being damaged. We must
hope that good luck will attend the future.
One achievement of the Lick telescope was to
discover a new satellite of Jupiter in 1892. Of
course the telescope could not have done it without
the sharp eyes of Professor Barnard behind it ; but
we must give the telescope its share of credit, for very
few others have managed to show the satellite even
now we know it is there, and it is much easier to
see something after it has been discovered than to
find it in the first place.
It was a condition attached by Mr. James Lick
to his gift that the public shall be allowed to use
the telescope one night a week, and a great number
of visitors go up the mountain on Saturday night
just to get a few minutes looking through this great
instrument. I am afraid many of them are dis-
appointed, for they expect to see all that they have
read about in books or seen pictures of. Now
what is set down in books is often the outcome of
very careful watching by skilled observers : it
cannot be seen every moment, but only on favour-
able occasions : and without a " seeing eye " it
cannot be seen at all. But some of the visitors
perhaps only want to say they have looked through
the telescope, without caring much what they see.
The largest lens in the world at present is that
at the Yerkes Observatory near Chicago. Perhaps
you would not call it very near, for it is about 70
miles away; but it was a millionaire of Chicago,
Mr. Yerkes, who gave the money for it and wished
no A VOYAGE IN SPACE
Chicago to have the credit of the largest telescope,
so we must regard it as belonging to Chicago. The
Observatory is on the shores of a lake which is
now called Lake Geneva, but which had a more
beautiful Indian name, Kish-wau-ke-toc, meaning
Big Foot, from its shape. The lake is a kind of
summer resort for the rich people of Chicago, and
when the train comes in, you can see crowds of
steam yachts waiting to carry them to their various
houses round the shores of the lake, just as we
might see a number of motor-cars at a big terminus
in England.
The great Yerkes telescope is 60 feet long and
has all sorts of beautiful and powerful mechanism
to work it. The floor on which the observer stands
can be raised or lowered, so as to give him a com-
fortable position for any object he wants to look
at ; and this is worked electrically ; so that pulling
a lever is sufficient to start the mechanism. Pulling
another lever starts a motor which turns the great
dome round : and generally, by pulling one lever
or another all the things can be done which in
smaller observatories are done by hand. To get
enough electric power for all these things there is
a " power-house " placed at some little distance
from the telescope. One curious visitor enquired
if they put it at that distance because they were
afraid it might explode and damage the 100,000
dollar telescope; but the astronomer said no, that
was not the reason ; they did not fear an explosion,
as might be seen from the fact that the power-house
was close to his own dwelling-house in which there
was a million-dollar baby !
JOURNEYING BY TELESCOPE
in
You probably want to know how much these
huge telescopes will magnify, and you may be sur-
prised at my answer, " As much as ever you like; "
but that is the actual fact. Any telescope can be
Fig. 29.
made to magnify a thousand times, say, but it does
not follow that you will be pleased with the result.
The magnifying depends partly on the big lens or
mirror about which we have been talking and partly
on the " eyepiece," the little lens you put your eye
112
A VOYAGE IN SPACE
to. If the object is not magnified enough you can
take out this little eyepiece and put in another
which magnifies more. Perhaps you have had
your photograph taken and " enlarged " : something
of the same kind happens with a telescope, the big
lens or mirror makes a little image and the eyepiece
enlarges it. Now you may have noticed that when
Fig. 30.
your photograph is enlarged, various defects appear
which were not noticeable in the original small
photograph : and the more you enlarge it the more
these defects are emphasized, so that to enlarge it
beyond a certain point is no advantage. Fig. 29 is a
picture of a horse and jockey from the Daily Graphic :
and an enlargement of the head of the jockey.
The enlargement (Fig. 30) shows that the head
is all composed of little dots, and though these
JOURNEYING BY TELESCOPE 113
do not trouble us on the small scale, they become
obtrusive in the enlargement. In the same way
if you try to enlarge the image made in a telescope
beyond a certain point the defects become glaring
and you get no advantage. These defects are due
partly to fault in the making of the lens or mirror
(because no workmanship can be quite perfect) and
partly to the tremulousness of our atmosphere.
All kinds of cross currents are continually passing
in the air through which we are compelled to look
at the heavens ; and they all tend to blur and con-
fuse the image. When we do not magnify it much
we do not see these blurs ; but a high magnification
makes them obvious.
This illustration of the jockey reminds us of
another fact about a large telescope : we cannot
see much at once. In the original picture we see
not only the jockey but the horse as well, but when
we enlarge it, we must be content with the jockey's
head if we are to have the patch of the same
size. Perhaps you will ask why we need have this
limited patch at all? Why not make the whole
picture bigger? I am trying to show you what
you actually see when you look through a big
telescope : and you will always find your vision
bounded by a ring, which limits the " field of
view " as it is called. In using a telescope you
have to put your eye to a small hole in the
eyepiece. Fig. 31 is a picture of the great nebula
in Andromeda, and the little hole in the covering
screen enables us to see just about as much of
it as we should see at one time in a large telescope.
If we move the telescope about, we can change
i
H4 A VOYAGE IN SPACE
the bit we look at. If you cut the screen loose,
you can move the hole about over the picture and
get an idea of the whole in that way. It is a
tedious process, but it was the only way astronomers
Fig. 31. The Great Nebula in Andromeda.
(See Notes to Illustrations.)
had of examining a large object like this when
they used their eyes and had not learnt to take
photographs : and you cannot wonder that they
got imperfect ideas of them. The eminent astrono-
mer Trouvelot drew a picture of this same nebula,
JOURNEYING BY TELESCOPE 115
before the days of photography : and he made those
dark rifts practically straight lines, which misses
the whole point of the structure.
When, however, we take a photograph, the eye-
piece is taken away and the light shines all over
the plate at once, so that we are no longer con-
fined to a small ring, but can photograph the whole
object at once, faithfully : we see that these rifts
are not straight but delicately curved, in a way
suggesting that there are several rings whirling
round the central bright nucleus. This has a very
important bearing on our ideas of the formation
of stars from nebulae : but the drawing made by
Trouvelot was meaningless.
Let us take another point about a large telescope.
We have chosen a telescope as our travelling car,
and we must learn all about it, not only how it is
used, but how it set up. You can well imagine
that it is rather a business to construct a telescope
like Lord Rosse's, big enough for a man to stand
up in. Fig. 32 shows one in which a man is not
standing up, but lying down, to do some part of the
fixing. This is the great Victoria telescope which
Dr. McClean presented to the Royal Observatory at
the Cape of Good Hope in honour of good Queen
Victoria, whom many of you may not remember.
We must hope that some one will give a big telescope
in honour of King George V, mustn't we ?
Now not only is such a telescope awkward to
set up, but it is rather awkward to work after it
has been set up, unless special arrangements are
made. One modern comfort is the rising and
falling floor which I mentioned in connection with
n6
A VOYAGE IN SPACE
the Yerkes Observatory. It was designed originally
by Sir Howard Grubb, for use with the great Lick
telescope; and has been generally adopted for all
large telescopes of that pattern. Presently we
will notice a different method of working a large
Fie. 32. The Victoria Telescope at the
Cape of Good Hope.
telescope, but before doing so I want to remind
you that since our Earth is steadily turning round
on its axis, a telescope would soon lose a star unless
there is clockwork to counteract the Earth's motion.
You know how the various constellations change
during the year : there is a little rhyme, made by
the good Dr. Watts, which enables us to remember
some of them, perhaps you know it already
JOURNEYING BY TELESCOPE 117
The Ram, the Bull, the Heavenly Twins,
And next the Crab, the Lion shines,
The Virgin and the Scales.
The Scorpion, Archer, and Sea-Goat,
The Man that bears the Watering-Pot,
The Fish with shining tails.
That is the order in which the constellations in
the Zodiac are arranged, and the Sun appears to
visit them all in turn once a year, owing really to
the fact that the Earth travels round the Sun. But
the Earth also rotates on its axis, as we learnt in the
first lecture : and in consequence of this rotation
all these constellations cross the sky every day.
Some of them cannot be seen to do so, because they
are too near the Sun : they cross in the daytime
w r hen we cannot see them ; but the others cross at
night, and if you care to notice at this time of year
(early January) you can see the Twins rise in the
east soon after sunset and cross the sky much in
the same way that the Sun does, setting in the west
long after you have gone to bed. If, then, we had
a fixed telescope it could only catch sight of the
Twins once during the night; but if we arrange
that it can turn round by clockwork it can keep
the constellation, or any part of it, steadily in view,
all night if necessary. Sometimes an astronomer
wishes to look steadily at the same object all night
in this way : he would probably be also taking a
photograph at the same time, of some very faint
light such as a faint distant nebula, which would
not appear on the plate unless he gave it a very
long exposure. So he points the telescope at the
object, sets his clockwork going, puts a photographic
plate to receive the image : and then in another
u8 A VOYAGE IN SPACE
telescope (firmly attached to the photographic one
and moved round by the same clockwork), he
watches to see that the clockwork is going smoothly
and accurately. If it is not, he has the means of
correcting it by pressing an electric button. But
it is very tiring to watch for long hours like this,
and he is generally glad when the first signs of com-
ing daylight warn him to put the cap on his telescope.
Sometimes even then he knows that he has not
given a long enough exposure; the object may be
so faint that even a long winter night of twelve
hours is not long enough for it. What is he to do
then ? Why ! he must cover up his telescope most
carefully so that no light can get to the plate during
the day : and when darkness comes again, he must
turn the telescope to the east where the object will
rise, set his clockwork going again, take off the cap,
and spend another night watching to see that all
goes well. Perhaps even two nights are insufficient,
so that he must add a third and a fourth : and I
need scarcely remind you that all nights are not fine ;
when there comes a wet or cloudy night, the astrono-
mer must keep his telescope covered up and wait
until the weather improves. So that sometimes a
plate has been kept in the telescope for many months
before the full exposure has been given. You may
not have realized what long hours of waiting some
of the beautiful pictures of the heavens which you
can now see so easily may have cost !
Now we can return to the way of working a big
telescope. One of the most recent modern im-
provements is to attach the clockwork, not to the
telescope itself, but to a mirror which reflects the
JOURNEYING BY TELESCOPE
119
light into it. The mirror is in this case a flat mirror,
not concave so as to bring the light to focus, but
made perfectly flat so that it simply turns the light
in another direction without altering it. All the
alteration is to be done by the telescope, which can
A Coelostat being prepared at Oxford for the
Eclipse Expedition of 1896.
now be firmly fixed : the mirror is turned about
by the clockwork so as to send the light always into
the fixed telescope. A mirror so arranged is called
a Coelostat, which means " sky standing still " :
and it is an actual fact that if you look into such a
mirror at any part of the sky, it will no longer appear
to move as the real sky does, but remain steady
120
A VOYAGE IN SPACE
and constant. You may ask whether we can pre-
vent a constellation setting in this way : of course
we cannot do that : a constellation will seem to
remain steady as long as you can look at it : but
the mirror is being turned round by clockwork, and
in course of time you will find that it will present
I The long shed for the Snow Horizontal Telescope
on Mount Wilson, California.
its edge to you, and then its back ! so that you can
no longer see the constellation, which takes the
opportunity to set below the horizon.
When a coelostat mirror is used in this way we
can fix the telescope in any position we like within
certain limits. One position which very naturally
occurs to us is the horizontal one. If we lay the
big telescope on the ground, there is no danger of
JOURNEYING BY TELESCOPE 121
its falling : and it is in other respects convenient
to work with. Hence several large telescopes have
already been built in this way. A good example
is the Snow telescope on Mount Wilson. You see
in the picture the long shed which is really the tele-
scope itself : and at one end there is the coelostat
mirror, reflecting the light (in this case usually the
light of the Sun, not that of a star) into the telescope.
But there is one great disadvantage about this
horizontal position. The ground gets heated during
the day and causes air currents to ascend from it.
Now currents of heated air blur the image (see p. 143)
and are to be avoided as much as possible, whereas
the horizontal position of the telescope seems to en-
courage them. Hence, Professor Hale tried the plan
of putting the telescope vertical instead of horizon-
tal. He built, in the first instance, a tower 60 feet
high. The coelostat mirror 1 was placed on the
top and the rays from the Sun or stars sent down
to the foot of the tower, where they could be ex-
amined or photographed. This new move was
found to be so successful that a more ambitious
experiment was tried. A tower 150 feet high has
been built to carry the coelostat at the top : by
it the rays of the Sun are reflected down to the
ground but do not stop even there, because a well
80 feet deep has been dug below the tower to
receive them, so that the whole length of travel
1 There are really two plane mirrors, one moved by clock-
work as already described, the other fixed. If we had only
the first mirror, then any part of the sky would indeed
remain stationary, but the telescope must be pointed to
it in a particular direction. The second mirror makes it
unnecessary to point the telescope.
122
A VOYAGE IN SPACE
is 230 feet. We have got back to the length
of those long thin telescopes which were found
so inconvenient two hundred years ago; but now
they are no longer inconvenient because they need
not be moved about, but can be fixed upright
The High "Tower '' Telescope on Mount Wilson.
once for all. That is of course not altogether an
easy matter, because a telescope must be fixed very
steadily; it will not do, for instance, for it to be
shaken by the wind. How are we to prevent a
.tower 150 feet high from being shaken by the wind ?
Professor Hale thought of a trick as neat and simple
JOURNEYING BY TELESCOPE 123
as a conjuring trick one of those dodges that is
so easy when once you know and so difficult before-
hand. If any of you likes to test his own brains
let him try to think for himself how this was done :
and to give you a little time to think I will not
tell you the answer just yet. 1 We will look
at this picture (p. 124) of the ladies who do the
calculations at the Mount Wilson Observatory,
ready to ascend the 150-foot tower in the bucket
which acts as a lift, and then we will leave the tower
for a time and go down into the well, where all kinds
of apparatus are available which it would take us
several courses of lectures to explain fully, but of
which we may get some idea without working too
hard.
Let us first think of the three essential parts of
a telescope any telescope we might take the
very first that Galileo looked through. There were
two lenses joined by a tube. Now these three
separate parts of a telescope have each had histories
of their own which we have partly reviewed already.
The tube went ahead first ; you remember how the
tube became immensely long, because of colour,
without any great alteration in the lenses : then one
of the lenses (the object-glass as it is called, because
it is turned to the object : the other is called the
eye-lens or eyepiece] became a mirror and was made
much larger, while the tube shrank back to normal
size. We thereupon followed the history of the
mirror, seeing how it became larger and larger :
1 The answer is given at the end of the chapter, but I
would urge all and sundry to try to guess it for themselves,
if they do not happen to have heard it.
124
A VOYAGE IN SPACE
and how, when the difficulty of colour was got over,
large lenses also began to be made. Meantime the
tube had to grow to keep pace with the growth of
the object end, and the old difficulties of unwieldi-
ness had to be faced afresh; and so we came back
The Computing Staff about to make their first ascent of the
" Tower " Telescope in 1910.
to the tube and learnt about the coelostat, and
ultimately about Professor Male's great 150-foot
tower. But all this time nothing had been said of
the poor eye end, which seems to have been left
quite out in the cold : and indeed its history did
not begin until later, but once begun, it has been far
JOURNEYING BY TELESCOPE 125
the most eventful of the three, because there has
been much more variety in it. The object end
and the tube have changed chiefly in size ; but the
eye end had become a photographic plate, or a
spectroscope, or a photometer, or a spectrohelio-
graph, or any one of a number of such things which
can be attached at the eye end. One of the practi-
cal problems of an astronomer is to change one of
these pieces of apparatus for another as easily and
rapidly as possible : and Professor Hale has devised
the best method up to the present. Perhaps some
of you have on your breakfast-tables a dumb-waiter
which can be turned round, so that you can get at
the butter or the toast or the marmalade when you
want it ? There is a contrivance of a similar kind
down in the 8o-foot well below the I5o-foot tower.
Of course it is much bigger and stronger than your
breakfast-table apparatus : but in just the same
way, and almost as easily, as you help yourself to
toast first and then jam, Professor Hale can put
on the spectrohelic ~aph first and then the polari-
meter, say, or what er he wants to use.
Up to the present, 'he only one of these things
that we have said much about is the photographic
plate. We can take away the eyepiece of a tele-
scope (and also the eye that looks through it) and
substitute a photographic plate, and some of the
advantages of doing this have already been mentioned,
especially that the plate can look at much more of
the sky at one time. We also mentioned that the
plate could be exposed for many hours, and in this
it has another advantage over the eye, because it
is steadily photographing more and more (that is,
126 A VOYAGE IN SPACE
fainter and fainter objects) all the time, whereas
the eye gains nothing by continuing to gaze it
rather loses by getting tired. Another advantage
is that when the plate has been developed, it can
be copied and the copies sent all over the world :
or it can be preserved for many years : or it may
be measured at leisure; and so on. The catalogue
of advantages is quite considerable, so that the
introduction of photography into astronomy has
effected nothing less than a revolution.
But scarcely less of a revolution has been effected
by the spectroscope, which tells us what the stars
are made of, by means of that very colour which
was at first simply a nuisance. A great tnan once
said that it was quite certain that we should never
know what the stars were made of : but it is now
quite certain that we do. You remember the ex-
periment with the prism which showed us the
different colours of the rainbow? I now want to
show you that these colours are not always the same,
We will first throw them on the screen with ordinary
light, when they have the familiar appearance; but
now we will put a salt of sodium in the source of
light and immediately you see that the yellow
becomes very bright much brighter than the rest.
(See spectrum No. 4, plate facing p. 273). That effect
can be produced by common table salt, which is a
salt of sodium; and we know when there is some
sodium in the light by this brightening in the yellow.
But we can improve the apparatus until we find
that the brightening is very local sodium does not
brighten all the yeUow, but only a particular part
or line of it. If we improve the apparatus still
I2 7
128 A VOYAGE IN SPACE
further the line splits up into two : and with further
and further progress these lines are so clearly separ-
ated that hundreds of other lines can be seen
between them. We might spend a lifetime examin-
ing these lines : but at present I want us to be
content with the very simplest facts which will give
us a general idea of the spectroscope.
Now let us try another experiment. We will
again put in some sodium and you see the bright
line in the yellow : but soon you see it changes to
a dark line instead of the yellow being brighter
than the other colours, it is much darker. (Look
at spectrum No. 10, " Sirius/' plate facing p. 273)
The reason is that the sodium has now been put in
in such a way that it gives off clouds of its own
vapour, and these clouds stop the yellow light though
they do not stop other light. The discovery of
that fact is one of the greatest discoveries ever
made, for it enabled us to read the heavens like a
book. Until it was made nobody could understand
the meaning of the dark lines which cross the spec-
trum of the Sun : they were thought at one time
to be boundary marks of the different colours.
Now we know that they vouch for the presence of
sodium and other chemical elements in a state of
vapour, stopping the light of particular colours
and letting other light pass on. It is as though a
French policeman were stationed at the gangway
of a steamer, stopping all Frenchmen from disem-
barking : all other nations might pass English,
Germans, Portuguese, Chinamen, Hottentots and
all others because the French policeman would
have no authority over them; but he would stop
JOURNEYING BY TELESCOPE 129
every Frenchman, and so there would be none in
the miscellaneous crowd issuing from the gangway :
and if we could notice this absence of Frenchmen,
we might say " Why ! there must have been some
French officials stopping them at the gangway."
So in the same way when we see the absence of a
sodium colour (or an iron colour), from the light
of the Sun, we say, " There must have been some
sodium vapour (or some iron vapour) stopping the
sodium light (or iron light) at the gangway (that
is, just where the light was leaving the Sun)."
It may seem strange that we notice the light
which is stopped rather than that which comes to
us. Why should we not rather look for a bright
light sent out by sodium, like that I showed you
first ? Well ! we often do : but the dark lines,
shown in the second experiment, are much commoner,
and have told us on the whole far more than the
bright lines. The fact is that the stars are mostly
very very hot, so that there are masses of vapour
surrounding them through which the light must
pass. The stoppages are therefore far the most
conspicuous features of their light.
You will understand from this brief description
what a spectroscope is. The chief part of it is the
prism 1 which spreads out the light into a band of
colour : but we must be careful to limit the light
to a narrow line or " slit," otherwise we shall not
see the stoppages distinctly. We can either examine
the result with the eye, or we can photograph it :
1 Instead of a prism we can use what is called a
"grating"; but the general result is the same.
130 A VOYAGE IN SPACE
and there are much the same advantages in photo-
graphing here as in the case of the direct light.
But we can also do something else. When the
light is spread out in this way we can throw most of
it away and keep only one colour, and then nothing
will shine unless it has that colour. Let us do
another experiment or two with our prism. Here
is the light spread out into colours as usual, and now
I will take a red ribbon and put it in the red light :
you see it looks red as usual, but in the green or blue
it looks black it gives us no light. Here is a yellow
ribbon which shines yellow in the yellow light, but
when I put it in the red or blue it looks black : and
so on.
We will try another experiment. Instead of
spreading out our light into colours, let us take light
of only one colour, to begin with. We can do that
by putting some common table salt into the flame
of a spirit-lamp, when it will blaze with that brilliant
yellow colour which we have seen is due to sodium.
Now here is our bunch of ribbons of all colours,
which perhaps one of my audience will kindly hold :
when we burn our table salt only the yellow ribbon
will show the others will all appear black but
we are not quite ready yet. Here are some letters
of different colours pasted on this board, which
perhaps two others of the audience will hold : and
here again is a picture in colours for some one else.
You see all these coloured objects in ordinary light :
now we will darken the room and put in the table
salt, and in the weird yellow light you see only the
yellows all the other colours appear black, but
we see at a glance where there is any yellow.
JOURNEYING BY TELESCOPE 131
On this principle is constructed one of the instru-
ments that replace the eye end of the telescope.
It has a terribly long name the spectro-helio-
graph : spectro from the colours, helio from the
Sun, which is usually the picture experimented upon ;
graph because the records are written by photo-
graphy. The thing it does is to make a picture of
the Sun in one colour, just as we used only yellow
light to look at this picture. Then all the parts
which have any yellow (or red or whatever colour we
choose) show up, while the rest remain black. We
can tell at a glance whereabouts there is any sodium
(or hydrogen or whatever gives that colour) in the
Sun. Suppose we could apply this process to the
Earth and take a picture of it showing just where
any gold was, would that be a good plan, do you
think? London would be a bright spot, and New
York and other big cities ; places like Alaska would
show up, if the gold is not yet all dug out ; and then
there would be other spots which nobody knows of
yet, where the gold is still in the ground, what
a rush there would be to them ! The pictures of the
Sun do not show us the distribution of anything
so exciting as gold, but what they show us is never-
theless of great interest to astronomers, and we
will say more of it when we visit the Sun.
I will tell you about just one more of the things
we can put on the end of a telescope something
rather simple? this time because perhaps you may
think we are getting too complicated for comfort.
The point of this instrument is merely its great
sensitiveness it detects variations in light which
the eye cannot notice. There is a star called Algol
132
A VOYAGE IN SPACE
which we may translate " The Wonderful " (" Al " is
the Arabic for " the," and " gol " is the word used for
genii or fairies in the Arabian Nights, for instance :
though it is mixed up with the word " ghoul," which
Fig. 33-
is not so pleasant in association) . This star behaves
like the light of some lighthouses, it shines steadily
for a time and then goes faint. The steady shining
lasts for about three days and then comes the dip.
Now the eye is able to see this without any telescope
at all; and we can interpret the dip to mean that
something comes in front of the light. Imagine two
JOURNEYING BY TELESCOPE
133
bodies, one of them bright, the other dark, revolving
round each other (Fig. 33). So long as the dark
body is clear of the bright one, or behind it, the
bright one shines steadily : but if the dark one gets
in front we have an " eclipse " and the light dips.
This imagination or suspicion has been confirmed
Punch's Representation of Algol, " An Astronomical Reprobate."
(See Notes to Illustrations.)
in a remarkable way by the spectroscope; and
from being only a hypothesis has become a serious
addition to our knowledge so serious that it
has been honoured with an illustration in Punch
by that clever artist Mr. E. T. Reed. By kind
permission of the proprietors I am able to show you
the picture.
But what I now want to tell you about is the new
134 A VOYAGE IN SPACE
knowledge we got simply by watching more care-
fully with a new kind of photometer invented by
Mr. Joel Stebbins in America. The principle of it
can easily be illustrated by an experiment. There
is a substance called selenium, which is sensitive to
light, so that when light falls upon it the resistance
alters, and that spot of light on the screen will move-
The selenium is shut up in this box, so that light
shall not disturb it, otherwise the spot would be
moving about instead of remaining steady. But
now we will darken the room and open the box :
so long as the box remains still in the dark the spot
remains steady; but now we shine a taper on the
selenium and you see the spot move off at once.
You will understand that by using this principle,
Mr. Stebbins was able to tell when a star was shining
on his selenium and also how bright the star was :
and though he had many difficulties at first, he got
over them so successfully that in the end the instru-
ment could tell the brightness of the star much
better than the human eye could. I will mention
just one of his difficulties. He found that the instru-
ment behaved differently in warm weather and
cold, so that to get consistent results he determined
to keep it always at the freezing temperature by
packing it in ice. In America there is always plenty
of ice to be got, but it is generally used for cooling
drinks : and when Mr. Stebbins sent in his bill for
all the ice wanted to keep his photometer cool, the
authorities pretended to think that he must have
required a great many drinks. But Mr. Stebbins
was so determined to make his photometer work
that he bore even practical jokes in the good
JOURNEYING BY TELESCOPE 135
cause. And now I want to show you what beautiful
results he obtained. We may represent what the
eye saw by the diagram of Fig. 34. The time
when the light is steady is represented by the
horizontal lines like CD and FG : the eclipses by
the sudden drops ABC and DEF. But the first
thing found by the selenium photometer was that
in the middle of the straight portions (or what had
been thought to be straight portions) there was
another little drop so small that the eye had
overlooked it. What does this mean? The big
drop ABC means
that the dark body
comes in front of
the bright one : the
little drop means
that the bright one
comes in front of
the dark. If it were
entirely dark, that Flg- 34 '
would of course make no difference : and since there
is a noticeable difference, it follows that the second
body cannot be entirely dark, but must be giving off
some light which is cut off when the brighter body
comes in front. This, then, was the first thing found
out ; but there is more to come. Not only is there a
little dip in the middle of each supposed straight
portion, but the remaining parts of those portions
were found to slope slightly in opposite directions,
as shown in Fig. 35. This means that even when
the two bodies are clear of one another, and no
eclipse is taking place, the light is still changing
in some way. Mr. Stebbins was able to give the
136 A VOYAGE IN SPACE
explanation very easily. He pointed out that the
bright body must be shining on the dark one, just
as the Sun shines on our Earth and our Moon : it
will illuminate half of it, and this half will be turned
sometimes towards us and sometimes away from
us, just as the Moon is sometimes full and sometimes
new. This is seen easily enough in the case of the
Moon, but in Algol we cannot see the two bodies
separate from one another, we only see their com-
bined light ; nevertheless by these delicate observa-
tions we can say when one of them is full and when
it is new just as
easily as we can
put the times of
full Moon on the
almanac. Is that
not a wonderful
result of patient
work? And there
is still more wonder
to come. Mr. Stebbins found that he was able
to calculate how large these two bodies were and
how far apart (Fig. 36), and he found that the
bright one must be 240 times as bright as our
Sun, and even the fainter one, which has hitherto
been called " dark," is 16 times as bright as our
Sun ! I think you will agree with me that this is
a wonderful addition to our knowledge ; we have
been able to find out that a star which always ap-
pears just as a speck of light is really made up of
two at a certain distance apart, one very bright
and the other faint, partly shining of itself and
partly being illuminated by the other ; and that
JOURNEYING BY TELESCOPE
137
even the dark side of the fainter body is 16 times
as bright as our Sun. And the chief part of this
knowledge comes, not from using a very large
telescope, but from putting a sensitive apparatus
in place of the eye end.
There is one more thing I must tell you in con-
cluding this lecture. I must answer the question
about Professor Hale's tower how it is kept from
shaking. We could keep a reed from being shaken
by the wind if we enclosed it in a tube : the wind
would blow on the tube and perhaps shake it, but
Fig. 36. Mr. Stebbins's Representation of Algol.
could not get at the reed. Professor Hale's idea
is to build a tower of rods and surround each rod
by a tube, joining all the tubes together to form an
outer tower or casing, on which the wind may blow
and which perhaps may shake, but which keeps
the wind off the inner tower altogether. The
coelcstat is of course attached to the inner tower,
so that it may not be shaken. " The proof of the
pudding is in the eating," and such an idea can
only be justified by trying it. It has been tried
and found completely successful. Professor Hale
finds that even in high winds the image of the Sun
in his big tower telescopo does not shake at all !
The Planet Mars, as drawn by N. E. Green.
(See Notes to Illustrations)
LECTURE IV
VISITS TO THE MOON AND PLANETS
You may perhaps think that we take a long time
getting started, seeing that we have spent three
lectures out of six in discussing where we are to start
from, in what we are to travel, how far we are to go,
and so on. But I find that this is the method ap-
proved by my great predecessors as celestial guides.
Jules Verne, you find, takes a whole book talking
about the start ; how the big gun is to be built
capable of shooting people to the Moon ; how, more-
over, a big telescope is- provided to watch them when
138
VISITS TO THE MOON AND PLANETS 139
they go ; and it is only at the end of the first book
that they really get started.
However, here we are now started ; and if we are
not exactly arrived on Mars, we are as near Mars as
the telescope will take us ; let us say about a thou-
sand times nearer than our eyes can take us. I told
you that the telescope would magnify as much as
you like, but that you might not be satisfied with the
result if you magnified too much. Roughly speak-
ing, the best telescopes we have will magnify about
a thousand times, so that we reduce the distance of
Mars to about one thousandth of the actual distance,
let us say 50,000 or 100,000 miles; and that is as
near as the telescope will take us. One peculiarity
of a telescope is that it usually turns things upside
down, so that the South appears at the top. The
white patch at the top of the picture is therefore
not the North polar cap but the South polar cap. I
wonder whether any one is at present making a South
Polar Expedition in Mars ? It is claimed that the
white patch represents ice and snow, such as we have'
at the poles of our Earth ; others, however, question
whether it can be ice and snow, because certain
observations made with the spectroscope render it
a little doubtful whether there is sufficient water in
the air of Mars to make ice and snow at the Poles.
One thing we know with certainty; that in the
Martian Summer the cap diminishes in size as though
it were ice and snow melting, and in the Winter it
increases in size as though it were ice and snow form-
ing. But, after all, other things besides water may
give this appearance. At the end of the lecture,
when we come to an experiment or two, we shall see
140 A VOYAGE IN SPACE
that liquid air may give a beautiful snowy appear-
ance, and it is possible that the Martian polar caps
are not made of ice and snow but of liquid air or
frozen carbonic acid gas, or some other gas which on
the very cold surface of Mars becomes liquid or even
solid.
There are other markings on the picture, but what
you will probably look for are the famous canals of
Mars. I am afraid there are none shown on this
picture because Mr. N. E. Green, who made it, did not
see any canals. Some people do not see them, and
claim that therefore they cannot be there. One
scarcely knows what to say about this claim : it is
rather like calling witnesses, after others have testi-
fied that they saw a theft committed, to declare that
they did not see the theft. We should probably pay
more attention to the first lot who saw something
and agreed among themselves about it. Never-
theless, we must be careful to listen to the others also,
in case there is more in what they say than might
appear at first. A jury must be very careful before
they find a man guilty, and one thing they must be
specially careful about is not to let their feelings of
affection or dislike influence them. So well known
is it that such feelings may interfere with true justice
that every effort is made to have the jury composed
of people who do not know the prisoner being tried,
because a friend might interfere with justice in one
way or an enemy in the other. Now in this case of
the canals of Mars it is rather harder than usual to
keep the friends and enemies off the jury. Some
people are very anxious to believe that the planets
are inhabited and they are inclined to jump at any-
VISITS TO THE MOON AND PLANETS 141
thing which seems like an indication of life on Mars.
Others are equally anxious that our little Earth
should be the only place in the whole universe with
life upon it ; does not this seem rather selfish ? Yet
a very great and unselfish man, who helped to make
one of the greatest of scientific discoveries, has
written a big book to try and establish this fact
that this little Earth of ours is the only place in the
whole universe on which there is life. All the thou-
sands of millions of stars in the sky may be suns like
ours, and each of them may have many planets
circling round it as we circle round the Sun, and yet
Mr. Alfred Russel Wallace sincerely believes that
there is not a sign of life on any of those thousands
of millions !
You see there are very different views about Mars
and whether it is inhabited ; and if we wish to be a
good jury we must listen to both sides carefully.
But we have not time to do so in this brief hour ; all
we can do is to notice that there is evidence on both
sides, so that we may be careful not to make up our
minds too quickly. If you wish to make up your
mind at all you ought to read a good many books
but it is easier not to make up one's mind at all, and
sometimes that is the best plan. Another plan is to
get some one to make up your mind for you, and per-
haps you hope that I will do it by telling you what
I think. I don't feel sure that that would be a good
plan in any case, but one thing makes it impossible
to act upon, I have not made up my mind myself.
At least I have not made it up about Mars. On the
big question raised by Mr. Wallace, whether our little
Earth is the only place for life, I have made it up ;
142 A VOYAGE IN SPACE
I don't think for a moment that it is. I have read
his book carefully, but cannot see that he makes out
a case for so strange an idea. That there is plenty
of life elsewhere seems to me practically certain ; but
whether Mr. Lowell has really detected signs of it on
Mars is more doubtful. Of one thing, however,
there is no doubt ; he has certainly written some
most fascinating books, which will teach you much
and tell you a story at the same time; I heartily
advise you to read some or all of them. The kind
of story he has to tell is this : Mars has become dried
up, so that it has no great oceans as we have, making
rains which fertilize the ground so that plants may
grow; there is very little water indeed on Mars.
Every one agrees to that. Mr. Lowell, however, says
that there is some water, mostly frozen up in the
polar caps ; when these melt at the edges with the
coming of Spring and Summer, the inhabitants draw
this water off quickly to the thirsty land in long
sluices; vegetation immediately springs up on the
banks of these sluices, making a wide belt of verdure.
Something of the kind may be seen in Egypt when
the Nile floods : the surrounding country, previously
brown and bare, becomes covered with crops. An
observer in the Moon or in Mars might not see the
thin river Nile in the dry season, but after the flood,
when the whole width of the country is green, he
would scarcely fail to notice it. So with Mars : the
" canals " disappear at times ; then, Mr. Lowell says,
there is little or no water in them and the surround-
ing country is bare and dry. The polar cap melts a
little, the water is drawn into the sluices, the crops
spring up on the banks, bordering the narrow sluice
VISITS TO THE MOON AND PLANETS 143
with a margin wide enough for our telescopes on
Earth to see, and they show us a " canal." From
the straightness and arrangement of the sluices thus
indicated Mr. Lowell infers that the} 7 were made by
intelligent beings in desperate need of water; so
that he considers the " canals " a proof of life in
Mars.
It may surprise you that, if there is really any
chance of there being wide tracts of cultivated land,
there should be any difference of opinion on the sub-
ject. Why can some people see them and others
not ? I can easily show you one reason. You see
here a clear picture on the screen, but you seldom
see a clear picture in a telescope because of air
currents. By a little trick the lanternist will imitate
some air currents for us ; and now the picture instead
of being clear and definite is all fuzzy, with the de-
tails blurred ; then perhaps there comes over it now
and then a clear moment, when you glimpse it
clearly ; but almost at once it blurs again, and you
wonder whether perhaps after all you really did get
that glimpse or whether it was a mistake. You
wait for another clear moment, and get the same
glimpse ; that reassures you that you did see it ; but
the best way of removing doubt is for some one else
to see the same thing. That is the way in which
telescope observations are rendered difficult in a bad
climate ; and that is why people go up high moun-
tains, in order to see whether they cannot reduce
the air currents. Even when, like Mr. Percival
Lowell, they build a big telescope in a specially good
climate to study Mars, they can only grasp a few
details at a time; and two pictures made at the
144
A VOYAGE IN SPACE
same time by different people may be very different.
Fig. 37 shows two drawings made by Mr. Percival
Lowell and his assistant on August 14, showing about
what they could see at that time ; and you see that
although they have tried as faithfully as they could
to put down what they saw, Mr. Lowell saw a straight
canal nearly due North and South, while his assistant
saw it slope to the East. When there is difference
of opinion of this kind the only way is to make vast
numbers of drawings, throwing away what only one
MARS \
1896 AUG 14
PERCIVAL LOWELL.
Fig- 37-
man sees, and keeping what everybody sees ; in this
way only can they really make a trustworthy chart
of Mars. You may say : Why not photograph the
planet ? But photographing is even less satisfactory
because the camera cannot always seize good mo-
ments as the eye can; it often photographs at the
wrong moment. By taking many photographs,
however, one after another, we can pick out the good
ones; this plan has only recently been introduced,
but it has given us already some good photographs
of Mars. Fig. 38 shows three taken by Professor
Barnard on which you see the polar caps clearly.
But, after all, we shall much better understand the
VISITS TO THE MOON AND PLANETS 145
difficulty of coming to conclusions about the planets
if now we have got to Mars we take the big tele-
scope that has carried us there and turn it on to
Fig. 38. Mars Photographed with the 4O-in. Lens of
the Yerkes Observatory.
the Earth ; and let us try what we can see of our
Earth from that distance. I propose to arm you
with the best telescope in our possession, and to ask
Fig. 39-
you to look with it at the Earth. Of course I must
provide you artificially with the view you will get ;
and Fig. 39 provides two views of our Earth, differing
in some respects, but both of them better views than
you would have any right to expect if you were really
looking from Mars, even with our very best telescope.
L
146
A VOYAGE IN SPACE
You see the outline of Africa and other well-known
outlines. ; these need not be regarded very closely ;
they are the same in both pictures. The only differ^
ence is in the British Isles. To make these two
pictures, a globe was photographed twice, in as
nearly the same conditions as possible, with one ex-
ception : the British Isles were cut out in paper, in
the first case in their usual shape, in the second case
Fig. 40.
in very much altered shape. You will see the alter-
ations in a moment when I put both pictures on the
screen ; but first I want you to try whether you can
see them from Mars. If such alterations were made
in England, could the Martians armed with our best
telescope detect them at all? Well, you are prac-
tically in that position ; what do you say ? I fancy
you cannot detect much difference. Now let us look
at Fig. 40 on a scale which will show us their
real differences ; you see that our well-known wavy
VISITS TO THE MOON AND PLANETS 147
coastline has been made straight, various islands,
such as Anglesea and the Hebrides have been swept
away, the Thames and Severn have given place to
two straight canals, and there is Home Rule in Ire-
land ! Of all this you as Martians could detect
nothing, even through the best telescope. How,
then, are we to find signs of life in Mars ? You will
perhaps agree with me that it is a very difficult quest.
With other planets we can see less still, either
Fig. 41. Jupiter, drawn by Mr. Scriven Bolton.
(See Notes to Illustrations.)
because they are too near the Sun, like Mercury and
Venus, or because they are even further away than
Mars, like Jupiter and others. Here are two pictures
of Jupiter showing the " red spot " (see Fig. 41).
We learn from it that Jupiter rotates on its axis as
the Earth does, because the red spot goes round : it
crosses the disc, disappears on the other side, and
then reappears again. It comes back to the same
place pretty regularly after about 10 hours, and
other spots on Jupiter, not so large as the red spot
but quite noticeable, also return to their places in
148 A VOYAGE IN SPACE
about 10 hours ; so that this must be about the length
of time in which Jupiter rotates. But now comes
a curious thing, quite different from what we should
expect. If the Martians are looking at the Earth,
they see the markings on the surface go round to the
other side and then reappear again in exactly 24 hours.
Everything comes round in precisely the same time,
England and Ireland and China and Australia, the
mountains, and the seas, all keep their places on the
Earth, just as they do on a globe of the Earth which
I can turn round. If they did not keep their places
we could not learn geography, which would be a ter-
rible distress to us all. But when we look at Jupiter
the spots do not keep their places : some of them
come round faster than others, so that we cannot
make a map which will do for more than a very little
time. It is as though the Martians noticed the posi-
tions, not of England or China or Australia, but of
the ships crossing our oceans. They are scarcely
likely to do this unless they have very much more
powerful telescopes than ours, because you have seen
how very small even a whole country like England
would appear to them, and a ship would be far too
small. But we keep building ships bigger and bigger,
and perhaps the Martians might see them at last ;
and if they watched carefully they would find that
after 24 hours exactly the ship was not quite in the
same place on the Earth's surface. Shall we, then,
say that the spots we see on Jupiter are ships moving
on its oceans ? It seems more probable that they
are cloud effects of some kind ; all we know is that
we are not looking at solidly arranged countries like
our Earth; and if we want to know what we are
VISITS TO THE MOON AND PLANETS 149
looking at, we have some choice of opinion. Most
astronomers would say the spots are cloud effects
high above the real surface of Jupiter, which is
hidden from view.
Another thing I would like you to realize about
Jupiter is his vast size. Here is a model of Jupiter
on the scale of 10,000 miles to the inch, and the
model is about 9 inches across, so that Jupiter him-
self must be 90,000 miles from side to side ; and now
look at our Earth less than an inch across, and our
little Moon less than a quarter of an inch. How
small they are compared with Jupiter ! Why, to go
round Jupiter is a longer voyage than from the Earth
to the Moon !
Another big planet is Saturn, which I hope you
recognize by his ring. In this model we have had to
imitate the ring in solid cardboard, but if the real
ring were solid like this, it would soon fall on to
Saturn, as a great mathematician (Clerk Maxwell)
showed by calculation. He went on to show that
the ring must be made up of millions of little tiny
satellites all separate from one another and revolving
round Saturn at different rates. They are, how-
ever, so close together that we cannot see them
separate even with our best telescopes; conse-
quently we do not see that one travels faster than
another, as we can see one of Jupiter's spots catch
up and pass another ; at any rate we cannot see this
in the ordinary way ; but by a wonderful property of
light, the spectroscope enables us to see it in another
way of which we will say more in another lecture. We
are able to measure the speed with which a shining
body is coming towards us or going away from us ;
150 A VOYAGE IN SPACE
and we find that though we cannot see the satellites
separate, their different speeds declare themselves
in the spectroscope, just as Clerk Maxwell said (see
p. 275). One more point about the ring : it does not
always appear to us in the same aspect ; sometimes
it is open and sometimes it closes up until we see it
edgeways, and then it disappears altogether. This
shows how very thin it must be, and we can infer
that the little satellites composing it must be very
small indeed. You might say they need only be very
flat ; but it is hardly likely that they would keep
their flatness always in the plane of the ring when
they are jostling round at different rates. It seems
most reasonable to think that they are small in all
ways.
Now please look at this diagram showing the dis-
tances of the planets from the Sun. We can only
imitate them on a very small scale, of course. You
remember that our Earth is about 93 million miles
from the Sun ; let us call this 10 inches or 10 milli-
metres or whatever we like. Then the other dis-
tances can be written down as follows
Bode's Law Distance
Mercury . . . . 4 -f o = 4
Venus . . . . 4 + 3 = 7
Earth . . . . 4 + 6 = 10
Mars 4 + 12 = 16
(Minor Planets) . . 4 + 24 = (28)
Jupiter . . . . 4 + 48 = 52
Saturn . . . . 4 + 96 = 100
Uranus . . . . 4 + 192 = 196
Neptune . . . 4 + 384 = 388
VISITS TO THE MOON AND PLANETS 151
These are not the actual distances, but they are so
close to them that the extra convenience of being
able to remember them or to write them down out-
weighs the disadvantage of inaccuracy for many
purposes.
The law which helps us to remember them was
first stated by a man called Titius, and we ought to
call it by his name if every one had his rights ; but
Bode made a sensational use of the law, and so it is
generally known as Bode's Law. You can easily see
what it amounts to : write down a set of 4's, and add
Saturn : photographed by Barnard in 1911, with the 5-foot
Reflector on Mount Wilson.
to them other numbers which are doubled every
time, beginning with 3 for Venus. The easiest way
to remember it is to try and remember that the
Earth's distance comes out 10; and perhaps also
that Saturn is 100 ; from these two facts you could
recover the law if you had forgotten it.
Let us spend a moment or two on the use made of
the law by Bode, about the end of the eighteenth
century. At that time there was a break in the
series, for none of the minor planets had been found.
Nowadays we know nearly 1000 of these little bodies ;
many of them are very tiny, only a few miles in dia-
meter. They are probably made of the same kind
152 A VOYAGE IN SPACE
of stuff as our Earth, and reflect the Sun's light as the
Earth and the other planets do; but being so tiny
they cannot reflect much of it and are consequently
very hard to see. Most of the brightest of them have
probably been found by this time, but every year
more and more faint ones are found, so that we
scarcely know whether there is any limit to their
number. As regards distance from the Sun, they
differ considerably among themselves, but with one
or two rare exceptions they agree to keep within the
gap between Mars and Jupiter. Now in Bode's
time this was a real gap since none of the minor
planets were known, and Bode thought that there
must be a planet to fill the gap. The idea of a num-
ber of little planets doing duty for a big one had not
at that time occurred to any one : it was merely
thought that there was one missing planet, and Bode
set the police on the track of the culprit. This may
sound a strange statement ; what he actually did was
to get a number of astronomers to agree among them-
selves to watch different parts of the sky for the
missing planet ; but the little band of searchers were
jokingly called the " astronomical police," so that
my statement is not far wrong. It must have been
mortifying to them when the culprit was first seen by
an outsider (sometimes that happens to our earthly
police in spite of their vigilance). Another astro-
nomer called Piazzi, who was not looking for the
planet at all, happened to find it ; he watched it for
a few nights to be sure of it, and sent a note of its
position to a brother astronomer one of the "police, "
but the post travelled very slowly in those days when
there were no railways, so that it was a long time
VISITS TO THE MOON AND PLANETS 153
before the other astronomer could look for the
planet ; Piazzi himself had fallen ill, and the planet,
seizing the opportunity (almost like an actual thief
with the crowd after him), dodged into the Sun and
was lost to view. Perhaps you will not understand
rightly what I mean by dodging into the Sun. You
know how the Sun goes the round of the Zodiac :
The Ram, the Bull, the Heavenly Twins, and so on
(have you learnt that verse thoroughly?). As it
visits them, each in turn disappears from view in the
glare of daylight so that you cannot see the Ram in
April, or the Bull in May. Indeed, not only one,
but two or three of them are invisible at any given
time of year. Now a planet moves about among
the constellations : sometimes it is in one that is
visible and sometimes in one that is lost in the Sun's
glare ; and Ceres, as the first-discovered minor planet
was called, passed from one to the other very soon
after the discovery. It seemed as though she had
been found only to be lost again. You may say,
could they v not wait till she came out and catch her
again ? Yes, but sometimes it is very hard to tell
when and where a burglar will come out if once you
let him disappear. He may get on to the roof and
crawl along to other houses, down through them and
out at some back door, when the police are watching
all the time in the wrong place. A planet is not quite
so clever and erratic as a burglar ; indeed, if we have
watched his or her movements long enough, we can
calculate almost exactly where he or she will be at a
future time. That is what Kepler and Newton have
done for us by finding out the great Law of Gravity
which controls the movements. But everything
154 A VOYAGE IN SPACE
depends on that if; if we have watched her long
enough. Ceres had only been watched for a very
little time before Piazzi fell ill, and those who were
accustomed to such calculations said it was not
nearly long enough to enable the orbit to be cal-
culated. You will understand the difficulty perhaps
from Figure 42 : if you have a large portion
ABC of a circle given, it is pretty easy to find the
centre O and draw the rest of the curve (dotted) from
A to C. If the given part is much smaller, like DE,
it is harder to find the centre P and draw the rest,
though it can still be done. But when there is only
a tiny bit like GH given you, you scarcely know
whether the circle should come out like GHK or like
GHL : you may be watching for the burglar to come
out at K when he is really at L. That was the diffi-
culty about Ceres ; she had only shown a very small
bit of her circle before disappearing. Look at the
minute spaces on your watch there are 60 of them
in the complete circle. If you had one of them
drawn on a piece of paper, and one only, and tried
from that to draw the whole watch dial, you would
find it pretty hard ; and Ceres did not even give the
astronomers a whole space she gave them less than
VISITS TO THE MOON AND PLANETS 155
half a space ! You will agree with me, I feel sure,
that the man who showed them how to draw the
whole dial in such circumstances was a remarkably
clever man a wonderful mathematician. His great
book, in which he explained the method, his Theoria
Motus, is one of those books which are likely to
last as long as the world does. The little planet
was found again, and to the astonishment of the
world, another was also found in the search : and
then two more four burglars when the police had
never suspected more than one ! However, they
were all securely handcuffed and tethered that is to
say, the mathematicians calculated their orbits very
carefully, so as to know exactly where to find them
when wanted and the world settled down again after
the excitement of the chase. I forgot to tell you
that Ceres was first seen on the very first day of the
nineteenth century, January i, 1801 ; and the other
three, Vesta, Pallas and Juno, were all found by
March 1807. After that no more were found for
forty years, and Hencke, who found the next, had
been looking for fifteen years before he found it.
Is that not strange when, as we know now, there are
hundreds of these little planets in the sky? It
shows the difficulties of finding them in the days
before photography; since we have been able to
take photographs, discovery is much easier; astro-
nomers merely take a photograph of part of the sky,
showing all the stars as fixed points, but if one of
them seems to have moved or " trailed " during the
exposure it is probably a planet.
But we must return to the more important planets,
as I feel sure you cannot afford much time to visit
156 A VOYAGE IN SPACE
these tiny bits of rock. It is quite possible that they
have their inhabitants on a small scale ; they may be
like dolls' houses, which are always most attractive ;
but we have to make calls at more important dwell-
ings. The main point of our visit was to notice how
the gap in Bode's Law was filled, and how it came in
consequence to be called Bode's Law ; and the next
two big planets in the list have also something to
say about filling vacant places. There were no more
gaps to fill in the middle of the series, but we can
have any number of places at the end. Before
Uranus and Neptune were discovered the series
stopped short at Saturn, with 4 + 96 = 100 ; and
it naturally seemed to confirm the law when Uranus
was found by Herschel, and its distance was seen to
agree closely with the next term, 4 + 192 = 196.
This had a good deal to do with Bode's formation of
" the Astronomical Police."
I have already told you of Herschel' s discovery of
Uranus, when we were talking about his telescope.
He thought at first he had found a comet, and it was
not until after some time that the true nature of the
discovery became clear. Then it created an im-
mense sensation, for never before in the memory of
man had any one found a new planet. All those
previously known had been known for ages; they
give their names to the days of the week, Satur(n)-
day, Sun-day, Mo(o)n-day are called after Saturn,
the Sun, the Moon. Our English names for the other
days do not remind us of the planets, but the French
names (Mar-di for Mars, Mercre-di for Mercury, Jeudi
for Jupiter, Vendredi for Venus) are probably known
to you. We have for some reason put instead of
VISITS TO THE MOON AND PLANETS 157
Mars, who was the Roman god of war, the corre-
sponding Scandinavian god of war Tuya, leading to
Tuesday ; and for Venus, the Roman goddess of love
and beauty, we have substituted the Scandinavian
Freya, leading to Friday. Perhaps as I have men-
tioned these days of the week I may as well explain
the curious order in which they come.
The ancients did not know the distances of the
planets away from us or from the Sun, but they could
SUN
MERCURY
SATURN
Fig. 43-
easily observe which of them moved most slowly
and which most quickly. They put them in the
order of speed, beginning with Saturn, the slowest,
then Jupiter, Mars, the Sun, Venus, Mercury, and the
Moon, which moves quickest of all (see Fig. 43).
The old astrologers then assigned each planet in-
fluence for an hour; Saturn started with the first
hour of his day, followed by Jupiter for the second
hour, Mars for the third, and so on. When all seven
planets had been used up, Saturn's turn came round
again, and in the first 21 hours of the 24 all the planets
158 A VOYAGE IN SPACE
would get 3 turns each. Then Saturn, Jupiter, Mars
would finish up the day, and the Sun's turn would
come in the first hour of the next day. In Fig. 43
let us show that the Sun follows Saturn by joining
up Saturn to the Sun by a line, missing two planets
(Jupiter and Mars). Then to find what planet will
rule the next day we miss Venus and Mercury and
join to the Moon : miss two and join to Mars, and so
on. If you follow round the star in the direction of
the arrows you will find the days of the week in the
right order, Saturnday, Sunday, Moonday, Mardi,
Merer edi, Jeudi, Vendredi. I have begun with
Saturday to make the explanation simpler; but
you can see that one may begin anywhere ; and the
Sun is so important that they began the week with
him.
Of course, when Uranus was found they could not
upset so well-established an order as the days of the
week to make room for him in it ; and a very good
thing it is they did not try, because there would have
been another upset half-a-century later when Nep-
tune was found.
That was a discovery even more remarkable than
the discovery of Uranus, because it was made by
calculation. We have already said that when a
planet has been watched long enough, we can calcu-
late just where it will be in the future ; we must take
into account, not only the attraction of the Sun,
but that of other planets as well, since they are
all pulling at one another. Though this is hard
and toilsome work it can be done, and was done for
the planet Uranus when it had been watched long
enough. But the curious thing was that Uranus did
VISITS TO THE MOON AND PLANETS 159
not follow the track calculated for it, and ultimately
two very clever men, Adams, an Englishman, and
Leverrier, a Frenchman, calculated that there must
be a yet undiscovered planet pulling it out of the
place; they independently calculated whereabouts
this unknown planet must be,
and in that position it was
actually found.
One thing I would like you
to realize is the very small
indication they had to go
J . , N? i. EXPECTED PLACE
Upon. YOU knOW Stories Of cannot be distinguished
. , , - . from REAL PLACE.
scouts who have been able to
track their prey by small in-
dications, a bent twig here, a To ^ eSUN
I
footprint there indications N? 2= 50 Times N? i.
which our untrained eyes MJ?SSf.
would pass unnoticed ? Well,
the indications left by Uranus REAL X PLACE
in his journey were small and TO the SUN EXPECTED
faint like those, so that very N? 3* so rimes N? 2 PLACE
clever scouts were required to =2 < 500Ti s n K
.. . , _ We coln ut last see hon
read them properly. Suppose the two places oiitfer
TT -,-,. ,! . From this tiny difference
Uranus travelling in the cir- Neptune w*s discovered
cular path at the top of Fig. Fi
44 ; let us put the real Uranus
and the Uranus-as-it-might-be (undisturbed) both in
their places ; then you could not see the difference
between them at all. More than that : suppose you
magnify that circle on the screen 50 times, so that
it becomes a big circle nearly the size of the room.
We shall only have space on the screen for a little
bit of it, which looks almost straight, owing to the
160 A VOYAGE IN SPACE
slow curvature. Even now you could not distinguish
the real Uranus from the theoretical one in the
calculated place. The disturbance can only be seen
when that again is magnified 50 times ; then you
can just see the difference between the real and ex-
pected places. That small difference in the picture,
diminished 2500 times, was all that Adams and
Leverrier had to go upon, but from that tiny
discrepancy they were able to infer the existence
of the mighty planet Neptune.
Another point worthy of notice is that Bode's
Law had by this time taken so firm a place in men's
minds that both Adams and Leverrier used it to help
them in their calculations. This is scarcely sur-
prising when we remember that, first of all, Uranus
had been found to fit in with the law; and that,
secondly, the gap had been filled by the minor planets.
But unfortunately this law, which they thought
would be a help, was only a hindrance, for it no
longer held. You may at some time or other have
been going down a dark staircase, perhaps in some
old tower, and the steps have been so even for a long
time that you think you know just how far down to
put your foot for the next, when suddenly there
comes a short step or an extra long step which gives
you quite a shock. It is often like that in scientific
work : you think you have found out some law which
will enable you to set your foot confidently on the
next step, but owing to some unknown cause the
next step is of a different length and you get a shock.
Sometimes you can find out the reason for the excep-
tion, which soon leads to a discovery; sometimes
the reason is not found for a long time. We do not
VISITS TO THE MOON AND PLANETS 161
yet know why the steps in Bode's Law are so nearly
regular up to Uranus and why there is then a short
step to Neptune, but so it is ; and both Adams and
Leverrier, who confidently put out their feet for a
step of the usual length, got a jar in consequence.
It was not enough to make them miss the step alto-
gether; in other words, they found Neptune all
right; but the stumble was so obvious that it ex-
cited many remarks. Some people even went so
far as to say that they had not found Neptune at all,
but that the discovery was made by accident ! It
would take too long to explain the full meaning of
this criticism ; you may like to read all about it some
day, especially if you like mathematics. But before
leaving the story of this great discovery I should like
just to tell you how it came to be connected with the
names of two different people.
J. C. Adams was quite a young man who had just
taken his B.A. degree at Cambridge when he carried
out his resolution of calculating where the planet
must be that was disturbing Uranus. He finished
the calculations, and took them to the Professor
of Astronomy in Cambridge. Unfortunately that
particular professor did not happen to be very clever ;
sometimes there are professors who are not very
clever, or are lazy, though you might not believe it,
and it is hard luck on the students. However, this
professor had enough sense to suggest that Adams
should get help from some one else, and he recom-
mended him to go to the Astronomer Royal (Airy)
at Greenwich, and then he thought that he had done
everything that could be reasonably expected of
him and went peacefully to sleep. At any rate he
M
162 A VOYAGE IN SPACE
seems never to have enquired whether Adams went
to Greenwich, or what happened to him there. He
did go to Greenwich, but the Astronomer Royal was
away in London on Government business; Adams
called again later, but he was at dinner, and his faith-
ful servants would not disturb him. The poor young
man was getting a little discouraged, but he left a
note of his results for the Astronomer Royal to look
at after dinner. " According to my calculations,"
it read, " the observed irregularities in the move-
ments of the planet Uranus may be accounted for by
supposing the existence of an exterior body, the orbit
of which is as follows." This note has been preserved
and bears the date " October 1845 " in the hand-
writing of the Astronomer Royal ; for Adams himself
put no date at all on this most important document,
and Airy must have supplied it later on when he had
already forgotten the exact day ; so that we may
judge he did not pay it very much attention at
the time. But he did reply to Adams, asking him
what he regarded as a test question. Adams got
the impression that his careful calculations were mis-
trusted, and was so disappointed and heartbroken
after all his work that he did not reply; and the
whole incident dropped. It was the very greatest
pity ; a little more persistence on the part of any of
these three men would have almost certainly led to the
discovery of Neptune at once. Some one like Halley
was wanted to keep them up to the mark; but no
successor to Halley appeared, and the great chance
was lost.
Meanwhile Leverrier had begun work on the same
problem. He was already a famous astronomer and
VISITS TO THE MOON AND PLANETS 163
published his results in a series of famous papers,
ending up by pointing to the place near which Nep-
tune must be. Now he pointed to very nearly the
same place in the sky
as Adams; as Airy
found on comparing
Leverrier's paper with
the note he had re-
ceived from Adams
nearly a year before;
and this made Airy
think there must be
enough chance of find-
ing the planet to be
worth trying. So he
woke up the professor
at Cambridge and told
him to begin search-
ing. The professor
set to work, but was
still half asleep, so
that though he looked
straight at Neptune
more than once, he
did not recognize it.
Meanwhile Leverrier
had set Galle, a Ger-
man astronomer, to
work (it is very curious
how all these people set other folk to work instead of
looking themselves, as they might easily have done),
and Galle found the planet on the first night ! You
can well imagine that there was then a great fuss
The statue of Leverrier at the
Paris Observatory.
164
A VOYAGE IN SPACE
as to whose planet it really was ; everybody blamed
somebody else, except Adams, who never blamed
any one but himself for being too shy and easily
discouraged. We need not follow the story further
now than to say that it was at last honourably
agreed to share the merit of the discovery equally
between Adams and Leverrier. But I scarcely
think we English
have done enough
public honour to
the part played by
Adams ; there is
indeed a plaque of
his head in West-
minster Abbey;
but in the centre
of the courtyard of
the Paris Observa-
tory there is a fine
big statue of Lever-
The plaque of Adams in
Westminster Abbey.
rier, with head
erect, pointing
with his finger at
" They order this
a globe representing Neptune,
matter better in France."
We must now turn from the planets to their satel-
lites. In one of the books which I mentioned at the
outset, Mr. Griffith's Honeymoon in Space, it is
suggested that instead of landing on Jupiter, which
may still be so hot as to burn our feet, we should land
on one of his satellites. We are not yet sure how
many he has, but we know that he has at least eight. 1
1 A ninth satellite to Jupiter was discovered by Mr.
Seth B. Nicholson in July 1914.
VISITS TO THE MOON AND PLANETS 165
Four of them can easily be seen with quite a small
telescope, and were seen by Galileo with the first
telescope ever turned to the skies. The other four
are very faint and were not discovered until recently.
Just as Jupiter is so much bigger than our Earth, so
is his system of satellites on a much grander scale
than ours. Our model of the little Earth has our
single Moon at a distance of 2 feet ; but though one
or two of Jupiter's satellites are about as near as
this, others would be outside this lecture hall on the
same scale, and one of them half-way down Albe-
marle Street !
Saturn has also a number of separate satellites
besides the great crowd of little tiny ones which make
up the ring. We know of nine or ten already (the
tenth has been announced, but has not yet been
satisfactorily identified), and they also are scattered
to great distances from the planet himself.
If we chose to land on one of these satellites we
should be liable to an experience which is quite un-
familiar on our Earth that of a long eclipse of the
Sun, when day would be turned into night. It is
just possible to have a total eclipse of the Sun even
on our Earth, but only for a few minutes. The Moon
is not large enough to cover up the Sun for very long.
Perhaps you have read an exciting book called King
Solomon's Mines? When Sir Rider Haggard first
wrote it, he introduced a total eclipse of the Sun
which was quite impossibly long; it lasted (if I re-
remember rightly) several hours, and the darkness
was so great that the people could only grope their
way about, being quite unable to see " their hands
before their faces." Somebody must have told him
this was all quite a mistake, because in later editions
166 A VOYAGE IN SPACE
of the book it was cut out. It has been my duty to
observe a number of total eclipses of the Sun, but
they never last more than a few minutes, and the
darkness is not so great but that one can read a watch
face. However, if we landed on one of Jupiter's
satellites, we could see such an eclipse as Sir Rider
Haggard describes; for instead of having only our
little Moon to act as a screen between us and the
Sun (see diagram on p. 232), we should have great big
Jupiter, and he would cut off the light for a long
time. We can imitate what would happen by using
a model. The Sun's light is represented by a beam
from the electric lantern, which lights up one side
of the model and leaves the other dark. On the
bright side it is day on Jupiter, on the dark side it is
night. But the darkness does not merely affect the
surface; it streams away in a cylinder or cone of
shadow. Now I will take a billiard ball to represent
a satellite, holding it by a string. While it is any-
where outside this cone of shadow it is illuminated
by the beam much as Jupiter itself is ; one side of
it is bright and has daytime, the other side is dark
and is having night time. But as I circulate it round
Jupiter, it comes into this cone of shadow ; the light
of the beam is cut off, and there is a total eclipse,
which lasts all the time the satellite is passing
through the shadow until it emerges on the other
side. Some of you may not be able to see this
emergence because Jupiter blocks your view; but
those in a different part of the room can see it quite
well, and so could the others if they changed their
position. In looking at the real Jupiter, we on
Earth are constantly changing our position as the
VISITS TO THE MOON AND PLANETS 167
Earth travels round the Sun; hence we can some-
times see the emergence and sometimes Jupiter
blocks the way. You see the two kinds of block-
ing : he blocks the sunlight in one case, to make
the eclipse ; but he may also block the view from
the Earth, if the Earth lies nearly in the same
direction as the Sun.
There are two different cases in which the Earth
may lie nearly in this direction; it may be on the
near side of the Sun or on the further side; and
because of the difference between the two cases a
very important discovery was made, viz. that
light takes time to travel. We know that sound
takes time to travel because of echoes; we shout
" Jack! " and after 'a perceptible interval a faint
shout " Jack ! " comes back to us from a distant
wall or hill. If we measure the interval between
shout and echo, and also measure the distance of
the wall from us, we can find how quickly sound
travels about noo feet per second. Perhaps you
have recognized this fact in another way; when
there is a thunderstorm, we first see the lightning
and we hear the thunder later. If we count the
number of seconds between the flash and the first
clap of thunder, and multiply by noo we can
find how many feet away the flash took place. (As
regards the rest of the thunder I suppose it is made
up of echoes, partly at any rate.)
Now, has it ever occurred to you that the strike
of a church clock never gives you the exact time
unless you are close to it ? If you are noo feet away
you will not hear the strike until a whole second after
it has occurred ; if you are a mile away, you will hear
168 A VOYAGE IN SPACE
the clock nearly 5 seconds wrong ! Many people set
their watches by a big clock striking without think-
ing of this : perhaps 5 seconds is not very important
to them. But now suppose a man had two houses,
his dwelling and his office, let us say, one three miles
away from the city clock, and the other only two
miles away; and suppose his watch and the city
clock were both keeping perfect time. Nevertheless
when he was at home he would always think his
watch was 15 seconds too fast, and when at his office
he would think it was only 10 seconds too fast. The
first time he noticed the difference, he might think
his watch had gone wrong ; but if he went on noticing
he would soon find out that the watch was all right
and that the discrepancy must be due to something
else ; and he would perhaps find out the real reason,
namely, that the extra mile between house and office
made the difference of 5 seconds because sound took
that time to travel one mile.
In this kind of way it was found that light takes
time to travel. For the striking clock we substitute
an eclipse of one of Jupiter's satellites as seen from
our Earth. For the dwelling and the office we sub-
stitute the two positions of the Earth, on opposite
sides of the Sun, one far from the eclipse, the other
nearer. The great Danish astronomer Roemer cal-
culated times for the eclipses (which we may regard
as his watch) ; and when he compared them with the
observed times (which we may regard as the clock)
he got a regular difference according as he was on
the hither side (office) or the further side (dwelling)
of the Sun, and he reasoned that light must take time
to travel. Compared with sound, it travels fearfully
VISITS TO THE MOON AND PLANETS 169
quick, no less than 188,000 miles a second, so that
you might think the difference would be very small.
But you must remember that instead of the single
mile between dwelling and office, we have now the
enormous distance between one side of the Earth's
orbit and the other, about 186,000,000 miles, or
nearly one thousand times the distance travelled
by light in one second. Hence Roemer found a
difference of nearly 1000 seconds or about 16 minutes,
and in this way the velocity of light was found for
the first time.
Now, you may say, that is very interesting as a
story, but you do not see any particular good can
come of knowing that light takes time to travel;
such a piece of knowledge is quite remote from our
practical everyday life. But very often scientific
discoveries of this kind lead to practical results of
immense importance. The great man whose statue
is. in the entrance hall, Michael Faraday, made dis-
coveries as to the behaviour of little bits of wire and
glass which seemed equally unpractical; and yet
they led to our electric railways, and motor-cars, and
aeroplanes, which could never have existed but for
Faraday's discoveries made with little bits of wire
in this Royal Institution. Another great man follow-
ing Faraday, James Clerk Maxwell, noticed that the
velocity of light, first detected by Roemer, was
the same as a measurement made with the electro-
magnetic apparatus due to Faraday; and he con-
cluded that light was electro-magnetic in its action.
And then another great man, Hertz, said, " If light
consists of waves, and is electro-magnetic, then we
ought to be able to get electric waves," and he found
170 A VOYAGE IN SPACE
out that he could ; and his discoveries made wireless
telegraphy possible. And so you see a practical
discovery which you probably regard as one of the
most wonderful of modern times can be traced back
step by step to this discovery of Roemer's which
seems entirely unpractical !
But we are getting rather away from the satellites
themselves, in this talk about their eclipses, and I
want us to think for a few minutes what a satellite
really is. Why should there be these moons, these
smaller bodies revolving round the planets? And
indeed why should there be planets revolving round
the Sun ? For the same sort of answer will do for both
these questions. The planets are satellites of the
Sun just as our Moon is a satellite of the Earth and
Jupiter's eight moons are his satellites ; in all these
cases we have reason to think that parts of the cen-
tral body have become detached from it to form
satellites. First of all remember that all these
bodies are rotating turning round on their axes.
We proved that the Earth was rotating by the pen-
dulum experiment in the first lecture ; we can see
Jupiter rotating by noticing his red spot ; we can see
the Sun rotating by means of sunspots, as we shall
remark in the next lecture. Now, when a body is
rotating, the outermost parts tend to fly off, and if
they can be detached they will fly off. When a
boatman twirls a wet mop, drops of water fly out
in all directions. There are several pretty pieces of
apparatus in the stores of this Institution which
illustrate this principle. In Fig. 45 a weight A
slides on a wire BC. A string DEFG attaches it to a
hanging weight G. When everything is at rest the
VISITS TO THE MOON AND PLANETS
171
hanging weight G drops to its lowest point, pulling
the slider A towards the centre E. But now let us
spin the apparatus about the vertical axis FK.
The slider will tend to fly away from the axis and
will pull up the weight G, unless we make G very
heavy. If G is heavy enough it will hold A near
the centre ; but if it is not heavy enough A will fly
F
o
A FLIES OUT
WHEN WHIRLED
AimD
3
WEIGHT PULLED
UP BY A
WHIRLING WHEEL
Fig. 45-
out. Whether G is heavy enough depends on how
fast we spin the apparatus. If we spin it slowly,
G will be strong enough to hold A in close ; but as
we spin faster and faster, A tries harder and harder
to fly out from the centre and is at last too much for
G. This is very much what happens in the forma-
tion of a satellite. If the parent body is spinning
slowly, its gravity or attraction (which we have
represented by the weight G and the string) will hold
172
A VOYAGE IN SPACE
all its parts in close together ; but if for any reason
the spin is made faster and faster, there will come a
point at which some of the outermost portions will
have a tendency to fly out too great for the gravity
or attraction, and a satellite will be detached. We
have still to explain why the spin should get faster
and faster, and we will come back to that in a mo-
ment; but as we have the turn-table here, let us
AT REST
Fig. 46.
SPINNING
Fig. 47-
first do one or two more experiments. In Fig. 46
is a hoop, or rather a pair of hoops, which when at
rest are circular, but flatten down as in Fig. 47 when
they are spun, because the outer portions AB tend to
fly away from the axis ; you can easily see that in
the second figure they are farther from the axis in the
positions ab than in the first figure. The hoops are
strong enough to hold together however fast we spin
them in this case; but you must remember that I
am not very strong and cannot spin very fast. If
VISITS TO THE MOON AND PLANETS 173
we set machinery to work and buzzed the hoops
round at a terrific pace we could make the portions
ab break away and fly off. It is only a question of
speed to break even the strongest flywheel.
Here is another
/\ j
experiment, which Q C o
shows us something a \ I
else. We generally
say that the satel-
lites go round the . .
planets, as though J- V
the planets them-
selves did not Fig>48>
move. But this is not strictly true. The Earth
and Moon are like a pair of partners waltzing, one
partner being very much heavier than the other.
The little partner almost flies round the big one, but
the big one has to move a little. Here is a big ball
A and a little one
Q B tied together
* I U HI 5 (Fig. 48), and both
A
are sliding on the
same wire. If we
put them at equal
distances from the
centre C and then
spin the apparatus,
the big ball immediately flies to the end a, pulling
the little one with it. But if we put A at the
centre (Fig. 49) and spin again, the little one flies
to its end b and pulls the big one. If we are to
get a proper balance we must put the big one,
not at the centre, but certainly nearer than the
.
i 7 4
A VOYAGE IN SPACE
other ; and if we try once or twice we can find a posi-
tion when they are just balanced, so that neither
pulls the other. It is quite a nice game to balance
them. Now I think we have them. The little one
is doing most of the revolving, but the big one is by
CLOTHES
PEG.
P
WEIGHTS (to W.) EXTENDED
Fig. 50.
no means at rest, you see. So you must remember
that our big Earth is itself waltzing a little with the
Moon, though the Moon does most of the dancing.
Our next spinning experiment shall be a rough
imitation of the detaching of a satellite. It requires
some delicate adjustment, and you owe your thanks
to Mr. Green for making us this mixture of alcohol
and water, so carefully made that a little oil will
VISITS TO THE MOON AND PLANETS 175
collect on the little table in the middle without
wanting to go either up or down. Now Mr. Heath
turns the table so that the oil is made to spin ; and
at once you see it flatten out as the hoops did. Now
he turns it a little faster and yes ! there you see
a drop of the oil flies off to form a satellite !
And so all we have now to consider is why a
planet should spin quicker. It was easy for us to
make the oil spin quicker ; all we had to do was to
ask Mr. Heath to turn the handle a little quicker.
But what turns the handle for a planet ? Well, the
simple reason is that the planet is continually shrink-
ing with the cold, and as it shrinks it automatically
spins quicker. We will illustrate that by experi-
ment. In Fig. 50 two wooden bars, CA, CB, are
loosely hinged at C. A string APB, passing between
the jaws of a spring clothes-peg at P, holds the bars
horizontal; and the whole is hung by a string and
can be set spinning round it. To make the experi-
ment more striking, there are two leaden weights at
A and B. Now if the spring of the clothes-peg be
nipped, the string is released and the ends A and B
drop into the vertical position (Fig. 51). This is
our way of representing the shrinking of the planet.
We might make the bars CA and CB actually
shorten themselves, but this is not easy to do, and
so we make the weights A and B come near the axis
in a different way, by dropping them downwards;
the main point is that in the first position they are
far out from the axis and in the second they are close
to it. In the case of a planet, the shrinking may
take millions of years ; but to save time, we do it in
a fraction of a second. Now I will spin the apparatus
176 A VOYAGE IN SPACE
slowly with the bars extended. Without interfering
with the spin, I now nip the clothes-peg and the
weights drop, when you see that the spin immediately
becomes much more rapid. Shrinking towards the
axis quickens up the spin just as well as telling Mr.
CLOTHES
PEG
r j
x 1 ^EIGHTS
DROPPED.
Fig- 51.
Heath to turn the handle quicker ; and so we get our
satellites formed because the planets shrink as they
cool. There is another and perhaps a better way of
making this experiment, using human arms instead
of wooden ones. We place the human being on a
turn-table (made to turn very easily by ball-
bearings) with arms extended and holding a pair
of dumb-bells (Fig. 52), and then give her a gentle
VISITS TO THE MOON AND PLANETS 177
rotation. If she drops her arms (Fig. 53) she will
at once spin much quicker.
I must now tell you about a very extraordinary
discovery of quite recent times. I don't know
whether you noticed that when we made a satellite
Fig. 52.
Fig- 53-
by spinning the oil, it went sailing round its planet
in the same direction as the spin; and this seems
natural. We should expect a planet spinning with
the arrows (Fig. 54) to have a satellite like A,
revolving in the same direction and not like B, re-
volving in the opposite direction ; and yet, to our
great surprise, such satellites have been discovered.
178 A VOYAGE IN SPACE
The story begins with the finding of Phoebe,
the ninth satellite of Saturn, by Professor W. H.
Pickering, of Harvard Observatory. He found it
by examining photographs of Saturn, with the stars
all round it ; and as he was able to identify it on a
number of such photographs taken on different
days, he calculated an orbit for it so as to be able to
find it again when wanted. You remember the case
of the little planet Ceres, which had been observed
for so short a time that astronomers despaired of
finding it again, until Gauss showed them how to
calculate the orbit even from a very few days' observ-
ations? Professor Pickering was luckier than
Piazzi; he had quite enough observations to cal-
culate the orbit so he thought; but the trouble
was that when he looked for the little satellite in its
expected place (after the Sun had hidden it, as it
hid Ceres) he could not find it anywhere ! This was
very puzzling indeed. Of course the little satellite
might have been destroyed in the meantime by some
unknown agency, but this did not seem likely; it
seemed much more likely that he had made a
mistake, and so indeed he had the mistake of
thinking that the satellite was travelling round
Saturn in the usual way, like satellite A in Fig. 54.
It is really travelling like satellite B, and when Pro-
fessor Pickering persuaded himself, very reluctantly,
to try this other supposition, he, to his great delight,
found the little satellite in its calculated place. You
may wonder why he made any supposition at all when
he had actual observations to go upon; it would
take too long to explain it here, but I may remind
you that both Adams and Leverrier, in their cal-
VISITS TO THE MOON AND PLANETS 179
dilations to find Neptune, made the supposition
that Bode's Law would go on in regular steps, and
they too got led astray by this supposition. Suppo-
sitions are often made in such work, in order to
simplify the calculations ; but it would appear from
these two cases far better to do without them if we
can.
This discovery of Phoebe ultimately brought
Fig. 54-
about an interesting situation. All the discoveries
of satellites were at first made by other nations
than England or America. Galileo started by
finding four satellites of Jupiter, and it was not
until nine in all had been discovered that anything
was done in England. Once having started, how-
ever, England had the credit of seven out of the next
eight, the other one falling to America. Then America
continued with two satellites of Mars in 1877, a
satellite of Jupiter in 1892, and Professor Pickering's
i8o
A VOYAGE IN SPACE
satellite of Saturn in 1899. Stimulated by this last,
Professor Perrine, of the Lick Observatory, gave
America the credit of two more satellites to Jupiter
in 1904, making the score England 7, America 7, as
you will see by the following table.
DISCOVERIES OF SATELLITES
Date.
Mars.
Jupiter.
Saturn.
Uranus. 'Neptune.
To 1685
4
5
Foreign
1787)
I789/
2
2
England
1848
I
America
1846)
1851 f
2
i
England
1877
2
America
1892
I
America
1899
I
America
1904
2
America
1908
I
England
I9I4 1
I
America
2
9
9
4
i
Score: Foreigners 9; England 8; America 8.
Now, who was to kick the next goal ? I am glad
to say that Mr. Melotte, of the Royal Observatory
at Greenwich, scored for England by finding an
eighth satellite to Jupiter in 1908. The Astronomer
Royal has kindly lent for your inspection this beau-
tiful model of Jupiter and his eight satellite orbits.
The four found by Galileo are comparatively close
to Jupiter, but the sharp eyes of Professor Barnard
found one inside them a very faint one in 1892.
1 The discovery of Mr. Nicholson in 1914 again brings the
score level.
VISITS TO THE MOON AND PLANETS 181
The other three are well away from these ; Mr. Per-
rine's sixth and seventh are at nearly the same
distance, and may be regarded as a twin pair. Then
comes this tangle of wires far outside everything else,
looking like a whole lot of orbits ; but it is really only
one orbit that of Mr. Melotte's eighth satellite.
The fact is that this satellite is so far from Jupiter
that the Sun gets quite an undue pull upon it ; it is
like the man in the Bible who was trying to serve
two masters, the result being unsatisfactory, as you
remember. But it is even more important to notice
that, like Phcebe, it is going round its planet in the
wrong direction. We now know of three l such
satellites ; that of Neptune has been known to go
round " retrograde " for a number of years, but as
it was the single exception to the usual rule no one
paid much attention to it. When, however, Pro-
fessor Pickering found Phcebe, and especially when
this going round in the wrong direction had been such
a trouble to him, he looked for some explanation;
and the one he suggested he illustrated by an ex-
periment with a gyroscope, which we will repeat
here. I have a gyroscope mounted in a wooden
frame which I can grip firmly with both hands, or
one of you can perhaps hold it after it has been set
spinning. Now you see when I turn slowly in this
direction (which is the same as that in which the
gyroscope is spinning) nothing much happens ; but
if I turn round in the opposite direction, the gyro-
scope turns upside down. Try it yourself : you
1 Since these lectures were given, a ninth satellite to
Jupiter, also going round the wrong way, nas been found
in America. This makes four retrograde satellites.
182 A VOYAGE IN SPACE
must grip the frame firmly, because when the gyro-
scope turns over it gives you quite a shock and you
must be careful not to drop it. The reason why it
turns over may be put in this way : that it insists
on spinning in the same direction in which the holder
is turning. If he turns in the opposite direction,
then the somersault of the gyroscope will practically
make it spin in the other direction. You can verify
this for yourself by turning your watch face down-
wards and imagining that you can see the hands
through the back ; you would see them moving
contrary to their usual direction. People who go
to the Southern hemisphere find the Sun going
round the wrong way; that is because they have
themselves turned upside down, pointing their feet
in the direction of the heads of northern folk; and
it would be the same if they remained at home, but
the Earth turned a somersault.
Suppose now that the Earth did turn a somer-
sault; the Moon would be going round us in the
direction opposite to our rotation, like Phcebe does
round Saturn. If, now, the Earth flung off a new
moon, we should have two moons, going round in
opposite directions. We explain the existence of
Phcebe in this way : she is Saturn's eldest child, born
at a time when he was turning on his axis in the
direction which Phcebe now indicates. Subsequently
he turned a somersault, and the other eight or nine
children, all born after the somersault, naturally
adopt the new direction. The only thing still to be
explained is what made him turn the somersault,
and Mr. Stratton has found a sufficient explanation
of this in the action of the tides. We must therefore
VISITS TO THE MOON AND PLANETS 183
suppose that our tides are tending to turn our Earth
over again, though the action is so slight and so slow
that we cannot detect it by direct observation.
The mention of our own tides reminds us that we
have shamefully neglected our own Moon. We have
been visiting other planets' satellites and neglecting
our own child all the time. Jules Verne and H. G.
Wells were very different : they spent all their atten-
tion on the Moon. Let us look first at some pictures
of the Moon's surface, which is very mountainous.
Now-a-days we can get such pictures by photography
very easily, but before we had this great help it took
a long time to make an accurate picture. A little
more than a century ago a great artist, John Russell,
R.A., spent about twenty years making a careful
drawing which is still preserved at the Radcliffe
Observatory at Oxford; we can now get as good
a picture by photography in a second or less. Per-
haps some of the fine detail would not be as good, for
the human eye still beats the photograph in drawing
fine detail ; but the more conspicuous features would
be more faithfully in their exact places. A photo-
graph is wonderfully accurate and faithful, as can be
proved by taking two photographs with different
telescopes say one in Paris and one in Chicago
and comparing the results. A great English astron-
omer whom we have recently lost, Mr. Saunder,
had the most careful measures made on two such
pictures and showed that they agreed extraordin-
arily well. Not only that, but he mapped out the
surface of the Moon so accurately that in some
ways we know it better than that of our own Earth.
He gave one very striking proof of the accuracy of
184 A VOYAGE IN SPACE
his knowledge. The photographs sent to him all
had marked on them the date and time when they
were taken, but Mr. Saunder was able to say that in
one case the wrong month had been given and in
another case the wrong day ! You can imagine that
the astronomer who had taken the photographs did
not like being told that he had made mistakes of that
kind, and at first he was inclined to dispute it ; but
Mr. Saunder's case was too strong for him. Now, I
want you to realize what this means : it is not as
though anything were in a different place on the
Moon itself from one day to another; so far as we
can judge everything that we can see remains per-
fectly steady; otherwise Mr. Saunder could not
make two different photographs fit. But on differ-
ent days the Earth looks at the Moon from a slightly
different angle, which can be allowed for if you know
the correct date. If the wrong date is given, the
wrong allowance is made and the measures will not
fit those of other photographs. This was what
Mr. Saunder found ; but I need scarcely say that if
his work had not been wonderfully accurate, he
could not have found it out ; and even as it was, it
took him several days' hard work at his calculations,
for all such work involves a great deal of arithmetic.
When we make a map of the Earth we may put in
the places of the mountains without saying how high
they are ; but the best maps tell you the heights of
all the hills. It is wonderful to think that we can
also find the heights of the mountains and hills of the
Moon, so that we can make not only a map, but a
relief model. The Royal Astronomical Society has
kindly lent us a beautiful relief model of a part of
VISITS TO THE MOON AND PLANETS 185
the Moon's surface, with a model of the neighbour-
hood of Vesuvius alongside it, so that we can compare
the two. You see that on the whole the Earth's
mountains are distinctly smaller than those on the
Moon. One reason for this is that our mountains
are being continually washed away by the rain which
falls on them, so that they are smaller now-a-days
Part of Moon. Vesuvius and Bay of Naples.
From Plate VI of "The Moon " (Nasmyth & Carpenter)
than they used to be. On the Moon there is no rain,
and so the mountains remain unwashed.
One way of measuring the heights of a mountain
in the Moon is to measure the length of the shadow
it casts. Of course a shadow does not always re-
main the same length ; as you walk past a lamp-post
you can see your own shadow grow shorter and
longer; but if you stay in the same spot near the
lamp it will remain of the same length ; or if you go
back to that spot it will come back to the same
186 A VOYAGE IN SPACE
length ; or if any one were told where the lamp was,
he could calculate the length. The lamp that casts
the shadows of the lunar mountains is of course the
Sun ; and we can find his position at any date from
the Nautical Almanac or a similar book, and then we
can calculate the length of shadow if we know
how high the mountain is; or, if we measure the
length of shadow, we can calculate the height of the
mountain. But there is one thing to be very careful
about : if the plain on which the shadow falls is not
quite flat we may be misled. We can see that by a
TO SUN
Fig. 55-
little experiment. Here is a rough model of a lunar
mountain attached to a flat board, and from the
lantern, which represents the Sun, we will cast a
shadow on the board. But now the board is made
up of two pieces hinged together, and if I slope the
outer piece a little, you will see how the shadow
alters in length ; I can lengthen it or shorten it by a
very slight inclination of the hinged piece (Figs.
55 and 56). I can, of course, see that I am sloping
the board, and make due allowance; but on the
Moon we do not know whether the plain is flat or
not, so that when we measure heights in this way,
we are liable to some uncertainties. The case is not
VISITS TO THE MOON AND PLANETS 187
really so bad as I have made it appear to you ; I have
exaggerated things for the sake of plainness. One
thing which helps us is that the Sun moves about,
throwing shadows in different directions and of differ-
ent lengths from the same mountain ; so that if the
supposed plain round the base slopes upward in one
place and downwards in another, the lengths of the
shadows will show it.
If there are mountains in the Moon like our own
mountains, are there also inhabitants like those on
TO SUN
Fig. 56.
the Earth? At the beginning of this lecture we
considered the case of Mars, and I showed you how
hard it would be to see any signs of life on Mars by
turning things round and supposing that we were
looking at the Earth from Mars. You remember
that we could not see violent changes in the British
Isles even with the best telescope from that enor-
mous distance. But the Moon is much nearer to us
than Mars is : only a quarter of a million miles in-
stead of (say) 50 million. If we use a telescope
magnifying 1000 times, it is as though we were look-
ing at things with the naked eye from 240 miles
i88 A VOYAGE IN SPACE
away. You see that this is still a considerable dis-
tance ; you would not expect to recognize much at
such a distance. Some of you have been at sea and
seen a ship on the horizon ; you know how small it
looks when it is five or six miles away, and how close
it must come before you can see the people on its decks.
You can well imagine that at 240 miles you might
not see the ship at all with the naked eye even a
very big ship. We have as yet been unable to see
any signs of life on the Moon, but that does not mean
that there is no life; still less that there has never
been any life. Perhaps life may by this time have
disappeared from the Moon, as there is apparently
very little air and water, if any. The Moon has dried
up. But that there was life on it once I firmly
believe. It seems reasonably certain that bodies
which have so much in common with our own Earth,
such as the planets and satellites of our system (and
probably of other systems, for the stars are Suns like
our own and probably have planets and satellites as
our Sun has), have also life on them as our Earth has.
It would be strange indeed if this peculiarity were
confined to a single, rather insignificant body.
But whether there are men and women like our-
selves on the Moon, or ever were, is quite a different
question. Let me remind you what a vast number
of forms of life there are even on our own Earth.
There are fishes which live in the water; there are
birds which fly in the air; there are insects which
crawl and there are rabbits which live underground.
Mr. H. G. Wells in his fascinating book has supposed
that the inhabitants of the Moon are a combination
of the last two classes ; he makes them large insects
VISITS TO THE MOON AND PLANETS 189
which live underground. Let me recommend you
again to read his book, which will teach you a great
Liquid Air Experiment,
i (See Notes to Illustrations.)
deal. His reason for making the inhabitants live
underground is that there is certainly very little air
on the Moon some people say none at all, but Mr.
Wells gives it the benefit of the doubt ; he considers
igo A VOYAGE IN SPACE
that there may be none on the outside for us to de-
tect, but there may be some in the interior; and
accordingly he puts the dwellings down inside the
Moon, so that the people can breathe. But he allows
some air for the outside, and he brings it into the
story in a very interesting way, all frozen. The
" first men in the Moon " land in a regular snow-
drift of frozen air, due to the fact that they land on
a part of the surface on which the Sun is not shining,
so that it is bitterly cold so cold as to freeze the
air. Not very long ago no one had ever seen air
frozen or even liquid; but you fortunate young
people of to-day can have it shown to you quite
easily, and we will conclude this lecture with one or
two experiments showing liquid air.
Here is a regular fountain of it at the back of the
room. It is frightfully cold, and though it will not
hurt you to put your hand in it for a moment, you
could easily burn the skin off your hands by exposing
them to it for too long. If we put an india-rubber
ball into liquid air it is frozen so hard that it breaks
when we throw it down. Even a flower dipped into
liquid air becomes quite brittle. But perhaps the
prettiest effect of liquid air can be seen by pouring
some into a bath of warm water (see illustration).
It makes a beautiful snowy cloud over the surface
of the water. Now if some of the audience will
kindly take these fans and fan away that cloud, they
will find underneath little frozen cakes of air, bub-
bling furiously. One can show that they are cakes
of air by the effect on a glowing wooden pipelight ;
we will light such a pipelight or a big wooden match :
then blow out the flame, but leave the end glowing.
VISITS TO THE MOON AND PLANETS 191
Now put the glowing end in the bubbles of one of the
ice-floes and you see the flame burst out again. If
the bubbles were water vapour this would not
happen ; they are really air, and the rush of air made
by the rapid evaporation lights up the match again.
LECTURE V
OUR SUN
TO-DAY we have to make perhaps the most
important visit of all a visit, as near as our tele-
scope will allow us, to our Sun. Now, there are three
special reasons why the Sun is so very important.
In the first place it is the source, as you know, of
all the light by which we see, very nearly all of the
heat that warms us, and almost of the life that
we live. We know how the plants live on sunlight
if there were no sunlight they would die ; we know
how animals live their lives when the sun is shining :
during the night they sleep. On the half of our
Earth not illuminated by the Sun the inhabitants
are chiefly asleep : certainly most of the animals
are asleep; and even human beings, especially
children, are mostly asleep; if they do lie awake,
they think the night is rather a dreadful time.
Perhaps you have read a poem called " The City
of Dreadful Night " ? Those who cannot sleep
generally long for the daylight. As the Earth
turns round they are carried towards the place where
the Sun rises; the birds begin to sing, the animals
get up and begin to feed; boys and girls get ready
for their breakfast and go to school and enjoy
themselves, and have dinner about noon and go
192
THE SUN 193
on with the other pleasures of the day : and finally,
when bedtime comes, they plead for just another
ten minutes because they know that going to bed
means getting into the dark. They like to keep
in the light as long as possible. Even if it is not
merely a question of no Sun and full Sun (a question
of night and day), when" it is a question of much
Sun and little Sun (summer and winter) , you know
very well which you prefer. When summer is
coming the spring brings leaves to the trees, and
little birds make their nests, and boys and girls
begin to think of the summer holidays when they
can go to the seaside. But when it comes near
the winter "the winter of our discontent," as
Shakespeare puts it then it is all cold ; and many
animals go into their winter quarters, perhaps to
sleep. Human beings have by this time found
many ways of alleviating the winter, especially by
means of fires and lights, but we must remember
that we owe even these to the Sun. Without the
Sun in times gone by, those forests would not have
grown which to-day give us our coal : the fires
which we light in the winter are in many respects
the work of the Sun. Hence it does not surprise
us that in the old days they used to worship the
Sun as a god. In our own country of Britain, the
ancient Britons have left monuments, such as those
at Stonehenge, showing the way in which the Sun
came into their religion. There is one great stone
standing erect at Stonehenge in a line with the
sacrificial stone, so that the Sun at sunrise on one
particular day in the year (June 21) just shines in
a line over these two stones, and for a moment it
o
i94 A VOYAGE IN SPACE
appears from the sacrificial stone as though the
Sun stood on the top of the big one; and at that
moment they used to make their sacrifice. We
can still see to-day how impressive a sight that is,
if we go to Stonehenge just at sunrise on Midsummer
Day. I have not seen it myself, but I am told
that hundreds of people assemble in their motor-
cars to see this great sight.
Of course Stonehenge was not the only place
where the Sun came into religious ceremonies. In
ancient Egypt they worshipped the Sungod (Ra) ;
and the Sun was used by the priests to consecrate
the King in a very striking way, if we may accept
the conclusions of some writers. Knowing how it
would shine on a particular day, the priests built
a special passage in one of the pyramids or temples
down which the Sun would only shine just that
once in the year, for a moment or two; and they
used that knowledge to impress the people when
they were going to appoint a new King. They
would take the people into the Temple when the
Sun was not shining; it was all dark. (At this
point the lights in the lecture-room were ex-
tinguished.) Then they arranged so as to have
the King in the right position, and at the proper
moment the Sun would rise and the people saw
their new King in the glorious blaze of sunlight !
(At this point a beam of light was thrown from
a special lantern to illuminate a small boy, dressed
in kingly attire, who had been placed on the lecture
table in the few moments darkness; his regal
bearing was deservedly applauded.)
The first reason for the Sun's importance, then,
THE SUN 195
is that he is the source of our light and heat, and
almost of our life itself. The second reason is
because he is in a sense the father of us all. The
Earth and planets are " chips of the old block "
in the ordinary phrase. They are parts of the Sun
One of the regular photographs of the Sun taken at Greenwich.
detached from him in the manner we illustrated
in the last lecture in talking of satellites. You
remember that they were detached as the rate of
spin increased. Now I want you to realize that the
Sun rotates. You know how we showed in the
first lecture that the Earth was rotating, and last
Saturday we showed that Jupiter and the other
196 A VOYAGE IN SPACE
planets were all rotating; and now we have to
realize that the great Sun is rotating on his axis
also. That was first found out by Galileo when he
observed sunspots. Here is a picture of the Sun
as it is taken at the Royal Observatory, Greenwich,
every fine day in the year. The two lines crossing
the picture are the spider lines of the telescope ; they
tell us which is North and South : but all the rest
of the bright part is the Sun. You notice that the
edge is darker than the middle; and that tells us
that the Sun has an atmosphere, of which we shall
have more to say presently. You see also the spots,
and you will please note their position on the first
picture, which was taken on February 18 ; and now
you will see how they have changed when we come
to February 19, and again to February 20, and
following dates. They do not move straight across
horizontally because the Sun's axis is tilted; but
careful measurement shows that there is a real
axis, which has remained so far as we can tell in
the same position since it has been investigated.
The path of the sunspot can be seen better
if we make a " composite photograph " of all the
days, by photographing them all on to the same
plate. We can then see how the spots are travelling.
From such a composite photograph I have made
(see Fig. 57) a drawing showing the position of the
big spot only and leaving out the others. The
positions for the different days are numbered
accordingly. On February 23, and again on Febru-
ary 25 and 26, the weather at Greenwich did not
allow a photograph to be taken.
Before we go on to consider how the Sun's rota-
THE SUN
197
tion leads to the formation of planets let us say a
word or two more about these spots. They were
discovered, so far as Western peoples are concerned,
by Galileo in 1610. The Chinese had noticed some
f??
I **
(83) 24 (25) (26) 27
Fig. 57. Composite drawing of a Sunspot from several
Greenwich photographs.
of the large ones which can be seen with the naked
eye from A.D. 188 onwards, but we had no know-
ledge of them at all till Galileo's time. Undoubtedly
they are regions of fierce disturbance. Even in the
old days, when Galileo first found them, the notion
of disturbance or conflagration suggested itself, as
198 A VOYAGE IN SPACE
old drawings show. Later Herschel thought that
they were holes in the bright surface of the Sun
through which his dark interior could be seen ; but
the idea of the sun being a dark body surrounded
by a bright envelope has long been given up. We
scarcely know as yet what the spots are, or how
they are caused; but there is no doubt at all that
they are regions of fierce disturbance where terrible
tornadoes are raging.
On one famous occasion (represented in Fig. 58)
two independent observers, Carrington and Hodgson,
saw an especially noteworthy disturbance. Two
intensely bright spots appeared at the positions
marked A and ~E* and then travelled in about five
minutes to the new positions C and D. The dis-
tances do not look very large on the picture, per-
haps, but on the Sun himself they represent nearly
100,000 miles, so that the rate of travel was about
300 miles a second ! That will give you some idea
of the fury of the storms that must be raging in
the Sun.
A curious thing about these sunspots is that
they wax and wane about every eleven years. At
the present time (i. e. Christmas 1913-14, when these
lectures were delivered) the Sun has been nearly free
from spots for at least two years. It is what is called
a time of " minimum." But we are expecting spots
to begin again soon, and they will increase in
number till they reach a " maximum," and then
they will die away again to a minimum about
eleven years from now : say about 1924, since we
are already probably past the minimum. We have
records which show us that this waxing and waning
THE SUN
199
has gone on at least during the last century and a
half. Before that the records are too scanty to
give us full or trustworthy information : people did
not pay enough attention to the Sun. We must
not be too ready to blame them because we our-
Fig. 58 A violent disturbance on the Sun.
selves do not pay nearly enough attention to him
even now, considering his immense importance to
us. Up till a year or two ago, no one had ever
taken the trouble to make sure whether light and
heat came to us from the Sun with strict regularity,
or whether they vary in amount as the spots do.
It has been calmly assumed that we can depend on
200 A VOYAGE IN SPACE
a strictly regular supply ; but Mr. C. G. Abbot has
now found out for us that this is not so : the light
and heat do vary, and it is most important for us
to watch the variations, considering that the future
history of the world may depend on them.
But let us return to the spots and let us look a
little more closely at the diagram (Fig. 59) showing
their fluctuations. When the curve rises to a peak
or maximum there were numerous spots : when it
falls to a valley or minimum there were very
few. The dates are shown along the bottom line
every ten years. You will see that the interval
between two minima is not always exactly eleven
years : thus there is only about nine years between
the minima of 1775-6 and 1784-5 ; but as much
as thirteen years between the minima of 1811 and
1824. The variation is not regular, and there must
be some reason for the want of regularity. I have
been studying this matter specially for the last
year and have found what I think is the key to
the puzzle : I think there is a swarm of meteors
revolving round the Sun, not in a nearly circular
track like our Earth, but in an elongated track
like that of a comet. I hope you remember the
way in which a comet moves loitering along slowly
when it is far from the Sun, quickening up as it
comes nearer, and whizzing round the sharp turn
when it is closest to the Sun what is called peri-
helion. Now I think this meteor swarm whizzes
round so close to the Sun's surface that some of
the meteors actually graze the surface and make
the sunspots. The swarm is collected mostly at
one part of the track, like a lot of people running
THE SUN
201
a race ; but if it is a long race, such as a mile, we
have seen some of the runners get far behind the
leaders, sometimes a whole lap behind. If you
stood inside the track close enough for the runners
to graze you as they went by, you would be touched
a good many times as the leaders went by, but only
occasionally by the stragglers : and I suppose that
the Sun is close up to the meteor track in this
112
Fig. 59. Waxing and waning of Sunspots.
way, so that he shows numerous spots when the
leaders are going by and only a few as the stragglers
tail off. But how does this notion help us to
explain why the leaders should come round the
track sometimes in eight years and sometimes in
thirteen years, sticking a sort of average of eleven
years? That is the really important point, and
supplies the reason for putting forward this notion
at all. Let me go back for a moment to Halley's
comet. His attention was first drawn to the
202 A VOYAGE IN SPACE
probability that it would return by the figures for
the years 1531, 1607 and 1682 being so much alike :
he inferred that it must be the same comet running
regularly round a track. Then he noticed that the
returns to the Sun were not quite regular : there
was an interval of more than seventy-six years
between the first pair and of less than seventy-five
years between the second pair. This was against
him unless he could give a reason for the difference.
With great acuteness he assigned the reason in
general terms : he said that when the comet was
far away from the Sun, loitering slowly along, any
of .the planets which happened to come by might
attract it out of its course a little, upsetting the
regularity of the return. It will do us no harm to
look at his actual words
The motion of Saturn is so disturbed by the
other planets, and especially by Jupiter, that
his periodic time is uncertain, to the extent
of several days. How much more liable to
such perturbations is a comet which recedes
to a distance nearly four times greater than
Saturn, and a slight increase in whose velocity
could change its orbit from an ellipse to a
parabola? ... I may, therefore, with con-
fidence predict its return in the year 1758.
If this prediction be fulfilled, there is no reason
to doubt that the other comets will return.
Now, the meteor swarm which I consider respon-
sible for sunspots can be pulled out of its course in
the same kind of way as Halley's comet : and if
we look for occasions when it was so attracted we
THE SUN 203
find plain indications of disturbance near the years
1766, 1799, l8 33 l8 66, 1899, which are a set of
occasions already well known to astronomers from
the great showers of meteors which were seen then.
It has been shown that these showers are all due
to a great meteor swarm called the Leonids, travel-
ling round the Sun in about thirty-three years.
My notion is that the sunspot swarm is disturbed
by these Leonids every time they come round : I
think that, as already stated, one end of the track
grazes the Sun, but the other end is close to the
track of the Leonids, so that it is particularly easy
for the Leonids to attract the sunspot swarm, and
to upset its regularity. Every time the Leonids
come round we open a new chapter of sunspot
history. Why should the end of the one track lie
so close to the other? Of course we can say that
there is no reason why it shouldn't, but it would
be much more satisfactory if we could mention a
reason why it should; and a very good reason
seems to me to be that the sunspot swarm was
broken off from the other near the point where
they now approach so closely. Here again it is
desirable to give a reason for the breakage, and
here again we find one in the planet Saturn, which
passes close to this same spot in its journey round
the Sun (Fig. 60). It does not always meet the
Leonids there, because when Saturn comes to the
critical spot the Leonids may be in quite a different
part of their track : indeed, they generally are. But
about every 265! years Saturn and the Leonids hit
off the same moment for being at the meeting-place :
and when this happens we find (by looking at the
204
A VOYAGE IN SPACE
Chinese records which go back nearly 2000 years,
as I have already mentioned) that there is a new
crop of sunspots. You will see, therefore, how I
have come to suggest that a collision between
Saturn and the Leonids causes meteors to fall into
Fig. 60. Suggestion for the explanation of Sunspots
by a Meteor swarm.
the Sun and make sunspots. The idea is a new one
and has attracted some attention : a clever draughts-
man has illustrated it in the Illustrated London News
(Fig. 61) ; I should myself have drawn the details
rather differently, but his picture is certainly
interesting. Let me give you another illustration
THE SUN 205
in words. On a ship when there is a head wind, the
bows will sometimes hit a wave with a resounding
smack, and a shower of spray is tossed into the air ;
the wind catches the spray, carries it down the boat,
and dashes it in your face perhaps though you may
be far away from the bows. In some such way I
suppose Saturn and his Ring to hit the wave of
Leonids : a shower of spray from the Leonids, or
from Saturn's Ring, or from both, is tossed up; so
to speak; is caught, not by the wind, but by the
Sun's attraction, and is carried down to hit the
Sun in the face, though he may be a long way from
the scene of collision.
Let me tell you frankly that other astronomers
have not as yet looked with a very friendly eye on
206 A VOYAGE IN SPACE
this idea : I think they will come round in time,
though at present they have not done so. On the
other hand, I may be mistaken, and some other
origin of sunspots may be found. But in any case
I hope you may have been interested to follow
the course of the story, seeing how one idea leads
to another. Unless we can get some chain of reason-
ing of this kind, which can be checked at various
points, we cannot advance our knowledge : when,
on the other hand, we can see our way to some check
and find that it works, it gives us confidence that
we are on the right road. It was a great pleasure
to me when, having seen that collisions between
Saturn and the Leonids ought to recur in about
265! years, I looked at the Chinese observations
for the check, and found it very completely shown.
Although the particular idea I have just sketched
is new, it is by no means a new idea that meteors
should fall into the Sun. At one time it was thought
that his heat and light were kept up in this way.
If a shot is fired into a target, both shot and target
are warmed. The heat comes from the stoppage of
the motion of the shot. Do you remember our
experiment to illustrate why meteors shine when
they strike our air? We whirled an electric junc-
tion through the air and it became warm, because
the air was resisting the motion and stopping it
partly. If a bullet went right through the target
and continued its course, nevertheless it would
be partly stopped it would scarcely continue its
course so quickly as before : and we should get
some heat : when the motion is wholly stopped we
get more. Thus it was thought that meteors
THE SUN 207
attracted to the Sun and falling into him with
tremendous speed, would develop enough heat to
keep him going. You see, the Sun is giving out
enormous quantities of heat; we get a good deal
on our Earth, and some goes to Mercury and Venus
and the other planets. But vastly more goes
straight out into space because there is no planet
in the direct line to catch it. From the amount
our Earth receives we can calculate the total out-
put, and it is truly terrific, enough to melt in a
single second a solid column of ice two miles thick
stretching from the Earth all the ninety-three
million miles to the Sun. How is this outpouring
kept up? One idea used to be that meteors fall
in; but it was calculated that so many would be
required that the Sun would grow visibly larger,
which is not the case. Hence, though meteors may
account for a part of the heat, there must be some
other supply too. I must mention here that the
Sun is not burning like a fire or a gas jet : it is
glowing like an electric glowlamp. You know the
difference between the two cases : a gas jet or
candle gets quickly used up; but the filament of
an electric light bulb, thin though it is, lasts for
hundreds of hours without being consumed. The
light comes, not from the consumption of the fila-
ment, but from the energy supplied to it from the
electric power-house. The question we are con-
sidering is what is the Sun's power-house, what
is the source of the energy supplied which makes
it glow?
The explanation considered sufficient until re-
cently is almost the opposite of that we have just
208 A VOYAGE IN SPACE
considered. Meteors falling into the Sun would
make it grow bigger : we believe, on the contrary,
that it is getting smaller" shrinking with the
cold." That notion was mentioned in the last
lecture in connection with the planets, and we
saw how it led to the further notion that they
would spin more quickly and ultimately throw off
satellites. If, as we suppose, the planets them-
selves are satellites of the Sun thrown off in the
same way, then at any rate he must have been
shrinking in time past, and we know of no reason
why the shrinkage should have stopped. But it
seems extraordinary that we should try to explain
his generous output of heat by saying that he is
" shrinking with the cold " : it looks as though we
are explaining a thing by its opposite. We can,
however, see how this may happen. Supposing
your father were to come home one day and say
to your mother : " I have been balancing accounts,
and I find we have spent a great deal of money this
year ; we must reduce our establishment ; let us
give up our motor-car " ; it might then happen
that giving up the motor-car saved more money
than was needed to bring the expenses down to
the right figure, so that in other ways the family
would be richer than before. It is rather like that
with the Sun : losing a lot of heat makes him reduce
his establishment makes him shrink, but the
very shrinking causes a development of heat more
heat, perhaps, than he lost, so that he is actually
hotter after the shrinkage than before.
This way of getting heat by shrinkage does not
appeal to our imaginations so readily as the idea of
THE SUN 209
getting it from fierce blows from meteors ; but it
is known to be just as real a way, and it can be
calculated how much contraction is required to give
out the heat we now experience. You might
scarcely credit it, but the amount is so small that
we could not have noticed it in the 200 years for
which we have been measuring the Sun's size
accurately. Until we had really good telescopes,
(that is to say until the beginning of the eighteenth
century), no measures could be made of the Sun's
size sufficiently accurate to be worth considering.
Hence we practically began measuring the Sun
200 years ago, and all the heat he has sent out
during these 200 years could have been produced
by his whole body shrinking ten miles inwards.
Now, ten miles seems a long distance when we have
to walk it, but it is quite imperceptible on the Sun,
owing to his great distance. An inch is easy to
see when close to us ; but put an inch 135 miles
away and how big would it look? Just as big as
ten miles on the Sun. The Sun might have shrunk
since astronomers watched him closely, not merely
ten miles but even 100 miles without our being
able to detect it.
Can this shrinking go on for ever ? It is a hard
question to answer. If it goes on steadily there
must come a time when the Sun is as compact
and solid as our Earth. At present we find that
he is not solid, and not nearly so dense as the
Earth. But even our solid Earth is still shrinking,
as we know from the occurrence of earthquakes
and the existence of mountains and valleys. When
an orange dries and shrinks, its skin goes into folds :
p
210 A VOYAGE IN SPACE
and so does the skin of our Earth as it shrinks.
The mountains and valleys are the crinkles : the
shrinkage is always trying to crumple them closer
together with a stronger and stronger grip : they
may hold out for a time, but at last they give way
suddenly and then we have an earthquake. If we
knew how much our Earth shrinks each year, we
might be able to look forward and calculate how
much smaller it will be in a million years from
now; but we do not really know enough as yet to
make the calculation. Or we could look backward
and calculate how much bigger it was a million
years ago, or two million or fifty million : and
similarly with the Sun. The case of the Sun is
easier because we have something to go upon; we
can measure the amount of heat he is now giving
out, and that tells us how fast he must be shrinking,
whereas we have no such information about the
Earth. In that way Lord Kelvin calculated back-
wards and thought he could find how many million
years ago the Sun could have started being a Sun
at all. You see as you go backwards you must
suppose him bigger and bigger until he is spread
out so much that he would no longer be like a
Sun at all, and Lord Kelvin thought this was
about 100,000,000 years ago. But he was assuming
all the time that the heat we receive all comes from
the shrinkage : that wonderful substance radium
had not then been discovered, and there seemed
to be no escape from Lord Kelvin's conclusion.
Since the discovery of radium, however, the matter
has assumed an entirely different aspect; we have
learnt of the existence of a new kind of power-house
THE SUN 211
for supplying energy, namely the breaking up of an
atom, of which no suspicion had entered any one's
head till recently. If, as we seem entitled to think,
there is this kind of power-house available in the
Sun, he need not be shrinking so rapidly indeed
he need not be shrinking at all. The calculations
of Lord Kelvin are, in fact, rendered so much waste-
paper, since they start with an assumption which
may not be correct at all.
I am sure you would all like to see just one
experiment with radium, as you have heard so much
about it. We will charge up this electroscope so
that the gold leaves stand apart from each other.
If now I bring a little radium near it, they close
together, showing that the electric charge has been
removed, and yet you see I have not actually
touched anything. The fact is that little particles
are shooting out from the radium : they hit the
gold leaves and carry off the electricity.
I must now come to the third reason for the
Sun's great importance. The first is that he gives
us light, heat and life in such profusion as led to
its worship in old days. The second reason is that
our Earth and the other planets are actually parts
of the Sun, who must be regarded as our parent
(as the Moon is our child), and the third reason is
that he is the nearest star to us. The Sun is really
a star : other stars are a very long way off ; but
the Sun is comparatively near. The actual distance
is 93 millions of miles, and you may think that
that is not very near; and indeed it would not be
very near if we were to go by an express train.
Suppose we travelled in a train at the rate of sixty
212 A VOYAGE IN SPACE
miles an hour we should take 175 years to get to
the Sun. I suppose a return ticket at the ordinary
British rates would cost us something under a
million pounds. It is curious to think that the
Earth is taking a journey three times as long as
that every year without charging us a penny. We
go round the Sun in the greatest comfort, with
Drawing of part of the Sun's surface by Sir W. Huggins.
sleeping compartments at night and restaurant cars
during the day, moving so quietly along that we
can play our games too. The big Atlantic steamers
have broad decks on which children can play
some few games ; but the Earth has a far broader
deck on which we can play every kind of game.
We ought to be very grateful indeed for all these
comforts and opportunities, and yet we scarcely
ever think of them.
But there is something that takes us much
THE SUN 213
quicker than the Earth, and that is Light. If we
agree to travel by the telescope, then instead of
taking 175 years to get to the Sun, we need only
spend eight minutes. Eight minutes is not a long
time on a journey. Even when the whole journey
Nasmyth's drawing of a Sunspotand the neighbouring surface.
is only a few hours long, when we are within eight
minutes of the end your mother will probably say,
" Now, children, get your wraps and things together :
we are just there." And so every one collects their
wraps and rugs. Why ! I have actually seen
people stand up for more than eight minutes at
the end of the journey, with bags and umbrellas
214 A VOYAGE IN SPACE
in their hands, so little do they reckon of eight
minutes. But even travelling by telescope, at
the enormous speed of light, we should take years
to get to the stars very different from the eight
minutes to the Sun. Hence you see how close the
Sun is compared with any other star. He is so
close that we can see details on his surface, whereas
even in our most powerful telescope the stars are
mere points of light.
Let us look again at some of the representations
of the Sun's surface made at different times. Sir
William Huggins, a great English astronomer,
whom we lost recently, drew the surface as a kind
of mosaic pattern, while Mr. James Nasmyth (the
inventor of the steam hammer, of which there is a
fine working model in the machinery museum at
South Kensington I hope you all know that
museum) drew a pattern of what he called willow
leaves. There is a good deal of difference between
their pictures, but they agree in claiming that the
surface is made up of numerous bright grains
another observer compared them to rice grains-
packed fairly close together excepting near a spot.
Small as they appear in the. pictures, these rice
grains or willow-leaves- must be of enormous size
in the sun, say 1000 miles long by 500 wide. That
these observers were not deceived has been amply
proved by photography, especially the photo-
graphic enlargements taken by a Russian astron-
omer, M. Hansky. By a tragic accident he was
drowned while bathing, and no one else has paid
the same attention to photographing these rice
grains on the Sun; but he obtained a sufficient
THE SUN
215
series of pictures to show us at what a great rate
they are moving about. Even in a few seconds
the pattern becomes quite differently arranged, as
you can see by comparing one of M. Hansky's
pictures with another. They must be moving
5h. 4m. 155.
5h. 6m. 2os.
Hansky's photographs of a small portion of the Sun's
surface, June 25, 1905.
with great speed, some of them perhaps at 100 miles
per second. We saw an instance of a great solar
storm observed by Carrington and Hodgson; but
we now begin to realize that the whole surface of
the Sun is in a state of constant turmoil. We get
another proof of this fact if we look at the edges
216 A VOYAGE IN SPACE
of the Sun, where we see great red flames shooting
up to enormous heights. Perhaps we ought not
to call them flames, because in our earthly fire-
grates a flame means that something is being burnt,
generally the gas from the coal which is being burnt
up in the air. The solar flames do not represent
anything burning; they glow like the filament of
an electric lamp, but we have no better word to
describe them than " flames."
We cannot see these flames in the ordinary way ;
but because they are of a special red colour we can
see them by means of a trick. You remember our
experiments with colours two lectures ago : we
found that a red ribbon would show bright in the
red, but in green light it appeared black; and do
you remember how we lighted the room with the
yellow light made by burning common salt, and
then only yellow things showed bright ? All other
colours in a picture we were looking at the reds
and blues and violets all disappeared. If we had
taken a photograph in this yellow light, the yellow
parts of the picture would have shown up, while
the others would have been quite faint or altogether
absent. And I then told you that this trick was
used to take photographs, with an instrument
called the spectro-heliograph, and with this instru-
ment we can photograph the red flames round the
Sun's edge; of course we use red light in order to
show them up. The general plan is simply this :
if we pass the Sun's light through a glass prism
we have seen that it is spread out into all the
different colours. Now let us block out all the
other colours except the special red we want ; for
THE SUN 217
i
this we need only a screen with a slit in it at
the right place ; all other colours will be stopped
by the screen, but the special red will shine through
the slit.
This special colour need not, of course, always
The " Red Flames " round the edge of the Sun.
(The whole of the ordinary Sun is blocked out.)
be red; we can alter the place of the slit so as to
use any other colour we please, which is lucky,
because red is not at all a good colour for photo-
graphy. You probably know that you can have
red light in the dark room without spoiling your
negative. And, further, we need not only photo-
graph the red flames at the Sun's edge, we can use
218 A VOYAGE IN SPACE
the instrument to photograph the details of the
surface, which Huggins and Nasmyth tried to draw
and Hansky succeeded in photographing in ordinary
light. When we make pictures of them in special
colours with this new instrument we get a different
picture for every new colour. Let me show you
a couple of such pictures of a small portion of the
Sun's surface near a spot. The sunspot is a great
. a.
Fig. 62. A Sunspot photographed in Calcium light
(Oct. 9, 1903).
convenience in this case, because the pictures of
the surroundings differ so completely that unless
you had the spot to go by, you would scarcely believe
that you were looking at the same part of the Sun ;
and yet it is exactly the same part, and the pictures
were taken within a few minutes of each other;
but in one the slit of the instrument was set for
calcium light, and in the other for a special light
made by hydrogen. You see how different they
are; and I hope you understand the reason. Sup-
THE SUN 219
pose we made a letter picture of those last three
words
UNDERSTAND
THE REASON
E
E E
A
A
Full Light. Yellow Light. Red Light.
colouring all the letters E with yellow, all the
letters A with red, and the other letters green.
Then if we took a photograph in yellow light we
should get only the E's as in the second diagram;
if we took a photograph in red light we should get
only the A's. The two photographs would be quite
different although we had actually photographed
the same picture. Perhaps some of you have
already taken colour photographs by the three-
colour process; if so you will know about this
principle without this explanation. The point is
that just as we find out in one photograph where
the letters E are distributed and in the other photo-
graph the letters A, so in the case of the Sun we
find in one photograph where the calcium is distri-
buted and in the other photograph the hydrogen.
Why should these two substances be arranged so
differently? We get a hint of one probable
reason from Fig. 63, in which we see a number of
curved lines. There is no such regularity in the
calcium picture; but the hydrogen picture shows
these curves, which remind us at once of curves
made by iron filings when near a magnet. Let us
throw a picture of them on the screen. We first
place a bar magnet on this sheet of glass and then
220
A VOYAGE IN SPACE
pepper some iron filings all over the glass. They
do not go into a pattern at once, because even the
smooth glass is not quite smooth; but if we tap
the glass gently so as to make the filings jump
up and down, then while they are in the air they
feel the pull of the magnet and get a chance to
obey it. He soon arranges them in the curved
lines (see Fig. 64). Now if we were to scatter
*-*
I
Fig. 63. The same Spot as in Fig. 62, but photographed in
Hydrogen light.
among these iron filings some filings of silver or
other non-magnetic substance, the silver would
not feel the pull of the magnet. Mr. W. S. Gilbert
made a pretty song about the " silver churn " if
you remember, in his opera Patience
"A magnet never by any endeavour
Can attract a silver churn ; "
and it is the same with silver filings; if they were
scattered on the glass along with the iron filings,
you might tap the glass as long as you like and
THE SUN 221
they would never get into curved lines ; they would
remain higgledy-piggledy. In the same way the
hydrogen on the Sun gets into curved lines because
it feels the magnetic attraction, while the calcium
remains higgledy-piggledy.
But, you will ask, what is it on the Sun that corre-
.ai^Sm^ 1 ^?"' vu
'' '
Fig. 64. Iron filings near a Magnet.
spends to the little magnet ? and the answer has
been given us in a beautiful manner quite recently,
by Professor Hale, the great American astronomer,
who has erected the Mount Wilson Observatory in
California. His proof depends on experiments with
polarized light which we cannot stop to explain
just now, though I will show you some pretty
experiments with it at the end of the lecture ; but
222 A VOYAGE IN SPACE
the gist of the matter is that a sunspot is an electric
whirlpool, which is as good as a magnet. Many of
you know already what an electro-magnet is; if
we take a coil of wire and send a current round and
round it, the coil will attract iron filings in the same
way as our little bar magnet. The wire may be
made of copper and need not have any iron in it
at all, though usually an iron " core" is added to
make it stronger ; but it would still act as a magnet
if there were only the copper wire and the electricity
circling round in it. Similarly, if we can get an
electric whirlpool in any other way we shall have
something which behaves like a magnet, and
Professor Hale has shown that the sunspots behave
in this way. Some of them are whirling in one
direction and some in the other he can detect the
direction by the magnetic action. You know there
are two ends to a magnet, often called the north
and south poles, a whirlpool in one direction acts
like a north pole and in the other like a south
pole.
We shall not be able to describe Professor Hale's
experiments, but I can show you some beautiful
pictures taken by his assistant, Mr. St. John, with
the spectro-heliograph, which show that a spot has
the sucking action of a whirlpool. Have you ever
read about the Maelstrom, the great Norwegian
whirlpool? A boat which gets caught in it circles
slowly round and round at first in the outer parts,
but is always being drawn nearer and nearer to the
centre, in a spiral. It is whirled more and more
rapidly and ultimately sucked down at the centre,
as though by some great octopus which waved its
223
224 A VOYAGE IN SPACE
long arms out and gradually drew the poor boat
to its greedy maw. The pictures taken by Mr.
St. John show us something of this kind. You see
the sunspot, which is the centre of the whirlpool,
about the middle of each picture, and near the
bottom is an object which in Fig. 68 resembles a
fish. At first this object is peacefully at rest, there
being little change between May 29 and June 2,
though the tail of the fish has turned towards the
vortex: on June 3 this turning rapidly develops
within a few minutes : it becomes clear that the fish
is caught by its tail, and in Fig. 69 we see him
being swallowed. Next day there is no trace of
him (Fig. 70).
These actual proofs that a spot is a magnetic
vortex are quite recent ; but for a long time we have
suspected some kind of magnetism in the Sun;
indeed, it has been much more than a suspicion.
Our magnets on the Earth are disturbed in a regular
manner which corresponds closely to the ups and
downs of sunspots ; that was noticed half a century
ago. But instead of calling your attention to these
more or less regular changes, I will show you how
beautifully Mr. Maunder proved that magnetic
" storms " on Earth originate in some way in the
Sun. These " storms " are quite irregular (like our
storms in the weather), we should never notice
them in ordinary life, but a telegraph clerk finds
them a great nuisance; if they are violent and
persistent they may make it impossible for him
to send or receive his messages. At the time when
Carrington and Hodgson noticed that great dis-
turbance on the Sun, of which I spoke early in this
THE SUN
225
lecture, there was a furious magnetic storm on
Earth, which was naturally attributed in some way
to the solar disturbance. But now there is this
difficulty : other disturbances in the Sun have been
noticed without any corresponding magnetic storm
on Earth; while on the other hand v we have had
storms which have driven the poor telegraphists
nearly frantic while the surface of the Sun has
betrayed no emotion whatever, so far as we could
see. Hence, the question was in a very unsatis-
factory state until Mr. Maunder made his ingenious
suggestion. To explain the nature of it, perhaps
you will let me first give an illustration of a more
familiar kind. You know that when you are at
the seaside the best time for bathing alters during
the day, according to the tide. The time of high-
tide falls later and later by about fifty minutes
each day, because the tides are caused by the Moon
and they follow it round. When the Moon has
made a complete turn round the Earth, that is to
say, in a month the tides have changed by twenty-
four hours, which comes to about fifty minutes each
day. Let us make a diagram (Fig. 71) in which
the twenty-four hours go from left to right, putting
the days below one another; and let us mark the
high tides, or the best bathing times on the diagram :
there will be two of them each twenty-four hours,
though one of them may come in the middle of the
night when not many people want to bathe; but
put them all down. Then the fifty minutes altera-
tion each day will cause the marks to slope to the
right. If we were to mark lunch time, which de-
pends on the Sun, the marks would fall exactly below
Q
226
A VOYAGE IN SPACE
one another, at the same time every day (that is
supposing you are always in time for lunch) ; but
since the tides depend on the Moon, the marks slope ;
and we could tell from the amount of slope that the
marks referred to something depending on the
BATHING TIMES AT MARGATE.
/9/3
MONT.
JUNE
I
SUN
2
M
3
T
4
M
5
TH -
6
F
M
7
S
a
SUN
*-
9
M
10
T
II
W
12
TH
13
F
/4
S
15
SUN
(6
M
17
T
IB
W
19
TH
20
F
21
S
22
SUN
25
M
24
T
25
IV
26
TH
27
F
28
S
29
SUN
30
M
/
T
NOON
MONT
Fig. 71.
Moon, because in a month they go right through
the twenty-four hours and start afresh. When the
marks form a long series like this, it is easy to
interpret the diagram.
We must notice just one more thing : the line
is not quite straight, but rather wavy; this is
THE SUN
227
because the Sun also helps to cause tides, but you
see the effect is small, and we will not notice it
further.
But now suppose the series of marks is, owing to
AMERICAN TOUR/ST'S BATH/NG T/MES
IN SAME MONTH.
MONT. 4 8 NOON 4- 8 MONT
SUN
M
T
JV
TH
F
S
SUN
M
T
W
TH
F
S
SUN
M
T
W
TH
F
S
SUN
M
T
W
TH
F
S
SUN
M
T
MARGATE
MARGATE
MARGATE
WEYMOUTH
WEYMOUTH
BRIGHTON
BRIDL/NGTON
BR/DLINGTON
BRIDL/NGTON
BR/DL/NGTON
ABERDEEN
ABERDEEN
ABERDEEN
ABERDEEN
B3IDL/NGTON
MARGATE
MARGATE
HASTINGS
WEYMQUTH
WEYMOUTH
WEYMOUTH
PLYMOUTH
PLYMOUTH
FOWEY
FOWEY
MARGATE
MARGATE
MARGATE
SOUTH IVOLD
SOUTHWOLD
SOUTH WOLD
Fig. 72.
some cause or other, broken up. For instance,
suppose that, instead of stopping at one place, you
travel about to different places. Then the diagram
would be altered as in Fig. 72, which I call the
" American Tourist's Bathing Timetable," because
228 A VOYAGE IN SPACE
Americans have a reputation for never staying long
in one place. So long as they stay anywhere, if
only for three days, we shall get three marks in a
sloping line of the right slope : even two consecutive
days will give us the right slope; single days, of
course, tell us nothing. The important point is
that wherever and whenever we get a few days
together, the proper slope shows itself, proving that
the marks must have to do with the Moon in some
way, though we may not be able to explain why
they are so broken up unless we happen to know
that they were made by an American.
Now I think we can follow Mr. Maunder's diagram
of magnetic storms. Each horizontal line refers,
not to the twenty-four hours in which the Earth
turns on its axis, but to the twenty-seven days in
which the Sun turns on its axis, as we saw at the
beginning of the lecture. The marks show when a
magnetic storm occurred on the Earth, and you will
see at once that they are apt to occur in groups of
three or four, one under the other. Whenever they
are exactly one under the other (like the marks for
lunch time in the other case), it means that when
the Sun has rotated exactly once the storm is
repeated, just as the striking of a clock is repeated
when the minute hand has gone round exactly
once. Mr. Maunder claims that the Sun stretches
out a long ringer like the hand of a clock, rotating
as the Sun rotates; and that this finger strikes the
Earth and causes a storm; goes round completely
once, strikes the Earth again and causes another
storm, and so on, until the finger changes its place
like the restless American tourist. Some astron-
THE SUN 229
omers cannot believe in the possibility of the
"finger"; but in any case it seems clear that
something on the Sun causes the storm, because the
storm is repeated when the Sun has turned round
once. But you will no doubt notice that the marks
are by no means always exactly one below another,
though they may be nearly so ; sometimes there is
ROTA " 330V tld uftf M ISf ,10 90* 6f 30* 330*
400 -_ - -- .
410. - """-""-
4ZO
430 "V
440 ' =L
450 ^
460 ~ =r
470 - -
480 -^
PART OF MAUNDER'S "MAGNETIC STORM SHEET
Fig. 73-
a slope in one direction and sometimes in another,
which means that the cause is recurring either a
little more or a little less rapidly than the time for
the Sun to go round. The real fact is that there is
no one special time which we can assign as that in
which the Sun rotates, because he is not solid.
When you stand on one of these nice new escalators
at the tube stations, all the platform moves together
because it is made of solids; but a liquid stream
230 A VOYAGE IN SPACE
moves faster in the middle and slower near the
banks, and the Sun is like the stream, rather than
the escalator. At any rate the spots move quicker
when they are near his equator, and slower near
the poles; and the slopes of the groups of marks
in Mr. Maunder's diagram correspond to the paces
of different spots ; there is no difficulty in finding a
place on the Sun which would suit any of the slopes
in the diagram. Hence he concludes confidently,
and we may agree with him, that the magnetic
storms on our Earth originate in some way on the
Sun; and this alone suggests pretty clearly that
the Sun himself must be magnetic, at any rate, in
certain parts. Professor Hale has carried the story
further and told us where to look for the origins.
He has enormously increased our daily work in
observing the Sun ; for, to take only one instance, he
has shown that we must photograph him not once
a day only, as is done at Greenwich, but many
times and in different colours, since each colour
gives us a different picture. But then Professor
Hale has also banded astronomers together into a
great union for observing the Sun, as Bode banded
them a century ago to discover the missing planet.
This new company of " astronomical police " is to
photograph the Sun in all sorts of ways as often as
possible; they are scattered round the world so
that when the Sun is hidden at one place it may be
shining in another. At present there is a rather
wide gap in the ring of observations; but we are
hoping that either Japan or Australia or New
Zealand, or perhaps all three, will establish solar
observatories; indeed, a rich and generous man,
THE SUN 231
Mr. Cawthron, has already promised 30,000 to
build a solar observatory in New Zealand ; and the
Australian Commonwealth have promised one for
Australia ; so that we hope the Sun will be presently
under constant police supervision, and will not be
able to have any disturbance of note without its
being recorded.
But there is one part of the Sun which cannot be
seen in the ordinary way, nor photographed even
with Professor Male's new instrument. The beauti-
ful corona which surrounds the ordinary Sun can
only be seen when there is a total eclipse of the Sun,
Fig. 74. A total Eclipse of the Moon.
and that is a comparatively rare event in our experi-
ence. Most people have never seen a total eclipse
of the Sun ; possibly I am the only one in the room
who has ; and although I have seen several, I have
had to travel many thousands of miles for the special
purpose.
Many or all of you may have seen a total eclipse
of the Moon, when the Earth comes between the
Moon and the Sun and cuts off the Sun's light from
it ; the Earth is so much bigger than the Moon that
it casts a shadow wide enough to envelope the
Moon for an hour and more. You remember
Jupiter's big shadow and how the little satellites
disappeared into and remained in it for a long time
232 A VOYAGE IN SPACE
before they came out on the other side. The
Earth's shadow is much smaller than that of Jupiter,
but it is still large enough to swallow the Moon
entirely. On the other hand, when the Moon
comes between the Earth and the Sun, its shadow
cannot swallow the Earth at all, it can only at best
make a little dark patch upon it ; if you can get
within that dark patch you will see a total eclipse
of the Sun, but it may not be easy to get there.
The patch does not stay in one place all the time
because the Moon is continually moving, and the
Earth also is turning on its axis; the patch, there-
Fig. 75. A total Eclipse of the Sun.
fore, makes a track on the Earth, and it may
interest you to see the way in which these tracks
arrange themselves on the Earth as years roll on.
The movements of the Earth and Moon round the
Sun are of such a kind that after about eighteen
years they come back to nearly the same relative
positions. If they came back to precisely the
same, then, of course, the track would be exactly
repeated in the same place; but the repetition is
not so exact as this. Moreover, the interval is not
eighteen years exactly, but eighteen years ten and
one-third days, and the one-third of a day is im-
portant, because it shifts the track just one-third
of the way round the Earth, You will see how the
THE SUN
233
track of 1824 is followed eighteen years later by the
track of 1842 to the left, and that again by the
track of 1860, and that by 1878. But now we have
made three steps, each one-third of the way round ;
and we come back after fifty-four years nearly to
the same place. We must keep putting in the
Fig. 76.
word " nearly," because none of these things happen
exactly; you see that the 1878 track is just above
the 1824 track, which has the 1770 track below it in
turn. So that these tracks make a regular pattern
on the Earth which we can almost draw for ourselves
when we have got a few of them.
Since the tracks are edging a little further north
each time, they will at last go over the edge of the
Earth, at the North pole, and that family of
eclipse tracks will be finished. It came in at the
South pole 1 200 years ago; travelled gradually
234 A VOYAGE IN SPACE
further and further north, and will disappear after
1932. There are altogether at any one time twelve
such "families" of eclipse tracks; six of them
are travelling northwards and six are travelling
southwards. Every now and then one of them
goes out at one pole or the other; but there is
always a new family born about the same time to
keep the number of twelve families complete. A
few years ago, in 1909, a new family was born at
the North pole which is of special interest to us.
At its next return, in 1927, the track will come
much further south and will cross the north of
England (Fig. 77). So in thirteen years time you
will have a chance of seeing a total eclipse without
leaving England; you are very fortunate young
people, for your parents and grandparents and
greatgrandparents have had no such chance since
1724, nearly 200 years. You must be careful to be
ready for it; I think you will only have twenty-
five seconds of total eclipse; but still that will be
long enough to give you a good view of the beautiful
corona.
When the time is so short (twenty-five seconds is
specially short, but even the longest total eclipse
only lasts a few minutes) it is naturally important
to make the most of it. Photography has helped
us considerably in doing so, because we can arrange
our cameras beforehand in exactly the right posi-
tions, and we can drill those who are to take the
photographs until they can make the exposures
quickly and without a mistake. A delay or a
mistake would waste the precious seconds terribly;
but the force of habit is so strong that after going
THE SUN 235
through the operations several times in rehearsal,
the short time available is used to the very best
advantage. The operations are as a rule quite
simple, nothing more than opening a shutter at the
right moment, and closing it; but even simple
operations require a little practice to get them just
right. We know the difference between a company
U\CK of TOTAL ECLIPSE of SUN
JUNE 29 19*7
Tta /(tit viittrle irv England SIM? 1^
Fig. 77.
of recruits and the same people after they have been
drilled a little; simple operations like " shouldering
arms " are done clumsily at first, but smartly after
drill. And after a man has learnt one kind of drill
it is easier for him to learn another kind. For this
reason the help of His Majesty's forces, naval or
military, is specially welcome in eclipse work, and
I am glad to say that it has on many occasions
been very freely accorded. Eclipse tracks often
236 A VOYAGE IN SPACE
lie in such remote parts of the Earth that we cannot
afford to send many astronomers there; but there
are few parts of the Earth so remote that the help
of British soldiers or sailors cannot be obtained;
and with such help even a single astronomer may
arrange his work so as to secure a number of valuable
records.
To bring home to you the exciting nature of work
of this kind, we will pretend to have an eclipse in
this room. Here is the astronomer come to choose
his site for observation (one of the " juvenile
audience," previously instructed, here selected a
site on the lecture room floor) ; he calls in the aid
of His Majesty's forces (here several boy scouts
entered the room) and directs them to erect his
piers and instruments (the boy scouts now brought
in a rough wooden model of a coelostat and telescope-
camera and set them up on boxes representing
piers). You see the astronomer proposes to use a
coelostat this mirror is to reflect the Sun's light
into the camera. There is the Sun on our lantern-
screen, and you see that the eclipse has already
begun, for the edge of the Moon has taken a small
bite out of the Sun's disc. It takes about an hour
to cover the whole disc; we will not keep you so
long as that, but while it is moving slowly across I
will tell you what these gentlemen propose to do.
The camera is pointed to the coelostat mirror in
exactly the right position; and the astronomer is
to take five photographs with different exposures.
The first exposure is to be only for one second ; this
will suffice to show the brightest parts of the corona
though not the faint portions; but even if the
THE SUN
237
eclipse only lasted one second before (say) clouds
came up, the astronomer would have got something
to take away. Hence, he puts that short exposure
first. Next he ventures on five seconds ; and then
when he has got those two he ventures on a long
exposure of forty seconds, so as to photograph the
The coelostat and telescope used by the Astronomer Royal
in Japan in 1896.
(By a Japanese Artist.)
faint parts of the corona. A ten-second and two-
second exposure complete the programme of five.
Altogether, therefore, the camera will be open for
i + 5 + 40 -f 10 -f 2 = 58 seconds, and since the
whole eclipse is to last 100 seconds it looks as though
we might take more time than this ; but you must
remember that it requires some seconds to change
the plate for the next photograph. There are four
238 A VOYAGE IN SPACE
changes to be made, and if every change took ten
seconds we should be running it rather fine. Fortun-
ately the changes can be made more quickly than
this. Special plate-holders are arranged which open
like a hinged door instead of like a sliding door
as usual; and to ensure rapidity the astronomer
has two assistants, one to hand him each plate-
holder when he wants it, the other to take it from
him when exposed. In this way he can change
plates in six or seven seconds. There are never-
theless a good many things to do in this time.
After giving word to the man at the lens to put on
the cap (which for eclipse work may be a light
Japanese fan held in front of the lens without
touching it) he must close the door of the slide,
take it out of position and hand it to one assistant ;
receive the new slide from the other assistant, put
it in place in the camera, open the door and (after,
perhaps, allowing a second for the instrument to
settle) give the word for the cap to be removed.
With a little practice this can be done in six seconds,
and these gentlemen have very kindly been practis-
ing this morning so that they may not fail to carry
through the programme smartly this afternoon.
One of them I have not yet mentioned the time-
caller. It is his business to look at a watch and call
out seconds in a loud voice from the moment the
eclipse commences, so that the astronomer may
make the exposures of the right length, and know
how time is passing. Thus suppose that when the
astronomer has put in the holder for the fourth
exposure of ten seconds, the time-caller is calling
seventy-nine; the astronomer will know that he
THE SUN 239
must close the cap at eighty-nine ; which leaves him
eleven seconds in which to get the last exposure
of two seconds. If, however, there has been an
unfortunate delay anywhere, and the time-caller is
calling eighty-five instead of seventy-nine, the
astronomer realizes that if he gives the full ten
seconds up to ninety-five, it will be practically
impossible for him to get the last exposure of two
seconds unless the eclipse lasts unexpectedly long
(as may happen) ; he can thus shorten the exposure
from ten seconds to five seconds if he likes, so as
to make sure of the last plate. He has probably
thought over what to do in such a case beforehand,
for it is desirable not to leave such decisions to be
made in the exciting moments of the eclipse.
Now I think I have explained the details, and
the time of total eclipse is drawing very near, as
you see. The daylight is perceptibly less (a few
lamps were turned out) , but it is by no means quite
dark at a total eclipse ; one can read the figures on
a watch-face, though the time-caller may prefer to
use a metronome which gives him the seconds by
ear, and leaves his eyes free to watch the eclipse.
By this time we ought to be feeling distinctly
chilly ; it has been said that a cold wind springs up
about now, though others say the feeling is merely
due to the fall of temperature. You know well
enough the way in which it seems cooler even when
the Sun only goes behind a cloud ; at a total eclipse
this effect is much more noticeable; and it must
be admitted that there is a very solemn feeling
about it all. Natives are often very much frightened ;
if they have not been told anything beforehand it
240 A VOYAGE IN SPACE
naturally takes them very much by surprise ; while
if they have been told, the tale has often come from
the mouths of their superstitious priests, who tell
it in their own way and for their own purposes. In
India an eclipse is a specially good time during
which to bathe in sacred waters; certain sacred
pools used to receive such masses of bathers at an
eclipse that hundreds of them were suffocated ; but
they died in ecstasy, believing the manner of death
to be such as would ensure eternal bliss. Our
careful English rule has changed all this by passing
the bathers with great rapidity as through a turn-
stile, down into the water, and out again the other
side quick, so that the next person may come
but even our tyranny has never contemplated
stopping the bathing. Animals have no one to
instruct them beforehand and are taken by surprise.
In the next lecture we will have the kinematograph,
and you will see how the hens go to roost when
totality comes on and come running out after it is
over. Even astronomers, who know exactly what
to expect, feel the strain, especially in these last
few minutes when there is nothing to do but wait,
thinking over for the hundredth time whether
everything is in exact readiness. Some of them
may have dreamt the night before that the eclipse
is beginning and they have nothing ready for it,
and I can assure you that is a very distressing form
of nightmare. But here you see we are all ready,
and in a few seconds more the total eclipse will
begin. I will ask you to keep silence, please, so that
the time-caller and the astronomer's instructions
may be clearly heard. Now !
THE SUN
241
[At this signal the picture of the round Sun on
the screen, over which the dark Moon had been
slowly creeping, was covered entirely up, while a
representation of the corona was flashed out by
means of a mechanical device in the slide. A few
more lamps were lowered to represent the com-
The total Eclipse of 1898.
paratively sudden decrease of daylight as totality
comes on. The time-caller counted seconds : one,
two, three ... in an even tone up to the 100
seconds agreed upon; the astronomer received the
plate-holders (which were imitated in wood and
cardboard), put them into the camera, called for
an exposure of the required length and its close.
242 A VOYAGE IN SPACE
handed over the finished plate and put in the new
one, all without a hitch or mistake of any kind. As
a mere piece of drill the performance was warmly
praised by a military man present in the audience.
During the long (forty-second) exposure, the party
of observers duly took the opportunity of regarding
the corona, according to instructions; but for the
rest of the time their " eyes were in the boat." At
the hundredth second the corona disappeared with
the reappearing crescent of the Sun, and the lamps
previously lowered were relit ; the eclipse was over ;
but all the photographs had been secured, with
several seconds to spare, an achievement which was
duly appreciated by the audience.]
Now the eclipse is over, and if our photographs
are not properly taken we must wait till the next
eclipse; we cannot repeat this one. Fortunately
we have had fine weather sometimes poor astron-
omers travel thousands of miles, get everything
ready, and then it is cloudy or raining ! but I feel
sure that when we develop our plates we shall find
good pictures on them. And now what is the use
of these pictures when we have them? Well, I
cannot bother you with all the details of eclipse
work, but I will try to give one or two illustrations
of the way in which knowledge may be gained by
the study of these pictures; and I will take the
illustrations from my own work at eclipses, not
because it is more important than that of other
people, but because I know more about it.
I have been measuring the brightness of the
corona at different distances from the Sun. Near
the Sun it is very bright, as bright as the full Moon
THE SUN 243
or brighter. But it fades away so rapidly that at
about a diameter from the Sun it is very difficult
to see it or photograph it. When it is photo-
graphed you get dense blackness on the negative
for the bright parts and hardly any effect for the
faint parts. My work was to measure just how
dense the different parts were; and the measure-
ments are best made when the grades are not
either too dense or too faint, but intermediate.
Hence, you see why it is desirable to take photo-
graphs with different exposures; for by giving a
long exposure you can make the fainter portions
affect the plate more ; it is as though they were not
so faint. On the other hand, the short exposure
of one second will suit the dense parts better.
But supposing these measures made, and the
brightness of different parts compared, what then?
What use will that be ?
Well, we want to find out what the corona is
made of; is it simply an atmosphere of gases like
our own round the Earth, or has it solid particles in
it, as our own atmosphere sometimes has in a fog?
And again, is the atmosphere at rest or moving?
If there are solid particles in it are they floating
quietly, or are they being shot up out of the Sun
with great velocity? Or are they, perhaps, falling
continually into the Sun ? If an eclipse only lasted
long enough, we might perhaps answer some of
these questions by watching the changes; but in
the few minutes of a total eclipse the changes are
too small to be noticed, and, by the time the next
eclipse comes, everything has been altered past
recognition. Hence, we have to work in other
244 A VOYAGE IN SPACE
ways. Suppose the particles are being shot up out
of the Sun; then they will scatter as they go, like
the sparks out of a rocket ; and being more diffused
they will send us less light per unit area. This will
explain in a general way why the corona should be
fainter as we recede from the Sun; but in science
we must not be content with general explanations,
we must see whether they fit particularly. We
can calculate not only that the corona ought to be
fainter but how much fainter it ought to be; and
we can measure on the photographs how much
fainter it is; and see whether the two "how
much's " agree : if not, then our idea of the particles
being shot up won't work and we must try some
other idea, perhaps that they are falling continually
down. When I tried these different ideas I con-
cluded that the particles were behaving as a fountain
behaves both being shot up and falling down again.
It would take us too long to explain why all I
want you to see is the kind of way in which we may
learn about the corona from photographs at an
eclipse.
But I have not explained why there need be solid
particles in it at all. Why should it not be merely
a mass of gas like our own air ? That again can be
answered from the photographs if we take them in
what is called polarized light. I am afraid I am
worrying you with rather hard notions this after-
noon, or at any rate hard names; there is nothing
very hard about the notion of polarized light, if
you will think of a ray of light as a flat thing like
a strip of cardboard which will bend quite easily
sideways but not edgeways. I ought, however, to
THE SUN
245
compare a ray of light not to a single strip of card,
but to a bundle of such strips with their edges in all
directions, not a tight bundle, but a loose bundle in
which one strip does not interfere with another.
Now what would happen if we tried to bend the
whole bundle in any particular direction? Some
of them would have their edges turned in that direc-
Fig. 78.
tion and would refuse to bend at all ; others would
have their flat sides and would bend easily; and
we may say that after bending we should have only
strips with their flat sides more or less in the right
direction. So it is with a ray of light which is bent
by being reflected from a solid particle. If there
are solid particles in the corona, then we see them
because the sunlight which shines on them is bent
246 A VOYAGE IN SPACE
or diverted to our eyes. In Fig. 78 I have tried to
draw four rays of light like strips of cards ; starting
from the Sun in the middle and going out sideways
from him, so that if they are not bent or reflected,
they will never reach the Earth, which is supposed
to be on the right of the picture. But I have
supposed them to be reflected (by solid particles in
the surrounding corona) at the places shown, and to
come earthwards. Rays which are edgewise and
cannot thus be reflected are left out of the picture.
Now, on reaching the Earth, if we try to " polarize "
these four rays by bending them downwards, the
top and bottom rays will bend, but those at the
sides are edgewise to this direction of bending and
refuse. Hence in our " polarized " picture we should
get light from the corona at the top and bottom
of the picture, but not from the sides. Another
picture with sideways bending would show the
opposite.
The sunlight is originally a bundle of rays with
their edges in all directions; but in the encounter
with the solid particle those rays which are edgewise
refuse to bend, leaving only the others, so that
the ray is now " polarized " as it is called. If the
corona is merely a mass of gas, the light will not be
polarized. If the corona is partly gaseous and
partly solid particles, we shall get some light from
the solid particles which is polarized and some from
the gas which is not. How much is there of each
kind? Here again we come to the question of
measurement, of finding out how much of a thing
there is, and by arranging the particular photo-
graphs to be taken we can find it out by measure-
THE SUN
247
ments of the same kind as before, viz. of the density
of the negative at different points. In Fig. 79 the
CORONA
SEEN BY
ORDINARY
LIGHT
PARTIAL POLARIZATION
Fig. 79.
light of the corona is supposed to be equal in all
directions as in the top picture. If on taking a
pair of pictures, one polarized for bending right
and left, and the other up and down, we got the
248 A VOYAGE IN SPACE
results shown in the middle, we should say that
the polarization is complete. If we get a pair as
shown at the bottom, we should say that it is only
partial. The actual pair I got in Egypt in 1905
is shown in Fig. 80. My conclusion is that there
are a large number of solid particles in the corona,
because there is a great deal of polarized light;
Fig. 80. Two pictures of the 1905 Corona, polarized in
perpendicular planes.
and other astronomers have found the same thing.
And now, since you have listened so patiently to
this tiresome explanation, I should like to show
you that polarized light can give us some very
pretty colours, which I am sure you will enjoy
seeing.
[The lecture concluded with an exhibition of
polariscopic effects from the magnificent collection
of apparatus at the Royal Institution.]
LECTURE VI
THE STARS
WE have now to make our longest journey of all
to the stars. Even as fast as the telescope will carry
us it will take us some years to get there. It is
difficult, perhaps, to realize that light seems to go
quickly, yet really is taking time to travel; that
really it takes time for light to travel, let us say,
from me to you not very long perhaps; but still
you do not see me as I really am, but as I was a small
fraction of a second ago. It is easier to realize the
interval in the case of sound; and several times in
this lecture I shall have to remind you of the likeness
between light and sound, because it is so much easier
to understand in the case of sound things which seem
very difficult in the case of light. For instance, we
all know that sound takes time to travel, because we
have heard echoes. We shout, and presently we
hear our shout coming back from some distant point.
The time in between is the time taken by the sound
to travel. When you look in your mirror in the
morning, you really have to wait a little time before
the light from your face goes to the mirror and comes
back again, so that you get the echo or reflection from
the mirror. I shall have something more to say
about these light echoes towards the end of the
249
250 A VOYAGE IN SPACE
lecture. Light, then, takes time to travel, but the
time taken is very, very small. I wonder how
small a fraction of a second you think it takes
to travel from me to you ? Well, you know how
long a year is; and how long a second is. Have
you any idea how many seconds there are in a
year? About 30 millions. Now suppose you pre-
tend that a second is itself like a year ; divide it into
its seconds, that is into 30 million parts ; one of these
tiny parts will be about the time that what we may
call a " wireless " wave of electricity takes to vibrate.
I do not mean the time that the spark takes, but the
time a single wave takes to be transmitted its whole
length. And then suppose you take one of these
" wireless " wave times, and divide that up into 30
million parts, you get about the time that a light
wave takes to vibrate. And the time that it takes
you to see me is about one wireless vibration, a very
small fraction of a second, but still containing 30
million light vibrations.
We might make a new set of tables for you to
learn
30 million light vibrations make one wireless
vibration.
30 million wireless vibrations make one second.
30 million seconds make one year.
And 30 million years what do they make ? Well,
I shall have something more to say about that
towards the end of the lecture.
For the present we will start with a year or two.
It takes a year or two to get to the very nearest star,
and many years to get, for instance, to most of the
bright stars of the zodiac. " The Ram, the Bull,
THE STARS
251
the Heavenly Twins " have you yet learnt their
names and order?
These stars of the zodiac are comparatively bright
and near. But there are many stars in the heavens
which are fainter, that we cannot even see with the
eye, especially those
in the neighbourhood
of the Milky Way.
In Fig. 81 you see a
very large number of
stars, some of them
big. Two of them
are so bright that
their light has made
a kind of halo by
reflection from the
back of the photo-
graphic plate. That
circle of light does
not belong to the
star; it is merely a
photographic fault,
showing how bright
the star really is.
But in the picture
there are also many fainter stars, and even dark
patches where there seem to be no stars at all. Now, do
you think it is possible that there are no stars at all in
those places ? The photograph rather deceives us by
being a flat thing ; as though the stars were scattered
over a flat surface; but that is not really the case.
When Mr. Daily Mirror came and photographed us
all the other day, his picture was also on a flat^plate ;
Fig. 81. Region near the Southern
Cross.
(The Cross is on the right, two big stars and
two much smaller.)
252 A VOYAGE IN SPACE
but that does not deceive us into thinking that this
room full of people is flat. We know that there are
some in the front row who come out big in the picture
and some in the back rows who come out smaller.
And so in the case of the stars ; big bright ones are
probably in the front row near the Earth : the very
faint ones in the back row. I say probably because
we cannot be quite sure. Even in this room there
are some very small people in the front row; and
they come out small in the picture; while their
grown-up parents may look large in the picture, in
spite of the fact that they are in a row behind. In
this case we can generally tell from the dress, or the
character in the face, that the difference of size is
due to age and not to the position in the room;
but the stars do not help us in this way by their
appearance, so that we are liable to mistakes if
we assign their distances from us simply by the
brightness.
But now what are we to think of the places on the
photograph where there seem to be no stars ? There
are some directions in which I can see no people in
this room, namely, when I look along the corridors :
but I have to be careful to stand in one special place
(A) to see this emptiness. If I move to the right,
as to B, then I lose the appearance of vacancy in all
three corridors (Fig. 82).
You can try a similar thing for yourselves with an
empty tube. If you point it straight to your eye
you can see the vacant space at the far end ; but if
you incline the tube, the far end disappears behind
one of the sides. If the tube is short, you may have
to incline it a good deal before the end disappears ;
THE STARS
253
but if it is very long, the slightest tilt will make you
lose the end.
Now, if these patches showing no stars are really
vacant spaces, they must be like very long corridors
or tubes pointed very straight at us : that they are
very long, we know, because we have evidence that
the stars extend to immense distances. The people
in this audience are confined within a definite room
quite a nice, large room, it is true, but still a room
LECTURE ROOM SHEWING EMPTY
SPACES FROM POSITION A. BUT NOT FROM 8.
bounded by solid walls. There are no walls con-
fining the stars : behind the front rows there are
others, and behind them others still, and again others,
and we never come to walls, so far as we know at
present ; so that the corridors through the audience
of stars must be enormously long. Therefore, if we
really see blank spaces at the end of them, they must
be pointed dead straight at us, because the very
slightest tilt would cause the end to disappear when
the corridor or tube is so very, very long.
254 A VOYAGE IN SPACE
Now, can we think it likely that so many tubes or
corridors or lanes through the stars should be pointed
actually dead straight at our poor little Earth, or
even at our tiny solar system? Here in this room
the corridors all point at the lecturer, because he is
a very important person. If there were no lecturer,
I suppose you would all go away, wouldn't you ? No
lecturer, no audience ! But we cannot think that
our Earth, or our Sun, is of any particular importance
to the audience of the stars millions and thousands
of millions of them, mostly far bigger and brighter
than our Sun. In times gone by our ancestors
thought that the Earth dominated the Universe ; but
we have gradually learnt modesty : nowadays we
cannot think that the Earth and Sun are of any
particular importance.
No ! we must seek some other explanation of the
blank patches ; and it is not very difficult to find one.
I can easily make a patch with no children visible
by holding a screen in front of my eye. If the
screen is near enough, quite a small one will do. A
threepenny bit shuts out quite a lot of you. And
similarly some kind of a screen, of dark nebulous
matter, say, will easily explain any of these dark
patches.
Hence we come to the conclusion that not only
are there stars of the last degree of faintness, but
there are in the heavens bodies of some kind not
shining at all " dogs in the manger," which not
only do not shine themselves, but stop the light of
other stars from reaching us. I don't know whether
the argument seems to you convincing, but per-
haps one or two other pictures may help it. Fig. 83
THE STARS 255
is a picture of a strip of nebula which seems curiously
split down the middle. Here, again, we must re-
member that the nebula is not a flat object like a
piece of paper, but a bulky object like a cushion. If
there is a split, it must be a slice right through the
cushion, and, besides this, the slice must be straight
in the direction of the Earth. Both these difficult
Fig. 83. Split Nebula in Andromeda.
suppositions are unnecessary if we suppose there is
a dark streak in front of the nebula ; we may go
further and presume that the streak is part of a flat
ring surrounding the nebula, stopping the light
where it passes in front.
Fig. 84 is a picture of a nebula of a different
kind. You see a gauzy mass running down the pic-
ture with a bright edge : and you will see that the
stars are much more numerous on one side of the
256 A VOYAGE IN SPACE
edge than on the other. It may be that they really
are more numerous; but, if so, there must be a
dividing wall pointed straight at the Earth, separat-
ing the region of many stars from that of few. It
is much easier to believe that there is some obscuring
stuff between us and the side which shows fewer
Fig. 84. Nebula in Cygnus.
stars ; so that the light of many of the faint stars is
blotted out. If we count the number of stars within
a given area on one side of the edge, and also the
number in an equal area on the other side, we shall
get a notion how many are lost by the stoppage of
light.
This suggests that it may be of interest to count
THE STARS 257
the stars generally; and it is not only a matter of
interest but of great importance. Let me take an
instance from this room. You know how the front
row is a little bit shorter than the next row and won't
seat so many people ; the row behind will seat a few
more, and so on till we get to the back row, which
will seat a good many. If we knew the distance
between the rows we could count how many people
there ought to be in each succeeding row. And it is
very much the same in the case of the stars. If we
count the bright stars, we know how many there
ought to be of the next order of magnitude; and
how many there should be next after that. Suppose
we count them, and there are not so many suppose
we counted the outside row and found there were
not so many people as it would hold, we would say,
" Why this audience is beginning to thin off at the
back ! " So with the stars, we are inclined to think
from the counts we make that they begin to thin off
in the distance ; and perhaps if we go far enough we
should not find any more stars. Look, for instance,
at this table l of the numbers of stars of successive
magnitudes. Up to and including the second mag-
nitude (let us say, the front two rows), there are
38 stars, and we can calculate (though I will not
bother you with the details) that when we add the
third row, or magnitude, we ought to have 151,
very nearly four times as many; but we only find
in. Adding the next row, or magnitude, if 38
is right, we ought to get 603, but only find 300, and
so on.
1 Taken from Chapman & Melotte, Mem. R.A.S., Ix.
p. 163.
258
A VOYAGE IN SPACE
Magnitude.
Observed.
Calculated
fronc, 38.
Calculated from 32,360.
2-0
38
(38)
8
3 -0
III
151
32
4 -0
300
603
129
5'0
95
2,399
513
6-0
3,150
9,550
2,042
7'0
9,810
38,020
8,128
8-0
32,360
151,400
(32,300)
9-0
97,400
128,800
lO'O
271,800
512,900
II'O
698,000
- .
2,O42,OOO
I2'0
1,659,000
8,128,000
13-0
3,682,000
32,360,000
14-0
7,646,000
128,800,000
15-0
15,470,000
512,900,000
16-0
29,510,000
2,O42,OOO,OOO
17-0
54,900,000
8,I28,OOO,OOO
By the time we get to the eighth magnitude the
numbers are so far behind the calculations that
it seems useless to go further. But we can make a
fresh start ; let us assume that there is something
peculiar about the first few rows perhaps they are
too closely packed, and let us start fair again with
the eighth magnitude and the number 32,360 as shown
in the last column. We can calculate both down-
wards and upwards, and we see that for the bright
stars the calculations give fewer than there are, and
for the faint stars many more.
We see then that the back rows are not properly
full ; and we get the idea that probably the stars do
not go on for ever ; they may not actually come to
a sudden stop, as this audience does, owing to the
walls of the room; because we cannot think there
can be containing walls for the stars ; but they seem
to thin out and ultimately fail, as a small audience
THE STARS 259
might in a very big hall front rows full, middle rows
rather scattered, back rows quite empty. It does
not need a great stretch of our imagination to think
of a cluster of stars of this kind, for we have many
instances in the sky ; Fig. 85 is a picture of one for
you to look at. Looking at them from outside
Fig. 85. The Star Cluster in Hercules.
we can very easily see the way in which they thin
out. When we ourselves are inside such a cluster,
it is not so easy; but by careful counting of the
different classes of stars, and reasoning upon the
counts, we may be able to do it still.
But we must be careful, and there is especially one
mistake of which we must be careful; a possibility
of mistake at which I have already hinted. There
26o
A VOYAGE IN SPACE
may be grown-ups in the back rows and small people
in front : in other words, there may be very bright
stars at a distance, and faint ones near to us ; and if
we do not remember this, our inferences from the
counts will be wrong.
Let me give you an illustration. Fig. 86 is a picture
of three men. You say unhesitatingly that the big
man is near us, the tiny man far away, and the middle
man in between. You say this because men are all
Fig. 86.
nearly of the same height, and when they appear so
very different in size as in the picture, the difference
must be due to distance. So we think. But now
I have deceived you in this picture by omitting
certain details; these are not men, but puppets
hung on a Christmas tree, as you see in Fig. 87.
They are really all at the same distance; and
their difference in size is real, because dolls and
puppets can be as different in size as we like.
What led us wrong was the quiet assumption that
THE STARS
261
the figures were those of men all much the same
in size.
We must be careful not to assume that about the
stars. They may vary as much as dolls or puppets
in actual size ; indeed we have found by experience
Fig, 87.
that they do, and here is one of the pieces of experi-
ence. Sir David Gill, now, I grieve to say, lying
very ill with pneumonia, 1 picked out a number of
stars of about the same brightness and actually
measured their distance away by the method of
1 Sir David Gill died on January 24, 1914.
262
A VOYAGE IN SPACE
" squint/' or parallax, which we dealt with in the
second lecture. He found the distances very
different indeed; so that the actual sizes of the
stars must be also very different.
SIR DAVID GILL'S DISTANCES OF FIRST MAGNITUDE STARS.
Name of Star.
Brightness
compared with
an exactly
first magnitude
star.
Parallax
or
"Squint."
Distance
in
" Light Years."
Diameter of
star com-
pared with
a 2 Centauri.
Sirius . .
13
0'37 9
6
Canopus .
6
O'OO ?
> 140
Rigel . .
2
O'OO ?
> 75
a 2 Centauri
2
'75 4
i
a Eridani .
I*
0-04 80
18
)8 Centauri
I
0-03 100
20
a Crucis
I
0-05 64
II
Spica . .
I
O'OO
p
> 55
In the first column is the name of the star : Sirius,
or the Dog Star, I expect you have heard of, even if
you don't know the others. In the next column
you see that Sirius is not exactly a first magnitude
star, it sends us actually thirteen times as much light
as the standard which has been adopted as first
magnitude, and others in the table are brighter than
first magnitude. In the next column you see the
amount of parallax or " squint " ; but I expect you
prefer to look at the column after that which shows
that Sirius is nine " light years " away, meaning that
light, travelling as we know at 186,000 miles per
second, actually takes nine years to come to us from
Sirius ! Can you now tell how many miles away
Sirius is? Remember that there are 30 million
THE STARS 263
seconds in a year, and you will find that Sirius is
more than 40 billion miles away using billion in
our English way for a million million. (In France
a billion means much less : they use it to mean
only a thousand million.)
Now this is far enough in all conscience ; but what
are we to say of Canopus and Rigel and Spica ? They
are so far away that Sir David Gill could not find any
" squint " or parallax at all, and had to write zero
in the third column. If the " squint "had been as
large as that for /? Centauri, or even half as large, he
could have measured it. These stars must be at
least twice as far off as ft Centauri, which is 100 " light
years," or, say, 440 billion miles from us.
In the last column of the table I have calculated
the actual sizes of the stars compared with a. 2 Cen-
tauri, assuming that their surfaces are all just
as bright as that of a 2 Centauri ; which again is not
a justifiable assumption, but we may venture to
make it for the sake of illustration, and in order to
bring out the next point. You will see that some
are very big and some very small ; they vary in size
at least as much as dolls, I think. I never saw a doll
more than four or five feet high say fifty inches;
and I never saw one less than about half-an-inch ;
so that the biggest would be about 100 times the
smallest. We may fairly say that Canopus must be
at least 100 times the size of a 2 Centauri.
Before we go further, let us think for a moment
how astonishing it is that light should reach us from
those vast distances at all. If the stars sent out
light in only one direction as a searchlight does, we
could understand it better; the rays of a search-
264 A VOYAGE IN SPACE
light are nearly parallel and in consequence they
can go a long distance without failing in brightness.
Let us make the lantern send us a parallel beam of
light like that of a searchlight : you can see that it
does not alter much in cross-section from the lens
right away to the wall ; and if the rays were strictly
parallel, it could go across a million or a billion miles
without being diminished in brightness. But the
light of a star does not behave like this : it spreads
out in all directions continually, like the light from
this other beam (see Fig. 88), getting, therefore, all
the time fainter. On leaving the lantern it spreads
out continually as wide as we like, if only we go
far enough. That is how the light of a star be-
haves, and accordingly it gets fainter and fainter
the farther it goes. You can see this very easily :
we let both the beams fall on the same screen, and
when we put the screen near the two lanterns we
can make the two patches of about equal brightness.
Now. they are about of equal brightness. But now
if we move the screen away, the patch from the
parallel beam remains about the same, while the
other gets fainter and fainter. Let us stop here a
moment ; and now we will put a dark glass in front
of the parallel beam lantern, so that its patch is made
as faint as the other once more. Again move the
screen further off, and the spreading beam again
gets fainter. If -we bring the screen back towards
the lanterns, the spreading beam recovers its bright-
ness, and finally it ends by being brighter than the
other. So in spreading out in that way the light of
a star is always losing itself; and it is wonderful that
it should get to us across those vast distances without
being so faint that we cannot really see it at all. The
THE STARS
265
light even of some of the brightest stars must travel
hundreds of years before we can see it. And we have
seen that there may be other things which might
diminish its light on the way, dark nebulae which
may screen or stop its light. So that it seems
wonderful that we should see stars at all. Never-
theless, we do ; we get light from even faint stars
across long distances ; and that shows there cannot
FIRST
SCREEN
be very much in the way between us and them.
There may be a little very fine " fog," as we might
call it ; according to our best information to-day
there probably is a little very fine " fog " between
us and the most distant stars ; but there cannot be
much ; otherwise we should not get light and even
some heat from the stars. The heat that we get
is far harder to detect than the light : it has taxed
the utmost resources of physicists at the present day
to measure it. Apparatus so delicate that it could
266 A VOYAGE IN SPACE
detect the warmth of a young lady's cheek some
miles away, failed to record any heat from the stars.
Perhaps you think the heat may get used up on
the way because it has to pass through such fright-
fully cold space. There is no doubt that if we
actually took our " Voyage in Space " instead of
only pretending, we should be chilled to the marrow
very soon indeed. It is terribly cold in space far
colder than we can possibly imagine. You remem-
ber the liquid air? Well, cold as that is, it is hot
compared with the cold of space. And yet the heat
does not get lost in passing through this cold any
more than light in passing through simple darkness.
If there is anything to stop the light anything
material, such as a dark nebula, however flimsy
that is another story; then light would be lost. But
if there is nothing of that kind in the way the light
can go on for hundreds and thousands of years, and
so also can heat. Let me show you rather a pretty
experiment. Here is a flask of liquid air very cold,
as you know already. We can pass a beam of light
and heat from the lantern through it so that the
liquid air will not only transmit the heat, but will
focus it like a " burning glass/' so that we can set a
piece of paper alight. There, you see, the paper is
on fire ! That is because the liquid air, cold as it is,
and the flask containing it, are transparent to heat ;
and similarly space, cold as it is, is transparent to
heat, so that we get the heat of the stars across these
immense distances, and though the early experiments
to detect it failed, others were more successful. The
heat of some stars has been definitely measured.
Now let us turn to quite a different thing con-
cerning the stars; to an announcement made two
THE STARS 267
centuries ago by Halley, that great Englishman who
first told us a comet would come back, and was so
proud that an Englishman should have been the first
to predict it. He also made an even greater an-
nouncement, namely, that the stars were moving.
From the beginnings of intelligence in man up till
then it had been believed that the stars were fixed;
they were called " fixed stars," and the name still
survives. Yet Halley pointed out that when old
observations were compared with those made in his
day, one could not but conclude that some stars had
actually moved. He chose especially Sirius, Alde-
baran and Arcturus, which had moved since the
time of Ptolemy more than the Moon's diameter in
one direction, while Betelgeuse, the brightest star
in Orion, had moved nearly twice that distance in
the opposite direction.
" What shall we say, then ? " he wrote. " It is
scarce credible that the Antients could be de-
ceived in so plain a matter, three observers
confirming each other. Again, these stars being
the most conspicuous in Heaven, are in all pro-
bability the nearest to the Earth, and if they have
any particular motion of their own it is most
likely to be perceived in them."
He seems almost apologetic : for he knew how
slow people are to accept a new idea, however
plain the evidence may be. After so many centuries
of believing the stars to be fixed, it was very hard
to unhitch the ideas. " Again, these stars being
the most conspicuous in Heaven, are in all probability
nearest to the Earth." We have already seen that
this is not quite true, but it is nearly true, and the
268 A VOYAGE IN SPACE
failure from truth is only a detail. The great
thing was to realize this wonderful fact for the first
time that the " fixed stars " were moving : and
I think Halley might have been even more proud
than in the case of his comet that the discovery
was made by an Englishman.
It might have been a Rooshian,
Or a French, or Turk, or Prooshian.
Mr. W. S. Gilbert has chaffed us about our pride
in being Englishmen; but there are some things
in which we may justly feel a national pride, and
surely one of them is that an Englishman (and I am
glad to add an Oxford man, writing from Oxford)
first set the " fixed " stars in motion.
A great many important consequences have
followed from this discovery of Halley 's. In the
first place we find that the movements of the stars
afford us a much better test than their brightness
for judging whether they are near us or far away.
If they are very far away their movements will
appear very slow : it is because even the nearest star
is so far away that no one noticed any movement
in them until Halley discovered it. They are
really moving very quickly, but owing to the
distance they seem to creep ever so slowly. Do
you know Tennyson's beautiful verses about the
Eagle ?-
He clasps the crag with crooked hands,
Close to the Sun in lonely lands,
Ringed with the azure world, he stands.
The wrinkled sea beneath him crawls,
He watches from his mountain walls,
And like a thunderbolt he falls.
THE STARS
269
In the fourth line Tennyson uses both the effects
of distance which we have been considering : it
reduces the apparent size of objects so that big
sea waves look like mere " wrinkles/' and it
reduces the apparent movement until the rolling
of the billows looks like mere " crawling." So the
huge globes of the stars dwindle at their vast dis-
tances into mere specks of light ; and their terribly
rapid movements perhaps a hundred miles a
second look like mere " crawling," so slow that
it wanted the eagle-eye of a Halley to detect them.
Astronomers have followed up the clue thus
found; by watching the stars very carefully for a
century and more they have measured the rates
at which a good many of them are " crawling."
Some seem to go very slowly indeed; others more
quickly; and Sir David Gill has found that those
which seem to move quickly are, generally speaking,
near to us. Here is a table of his results
SIR DAVID GILL'S DISTANCES OF RAPID STARS.
Star's Name.
Brightness
compared
with a first
magnitude
star.
Annual
Motion.
Parallax
or
"Squint."
Distance
in
"Light
Years."
Diameter of
star com-
pared with
0-2 Centauri.
ZC.Vh2 43 . .
I/IOOO
8-7
0-31
II
I/I 7
Lacaille 9352
1/250 7-0
0-28
12
1/8
Indi . . . 1 1/33
4'7
0-27
12 1/3
o 2 Eridani .
1/25
4'i
0-17
19 2/3
e Eridani . .
I/2I
3'i
0-I 5
22 I
Hydri . .1/6
2'2
0-13
25
2
Tucanae . 1/21
2-0
0-14
2 4 I
r Ceti . . .
I/II
2-0
0-31 II 2
Lacaille 2957
I/IOO
1-7
0-06 55 i
270 A VOYAGE IN SPACE
The names in the first column need not concern
you further than to notice that they are mostly of
a weird kind; clearly these are not familiar stars,
but small fry; and the brightness in the second
column shows this even more clearly. They are
stars which are chiefly of interest to astronomers
because of their rapid movements shown in the
third column : and you see that they all have
measurable " squint " and are within 100 light
years of us. The last column shows that they are
probably much smaller than the former stars.
The table therefore shows that if a star seems to
be moving quickly it is probably nearer us than
other stars : if very slowly it is probably far away.
But again we must not be too sure in either case,
because appearances are often deceptive. Have
you ever noticed a train coming nearly straight at
you from a long way off? When you are waiting
at a station sometimes the lines curl round a corner,
so that you do not see the train till it is pretty
close; but sometimes they run straight for a long
distance and then you can see your train coming
from a long way off. It looks no bigger than a toy
train, and moreover it seems to be quite still. You
know that it is coming towards the station (at least
you hope so if it is the train you want to join), but
you cannot see it move because it is coming at you
end-on. It might even be going the opposite way
for all you can tell by looking at it. Similarly, if
a star seems to us to remain nearly still, we must not
be too hasty in thinking that it has very little
movement of any kind, and therefore it is very
far away, for it may be coming directly at us, or
going directly away from us; by merely watching
THE STARS 271
it we cannot tell. Fortunately, a beautiful method
has been discovered by which we can determine
whether this is so whether the star is coming
towards us, and how fast it is approaching. Halley
showed us how to watch the stars moving across
our line of sight, and another great Englishman,
Sir William Huggins, showed us how to find out
with the spectroscope whether they are coming
end-on to us, or moving away from us : and I want
now to say something of how he did it.
Let us return to the train for a moment and
suppose it whistles as it approaches the station.
It will whistle a high note; let us say high D on
the piano. Now if the train is coming at us the
note will not sound like D, but like a note higher in
the scale, say D sharp ; if on the contrary it is
running away from us, the whistle will sound lower
in the scale, say D flat. Sometimes a train rushes
right through a station whistling all the time, and
then you can notice the change from D sharp to
D flat quite easily. Many people have thought
that the train changed its whistle as it went through,
but that is not the case, it keeps on whistling the
same note all the time; but while it is still
approaching you hear a higher note ; immediately
it has passed you and begins to run away you hear
the lower note.
You can hear much the same kind of thing with
a passing motor-car if it happens to sound its horn.
Or without the horn at all, if you listen to the hum
of the machinery you will hear it change its note
as it passes. If the motor-car is going slow, the
change will not be much; but if it is going very
fast, the change will be great. It would be quite
5000 5500 6000 7000
z Sun
9 In
10
Sirius
rrn
on
VARIOUS SPECTRA.
(See Notes to Illustrations.)
References to this are on pp. 126, 128, 275.
\Tofacep. 273.
THE STARS 273
day, but Sunday's loaf was delivered late say on
Tuesday ; if all the others were two days late also
then you would still get a loaf every day. But if
they got less and less late until Saturday's loaf
came as soon as it was baked, then you would get
a whole week's loaves in the five days. After that
we may suppose the delivery begins to get late
again until the next Sunday's loaf is once more
two days late seven loaves in nine days, which is
like the low note of a train leaving a station.
If this does not seem clear, never mind : you
will perhaps find a better explanation some day.
But now I want you to think of light instead of
sound, because the stars do not whistle to us, but
they do send us light. You remember that we
were to compare light and sound several times in
this lecture? And I want you to realize that just
as there are scales in sound, which you have to
practise on the piano, so there are scales in light-
nothing more nor less than those beautiful colours
which a prism makes for us. We will throw one
of those rainbow-coloured strips on the screen;
and to fix the likeness of colours to scales in your
minds I will put this harmonium underneath the
strip and play a scale. The low notes in the bass
correspond to the red colour, and as we go up the
scale we pass through orange, yellow, etc. all the
colours of the rainbow up to violet. You might
sing the names to a scale so as to remember this fact.
I \-
- . r i \
~J I" r
Red Orange Yellow Green Blue Indigo Violet Ultra
T
274 A VOYAGE IN SPACE
Perhaps you don't know that last colour ; it is
mainly put in to fill up the scale, which would
otherwise stop awkwardly short ; it does not mean
ultra-marine, but ultra-violet. I don't mean that
the colours correspond to these notes in detail,
but that in passing from red to violet the pitch rises.
If a star is whistling a green note, that is to say
sending us green light, then if it is also coming
directly at us the green note will rise in the scale
towards the blue; if the star is running away the
green note will fall in the scale towards the yellow.
How much it rises or falls depends on the rate of
movement, and I may say at once that the rise
or fall is very slight indeed, because however quick
the movement of the star may seem to us, it is
very, very slow compared with the enormous velocity
of light. But the rise or fall is noticeable and
measurable; by noticing it and measuring it
astronomers have found out which stars are coming
at us and which are running away, and how fast
they are coming or going.
Some of the best illustrations of this way of
measuring velocities, whether by sound or by
light, are afforded by bodies which are whirling or
turning round, because sometimes they approach,
sometimes they recede. Here is an instrument
called a " bull- roarer " which some of you may
have made for yourselves a shaped piece of wood
with a string tied to it. If I whirl it round it makes
a humming noise. Now I can whirl it round so
that it remains always at the same distance from
you, and then the hum is steady ; but if I turn end-
on to you, sometimes it approaches and sometimes
THE STARS 275
it recedes, and you can hear the note change up
and down.
Similarly, if one star is whirling round another, it
may do so in such a way as to remain nearly at the
same distance from us, when its light remains of
uniform colour ; but the orbit may also be end-on to
us, when we see the colours go up and down. I
am using the word colour in a special sense : when
we spread out the light of a star into colours by a
spectroscope, we see the band crossed by a number
of dark lines which mean special colours : and it
is these lines which are displaced towards the violet
or towards the red (see Plate facing p. 273). And
now we will look at a very beautiful instance of
this displacement in the case of the planet Saturn
and its ring.
Let us first consider the planet himself or the
"ball," as it is usually called to distinguish it
from the " ring." Suppose we have cut a slice
right through the centre O ; the slice will be turning
round O (Fig. 89). The left side A is coming at
us and the right side B is running away ; so that if
we pass the light of this slice through a spectro-
scope, one end of the colour-line will be displaced
one way and the other the other, in consequence
of which we get a sloped line XY instead of a
horizontal one PQ. Next, if the ring were solid
and attached to the ball, the same state of things
would continue in the ring ; the line XY would be
merely extended to HK. But the ring is not solid,
as we have seen : it is made up of little satellites,
the outer ones moving, not faster than the inner
ones, as they would if the ring were solid, but
276
A VOYAGE IN SPACE
ROTATION OF SATURN
slower than the inner ones. Hence the ends of
the line, instead of being more displaced, are less
displaced, and we get an elongated Z shape VXYW.
This was predicted from knowledge of the move-
ments, and when the spectroscope was actually
turned on to Saturn the prediction was beautifully
verified ! So that we may feel all confidence in
the power of the
spectroscope to tell us
about these move-
ments of approach
and recession.
We have, then, two
different ways of
measuring the move-
ments of the stars;
it is a piece of great
K good luck for us that
we have two, for we
can often test what
one method has told
us by using the other.
And now I want to
tell you some of the things we have found out by
watching the movements of the stars in these two
ways.
First of all let us pay a few visits to the double
stars cases where two stars are seen close together,
and where careful watching has shown that they are
revolving round each other. Sometimes one is so
much the larger that we may almost say that the
smaller is revolving round it, like our Earth round
the Sun ; but there are other cases when the stars
1 j
Li
tl
t
SHAPE OF
THE STARS 277
are so nearly equal in size that we cannot give one
the preference in this way; and in all cases it is
more correct to regard both stars as revolving
round some point in between them than to regard
one as fixed and the other as revolving round it.
These double stars are often beautifully coloured ;
one of them being of a different colour from the other.
But our chief concern to-day is not with the colours,
but with the movements of the two partners.
They are waltzing round one another just like a
pair of dancers in a ballroom : the idea of waltzing
will help us very much, because the dancers not
only turn round one another but move down the
room at the same time. At the corners of the room
they have to turn, and it is possible that our waltzing
double stars may have to turn at some corner or
other in the future : at present we cannot say
because we have not been watching them waltz
long enough; we have only had about a century
to watch them since they were discovered, and even
a century is but a short time in the life of a double
star. Some of them take as long as that to make a
single waltz-turn, and others take much longer
so much longer that in the century since they have
been watched they have scarcely turned at all.
You may ask how we know that they are partners
in that case. Well, sometimes you see a pair of
partners in a ballroom not waltzing round at all,
but going down the room without turning. We
know that they are partners because they keep
together as they move down the room; and so it
is with these double stars which do not seem to be
turning they are nevertheless moving together
278 A VOYAGE IN SPACE
among the other stars hand-in-hand so to speak;
so that we may safely regard them as partners.
Thus a ballroom helps us in several ways to think
of the movements of the stars, but now I come to
a point where it ceases to help us. Partners in a
ballroom are always in pairs; I have seen three
people waltzing together for fun, but you would
not see it often; and I doubt whether you would
ever see four or five together, let alone twenty or
thirty. But with the stars it is different. The
general rule is for a pair of stars to waltz together,
but we often have three or four, and lately we have
been finding groups of thirty or forty together, or
even more. We detect the partnership because
they move together in the ballroom : when there
are only three or four, we can sometimes see them
also turning round one another, but the movements
are apt to be complicated when there are more
than two, because it is difficult to know which of
the several partners you are to pay attention to at
any one time. All the partners in the group are
pulling at one another, and though we know the
exact law of the pulls (which is simply that great
Law of Gravitation which Newton found out for
us), it is quite impossible for mathematicians, with
all their present skill, to trace out the consequences
of the law. Perhaps they may have better success
in the future, but meantime we need not trouble
so much about the treatment of one partner or
another because what I want you to notice chiefly
is that the whole group moves together. Look
(Fig. 90) at the picture of the group called the
Pleiades, in which arrows are drawn to represent
THE STARS
279
the movements of the separate stars in a given
time : you will see that many of these arrows are
about in the same direction and about of the same
length. The stars are shown as round black dots
(as they would be on a photographic negative) in
their present positions, and the arrow-heads show
where they will get to after thousands of years.
Fig. 90. Movements of the Pleiades.
Since the arrows are not quite equal in length, and
quite the same in direction, the arrow-heads form
a figure rather different from the arrow-tails : that
is to say, after this lapse of time the partners will
have taken up rather different positions. They
have felt the pulls of gravitation and responded to
them at any rate this is a reasonable explanation.
They are waltzing to a certain extent, but not
enough to prevent them all keeping together and
moving down the room as a group of partners.
280 A VOYAGE IN SPACE
This noticeable fact about the Pleiades is not
very new. The late Mr. R. A. Proctor called atten-
tion to it forty or fifty years ago. But another
group of stars in the constellation of the Bull was
only noticed quite lately, by Mr. Lewis Boss, a
great American astronomer. The movements of
these stars are so slow such a snail's " crawl "
that it required very great care and skill to measure
DEC
X*"* 1-20
BOSS'S STREAM IN ,<
TAURUS %-- ^
- **=*-*_
x<^ ^'^' --fio
i 5"
Fig. 91.
them. The main point about the arrows in Fig. 91
(which again represent the movements in a given
time about 50,000 years) is that they are not
parallel to each other, but would meet in a point
if carried far enough. It looks as though the stars
would collide with one another at some time in
the future, but that need not be the case. If you
stand on the railway, where the lines run straight
they will seem to meet in the distance, whereas we
know that they always remain the same distance
THE STARS 281
apart. The apparent meeting is an effect of " per-
spective." Some of you have no doubt learnt
perspective drawing and you know that parallel
lines have to be drawn to meet in a " vanishing
point." So Mr. Boss's group of stars may be
moving along parallel lines, without closing up :
and indeed we can find out that this is the fact,
Fig. 92.
(See Notes to Illustrations.)
by using the second method of watching their
movements which we noticed, with the spectro-
scope. The " vanishing point " represents the
direction in the far distance to which they are
migrating, like a flock of birds such as we see in
Fig. 92.
There is one more group which I should like you
to know of, part of which was also identified long
ago by Mr, Proctor. He pointed out that if we
282
A VOYAGE IN SPACE
omit the first and last stars in the Great Bear or
Dipper, the five remaining in the middle form a
group flying along together. But it has recently
been discovered that several other stars belong
to this group, notably Sirius the Dog-star. Now I
hope you know where to look for Sirius away in
the south. We in Europe never see him very far
I > > i | .
O 50
SCALE OF
100 150
LIGHT-YEARS
URSA
THE
^
THE 5IKIU5-UK5A MAJOK CLUSTER
SEEN tN PLAN
Fig. 93-
away from the south : and, as you know, the Great
Bear is in the north. Does it not seem strange
that Sirius in the south should belong to the same
group of stars as the five Great Bear stars in the
north? The group is like a flock of birds flying
just over our heads, some of which have passed
us while others have not yet come up. There
seems to be no doubt about the association of these
stars, for their movements have been tested in
THE STARS 283
both the ways we described; and besides that, the
distances of some of them (especially Sirius) have
been measured; and all the facts fit in well to-
gether, so that we can actually make a model of
this system of stars. Miss Bellamy has kindly made
one for us by sticking hatpins into this cork mat :
and please notice how very flat it is. The pinheads
are nearly all at the same height from the mat
not quite the same, but nearly enough to be note-
l . , i . | . i . | i ,
O 50 fOO 150
SCALE Of LIGHT-YEARS
JbLEQHtS
URSA MAJOR
THE SIRIUS -URSA MAJOR CLUSTER
SEEN E75G-EWI5E
Fig. 94.
worthy. We may almost compare this group of
stars to a fleet of ships sailing on the flat ocean :
but the fleet is of a stupendous size its length is
about 150 light years; that is to say that if the
admiral on the leading ship signalled to his fleet,
it would be 150 years before the last ship would
even see the signal ! How long it would take us
to go round and pay a visit to each ship in turn
you can perhaps imagine for yourselves.
We have now noticed three groups of stars and
found that, though they may be altering their
places a little in the group, each group is never-
284 A VOYAGE IN SPACE
theless steadily moving in a particular direction
making a sort of migration. Whence did they come
and whither are they going? Will they go on for
ever or will they come back after a time as migrating
birds do ? These are questions which suggest
themselves at once, though they are not so easily
answered. I think, however, that an answer can
be given : an answer which suggests itself if we
go back to that notion of our belonging to a large
cluster of stars to which we were led earlier in the
lecture. You remember that our " audience " of
stars, as we called it, thins off in the back rows,
giving us the notion that we form part of a definite
cluster such as we see in the picture (Fig. 85).
Now let us think out the consequences of this
arrangement, remembering that by the great Law
of Gravitation each star is pulling every other star.
We have already said that even with three or four
stars it is so difficult to tell what will happen in
detail that the best mathematicians have not yet
been able to work it out : so you may think that
with millions and millions of stars it would be
quite hopeless. But curiously enough it makes the
problem in some ways simpler to have a very large
number, and I do not think we shall find it very
hard to think out some of the consequences. For
instance, it is easy to see that a star right in the
middle of the bunch will be pulled in all directions
at once to right as well as to left, up as much
as down so that it will not know which way to
move ; and if it had already no movement it would
not acquire any. But it would be very different
for a 3tar on the boundary : all the pulls would be
THE STARS 285
on one side, namely, more or less towards the centre
of the bunch. Some stars would pull it rather to
one side of the centre, others rather to the other
side : but there would be no stars at all to pull it
away from the centre, "and hence it would know very
definitely in which direction to move, namely
towards the centre. Let us go with it in its journey
and see what happens. After a little time it would
be no longer quite on the boundary, but would have
penetrated a little way into the bunch. There
would now be a few stars pulling it away from the
centre, but only very few compared with those
urging it on; it would still be urged centrewards
and would quicken its pace : and without troubling
about the details you will see that this sort of thing
would go on until the star arrived at the centre,
when there would be no force on it either way, as
we said before. But that does not mean that it
would stop, for it has got " way on." Let us look
for a moment at this pendulum. When I let it
hang vertically it is like a star at rest in the middle
of the bunch; there is nothing to make it move.
But if I pull it to one side it is like our star on the
outside of the bunch it is being pulled towards
the centre, and if I let it go it swings thither. But
on arriving at the centre it does not stop, because,
although there is no force moving it forwards, it
has got " way on " and so passes through. Directly
it has passed through, the force begins to call it
back, but it cannot stop it all at once the pendulum
swings out to the other side before returning. So
with our star which we started from the outside
of the bunch or cluster : it will arrive at the centre,
286
A VOYAGE IN SPACE
pass through it, and swing out to the other side.
Then it would come back again as the pendulum
does; and that is what I think the stars are doing
in our great cluster, some of them moving alone,
some in pairs (the double stars) and others in
groups ; but all swinging to and fro through the
centre of the whole bunch.
Fig. 95. The experiment with the billiard balls, idealized
by a member of the audience.
But now how is it that they keep clear of one
another ? If I take, instead of a single pendulum
as before, a whole lot of pendulums all swinging
through the same centre, they will be liable to
strike one another. Let us try the experiment.
Here we have several billiard balls hung by long
strings to the theatre roof, and we will see how
THE STARS 287
much they hit. Indeed, we will try to make them
hit. If several of my young friends will each hold
one ball in different directions, and then all let
go at exactly the same moment I think we shall
get a good crash at the centre when they meet :
but if they do not let go at exactly the same moment,
then they will miss. 1 Now are you all ready to
let go when I say " three " ? Don't fling the balls
at all, just open your fingers and let them drop.
One, two, THREE !
Some of them hit, you see, but one or two missed :
and now you see that after a few turns they seem
to avoid one another very cleverly partly because
some of them have been knocked a little sideways
and do not pass accurately through the centre, and
partly because some arrive there at one time and
some at another : so that though they are all still
oscillating through the centre in a general kind of
way, collisions are comparatively rare.
I think the stars in our cluster are moving some-
what in this way oscillating to and fro through
the centre and ' I have tried to represent the
movements very crudely by means of the kine-
matograph.
[Exhibition here of the results with kinemato-
graph. The opportunity was taken to show also
some features of a total solar eclipse ; such as fowls
going in to roost during the darkness and coming
out afterwards.]
I think there are occasional collisions, which
perhaps make what we call " new stars " : and in
1 A fanciful representation of this little experiment is
shown in Fig. 95.
288 A VOYAGE IN SPACE
times gone by these collisions may have been more
serious and more frequent. But I do not want to
trouble you about the collisions so much as the
general movements to and fro. That very phrase
" to and fro " reminds us of one most important
feature of the movements, namely that they are
sometimes " to " the centre and sometimes " fro "
or from it. At any given moment, therefore, we
can divide the stars into two lots, one lot going
to the centre and the other lot coming from it. Let
us think of the first lot alone. If we watch their
movements and represent them by arrows, then all
these arrows will point to the centre : they will
converge towards a point just like the arrows in
the picture of Mr. Boss's cluster (Fig. 91). Now,
in the case of this cluster, we said that the stars were
not really moving towards a point, but in parallel
lines : we knew that because we had the second
method of watching the velocities to help the first.
But if we had not had that second method to help
us, we could not really have told whether the move-
ment was in parallel lines or to a point : the per-
spective appearance wonid be just the same. And
we might easily make a mistake in concluding,
when we see arrows representing star movements
directed to a point, that the movements are really
parallel. They may really be to a point. Lots of
boys christened Henry are called Harry " for
short," but you might easily make a mistake if
you concluded that every Harry had been really
christened Henry, for some of them are christened
Harry direct. And I think that astronomers are
liable to make just this very mistake about the
THE STARS 289
star movements. They see one lot going to the
centre of the system and another lot coming away :
and they have argued that since the first lot seem
to be converging to a point their movements are
really parallel. They might be parallel just as
Harry's real name might be Henry; but they also
may be actually moving towards a point as they
seem to be, just as Harry's name may be really
Harry outright. And when there is a very good
reason why they should be moving to a point,
because all the stars belonging to the system are
all being pulled towards the centre, as we know,
then it seems better to take that as the explanation
rather than go out of our way to treat the move-
ments as parallel. The same may be said of the
outgoing stream : their arrows seem to diverge from
a point, and they might be merely a perspective
appearance of movements that were really parallel ;
but it might also be true that the stars are moving
outwards from the centre (like the pendulum that
passes through the centre because it has got way
on), and when there is a good reason for the latter
alternative it seems sensible to adopt it.
In mentioning this great fact, that we can divide
the stars into two lots, one going one way and one
going the other, I have omitted several very im-
portant things so as to keep the argument simple.
One important thing is who first found it out. It
was first announced by Professor Kapteyn of
Groningen in 1904, not yet ten years ago : and it
reflects the very greatest credit upon him that he
made the discovery. Many other people have
confirmed it since, and the evidence is so clear that
u
290 A VOYAGE IN SPACE
now it seems wonderful that it should have been
overlooked so long. That is often the case in
science : once the way is shown, it is easy enough
to follow it ; but it may require a genius to show
the way in the first instance. Have you ever hunted
for seabirds' eggs on the shore ? They are so nearly
the colour of the stones that it takes a very sharp
eye to find them, though when once they are pointed
out they are easily seen.
Another thing is that the arrows of the star move-
ments do not converge accurately to a point : in-
deed many of them are wildly wrong. That is
what made the great discovery so difficult. It is
only by taking careful averages of hundreds of
stars that the general tendencies were discovered.
This means that, although there is on the whole a
tendency to the centre of our cluster, many of the
stars have had knocks which carry them to one
side or the other, just as our billiard balls had
after the first few swings. There is no difficulty
in this, however ; indeed, it removes a difficulty that
might have otherwise existed, for it shows how
too-frequent collisions are avoided by the stars
passing to one side or other of the centre. Some of
them, indeed, may never go near the centre at all ;
they may whirl round and round it just as I can
make this pendulum-bob whirl round and round
instead of oscillating to and fro. To make it de-
scribe a circle I need only give it enough side-
ways motion, and now you see it never goes near
the centre. But if all the stars moved like this,
there would be none near the centre, except some
almost stationary, as the pendulum is now. There
THE STARS 291
must be a certain number, at any rate, which swing
backwards and forwards through the centre, and
though these are mixed with others which swing
wide, there is still enough oscillation to enable Pro-
fessor Kapteyn to make his great discovery. I must
not, however, mislead you into thinking that he
agrees with my interpretation of it, for he does not.
He prefers to regard the movements as parallel.
There is another point to be noticed about the
stars which circle round the centre of the cluster
instead of passing to and fro through it or near it.
They remain at a distance from the centre, possibly
at a great distance ; if so they may form a separate
ring round the central cluster. This might explain
the Milky Way, that beautiful soft track of light
which you can see crossing the sky when there is
not too much moonlight. It makes a complete
ring round us, though we cannot realize that without
going into the southern hemisphere as well as the
northern : and when we look at it through a power-
ful telescope, or take photographs of it, we find
that it is dotted over with thousands and thousands
of stars. Perhaps it is a ring of the stars which
always keep far away from the centre, but as yet
we cannot tell, because we have not sufficient
knowledge of the movements of these stars. To
measure the movement of a star we must record
its position carefully at some selected time, then
wait a number of years and note how far it has
changed. If the star is far away, the change will
be very slow and we must wait proportionately
longer in order to detect it. For the stars near
us a few years may suffice : there are one or two
292 A VOYAGE IN SPACE
stars moving so rapidly that we can see their change
of position in a week almost in a day; but such
cases are quite exceptional, and we may go much
further and say that it is quite exceptional to find
a star which has moved appreciably even after ten
years. We have spent a great deal of time at
Oxford lately in looking for such changes, by com-
paring photographs of the same region in the sky
taken at intervals of at least ten years, and out
of the hundreds of stars on the plates we only
find one or two which have moved appreciably
in ten years. Moreover, we find about the same
number on a plate whether the region represented
be in the Milky Way or far from it. In the former
case there may be one thousand stars shown on
the plate : in the latter case only one hundred : and
yet we are just as likely to find two stars which
have moved appreciably in the latter case as in
the former. The inference seems to be that the
extra 900 stars are all too far away to show the
slightest motion in ten years : we must wait fifty
or one .hundred years to see them move. If any
one had photographed them fifty or one hundred
years ago, we might to-day have known something
of their movements ; but unfortunately astronomers
had not learnt how to photograph faint stars at that
time ; they began only twenty or thirty years ago.
It is time that we said a word as to the position
of our own Earth in all this organized movement.
Our Earth and all the other planets and satellites
must be regarded as being carried along with the
Sun. Compared with the distances of the stars
our distance from the Sun is really very small,
THE STARS 293
You remember how we said in the last lecture that
it takes us only eight minutes to get to the Sun on
the wings of light, whereas it takes years to get to
the nearest star. Another way to think of it is
this : suppose our path round the Sun were repre-
sented by a wedding-ring, then on the same scale
the nearest star would be some miles away. Hence
we can regard all the planets and satellites as a tiny
little bunch travelling with the Sun, and we can
talk of the Sun alone as travelling, carrying us with
him in his pocket so to speak. Now how is our Sun
behaving in this matter of migrations ? Is he one
of the stars that pass to and fro through the centre,
or does he remain at a distance from it ? The
answer seems to be, from the best evidence we can
get at the moment, that he travels to and fro, and at
the present moment he has just been through or
near the centre only about a million years ago.
Perhaps you think that is so long ago that we
must by this time have come very far away; but
a million years is probably only a small fraction of
the time for a whole swing, which may be estimated
roughly at about 200 million years. And that
brings us back to that little table which we made
at the beginning of the lecture; we had to leave it
incomplete, but now we can complete it.
30 million light vibrations make one wireless
vibration.
30 million wireless vibrations make one second.
30 million seconds make one year.
30 million years make one " migration."
I have now put 30 million years for a migration
294 A VOYAGE IN SPACE
instead of 200, firstly to make the table simpler by
having all the figures the same, and secondly to
remind us that when I said 200 millions just now it
was little more than a guess. It was the best guess
I could make, but it may be wildly wrong ; perhaps
30 million will do just as well, and it has the advan-
tage of making our table simple.
We on our Earth, then, are accompanying the Sun
in these migrations we are all making this great
VOYAGE IN SPACE, not metaphorically by telescope,
but actually and in reality. We are doing it in
a leisurely manner, perhaps you will think, seeing
that it takes so many million years to make even
one " migration " : we cannot, even the youngest
of us, hope to see the end of a turn, or even to see
the scenery change much : it takes us all our
time and attention to detect that we are moving
at all. Have you read how Mark Twain once
boarded a glacier to travel downhill? He had
learnt that a glacier was really a moving river of
ice, and solemnly pretended to use it for a journey,
and to "be astonished and bitterly disappointed
when he learnt that it would take years to go a
few yards. I hope you will not form any similar
expectations about our journey. We are certainly
making it, but as regards change of scenery in our
short lives it is slower even than a glacier.
And I have used the word " migration " to denote
the time taken by the Sun to swing from one end
of its tether to the other. It seems possible that
all of the stars in our cluster take about the same
time to make their " migrations," just as the
billiard balls did when hung by strings of the same
THE STARS 295
ength. It is not at all likely that the " migra-
tions " are accurately in the same time : all I mean
is that they may not differ so much as the periods
of our planets, for instance, the innermost of which
(Mercury) takes only eighty-eight days to go round
the Sun, while the outermost (Neptune) takes 160
years. It would take too long to explain the reason
for and against the " migrations " being all nearly
equal in point of time : perhaps you will allow me
to state it without further explanation. And now
let me add that this does not mean that the swings
are of the same length as regards distance. We
can pull a pendulum far out from its position of
rest, and then when we let it go it makes a big
swing : or we can pull it out only a little way and
then its swing will be small ; but the time of swing
will remain the same. This was Galileo's great
discovery about the pendulum. In the same way
I fancy there are stars with big swings and stars
which take only little swings and so never get far
from the centre. On the whole the big bright
stars will swing out farthest, and the little faint
ones will swing quietly; but all much in the same
time.
The question arises whether there is likely to be
a collision occasionally between the stars migrating
in opposite directions. This would doubtless make
a considerable " flare-up," and it is tempting to
suppose that the " new stars " which we see sud-
denly appear in the sky represent a collision of this
kind. The chief difficulty is that stars are so small
compared with the enormous distances between
them that a collision between two must be extremely
296 A VOYAGE IN SPACE
rare. What seems to me much more likely is that
a star may collide with a dark nebula such as we
concluded must exist in order to account for the
patches empty of stars. To take an illustration
from a well-known game, it is extremely unlikely
that two golf balls will hit one another in mid-air,
though on links like those of St. Andrews they
may be struck off in opposite directions. I believe
such a collision has happened, but thousands and
thousands of balls must have passed more or less
near one another without hitting. On the other
hand it is pretty easy for a golf ball to hit a furze
bush, as all golfers know. Now I think these dark
nebulae are like the furze bushes and bunkers on
a golf course, which almost seem sometimes to
attract the ball out of perversity. Up till a few
years ago such an idea would have been mere
speculation : but in 1901 we had a splendid " new
star " which gave us direct evidence of such an
occurrence. Nova Persei, the " new star of the
new century," blazed up suddenly, and died down
slowly like all " new " stars : but after it had
become faint again, a photograph of it showed a
nebula surrounding it : and by comparing photo-
graphs the nebula was seen to be changing its
shape (Fig. 96). At first it was thought that there
had been an explosion of some kind and the nebula
represented the scattering fragments ; but it was
presently realized that the rate of scattering was
far too great for this explanation; and ultimately
the startling truth was realized that we were wit-
nessing some of those light " echoes " to which I
referred at the beginning of the lecture. Let us
THE STARS 297
think of actual echoes first. Suppose I repeat
Tennyson's lines from the " Charge of the Light
N Brigade " in an even voice thus-
Game through the jaws of Death,
Back from the mouth of Hell,
All that was left of them.
We might take that to represent the light of a
star shining steadily, because I said the words in
September 20, 1901. November 13, 1901.
Fig. 96. The expansion of the nebulous appearance about
Nova Persei.
an even monotonous way. But now let me say
them to represent the way a new star blazes up
suddenly
Came through the jaws of Death,
BACK from the mouth of Hell,
All that was left of them,
Left of six hundred.
You see I began faint and steady, because there
seems to be generally a faint steady star which
blazes out into a new one : then suddenly I mounted
298 A VOYAGE IN SPACE
up to the word " BACK," which represents the fiercest
outblaze, and then gradually down again to faint-
ness.
Now suppose when my voice had become faint
again we were to hear the word "Back," "Back,"
"Back," repeated several times : we should say at
once that those were the echoes of my biggest shout
from some distant hills or buildings. And that is
what probably happened with light echoes in the
case of Nova Persei. A fair explanation seems
to be that a faint star in its travels came across a
dark nebula, one of the " bunkers " of space; and
thereupon blazed up. The blaze lit up the pre-
viously dark nebula or bunker for us to see ; but
as light takes time to travel, it was not until some
months after the flare-up that we got the light
echoes from the nebula. We identify the illumi-
nation as an echo much as we could identify the
sound echo : the word " Back " was repeated, not
" mouth " or " hell," and this is reasonable because
" Back " was the loudest word. Similarly the
light of the echo, when analysed with the spectro-
scope, was found to correspond with the light of
the greatest flare-up : the chain of evidence is
complete. There is just one alternative supposition
in detail which I will mention, but cannot dwell
upon. It is possible that the big " flare-up " was
caused, not by the entry of a wandering star into
the nebula, but by the contraction of a part of the
nebula under its own gravitation. The rest of the
explanation would then follow as before.
But now I have tried your patience quite enough.
Our visits to the stars have been rather more
THE STARS 299
arduous than those which have gone before; just
as visits to very distant acquaintances are apt to
be more tedious than family parties. You have,
however, listened very patiently, and I offer you my
best thanks and best wishes for a Happy New
Year.
INDEX
ABBOT, C. G., 200
Adams, J. C., 159-164, 178
Aerolite, Appley Bridge, 84
Aeroplanes, 67, 7173
Air Currents, 143
Airy, Sir G. B., 161-163
Aldebaran, 267
Algol, 131-137
Alice in Wonderland, 20
American and English dis-
coveries, 179, 1 80
American tourist, 227
Andromeda, Great Nebula in,
II3-H5
Andromeda, Split Nebula in,
255
Apparatus for Gravity, 22, 45
Appley Bridge Aerolite, 84
Approach and recession, effects
of, 270-277
Arcturus, 267
Argon, 78
Aristotle, 5, 13
Astrologers, 157
Astronomer Royal, 161-163,
180, 237
"Astronomical Reprobate, "133
Atmosphere of Earth, 63-81,
143
Atmosphere of Mars, 138-144
Atmosphere of Moon, 188
Atmosphere of Sun, 241248
Atwood's machine, 29
Australian Solar Observatory,
231
Ball, Sir Robert, 18
Balloons, 67, 68, 74-78
Balls falling, 7-10
Barnard, E. E., 42, 109, 144,180
Base for measuring distances,
51-57
Bayeux tapestry, 40
Bellamy, F. A., 77
Bellamy, Miss E. F., 283
Betelgeuse, 267
Billiard balls, 44, 166, 286
Birds flying, 281-283
Black drop, 60
Bode, 150, 156, 160, 179, 230
Bolton, Scriven, 147
Boss, Lewis, 280, 288
Brahe, Tycho, 14-16
Bright stars, 251, 252, 261-263
Brightness of Corona, 243, 244
British Association, 78
Britons, Ancient, 193
Bull-roarer, 274
Burning-glass, 103, 266
Calcium Light, 218
Camberwell Beauties, 79
Cambridge, 26, 27, 161, 163
Camera, 91
Campbell, Elizabeth, 70
Campbell, W. W., 69, 70
Canals of Mars, 140
Cape of Good Hope Observa-
tory, 57, 116
Carrington, R. C., 198, 199,
215, 224
Cavendish, Henry, 13
Cavorite, 3, 87
Cawthron, J., 231
Centre of our Cluster, 284-292
Ceres, 153-155, 178
Chamonix, 88
Chinese Sunspots, 197, 204, 206
Chromosphere, 216, 217
Cigar, lighting a, 50
Clark, Alvan, 106-108
Cluster, Gravity and centre of
our, 284-292
Cluster in Hercules, 259
Cluster in Taurus, 280
Cluster in Ursa Major, 282-283
Coelostat, 119-123
Coin and Feather, 10
300
INDEX
301
Cold of space, 68-69, 266
Collisions of Stars, 287, 288,
296, 298
Colour, 91, 101-104, 126130,
273-277
Colour and musical scale, 273
Colour change with motion,
2 73-277
Colours of Double Stars, 277
Comets, 19, 35, 42
Common, A. A., 97
Convergence of movements,
280, 288-291
Cook, Captain James, 62
Corona, 241-248
Counts of Stars, 256-258
Coxwell, 75
Crabtree, 62
Crawling as effect of distance,
269, 280
Czar of Russia, 107
Daily Graphic, 112
Daily Mirror, 251
Damping oscillations, 33
Dark stars, 132-137, 252-257
Darwin, Charles, 9
Darwin, Sir G. H., 45
Davidson, George, 108
Dawes, Rev. W. R., 106
Deception in size, 260263
Dewar, Sir James, 30, 80
Diamonds, formation of, 83
Diffraction, 61
Discoveries in England and
America, 179, 180
Distance measurements, 49-63
Distance of Planets, 150
Distance of Stars, 89, 214, 262-
270
Distance of Sun, 88, 211
Disturbance on Sun, 199
Double Stars, 276-279
Eagle, 268
Earth seen from Mars, 145, 146
Echo, 167, 249
Eclipses, 119, 132-137, 165-
169, 235-242
Eclipse imitated, 235-242
Eclipse of Moon, 231
Eclipse of Sun, 232-242
Eclipse Tracks, 233
Egypt Eclipse, 248
Ellipse, 17, 28
England and America, dis-
coveries, 179-180
England, next Eclipse in, 234,
235
Englishman, 38, 267, 268
Escalators, 229
Eyepiece, in, 123
Faraday, Michael, 169
Field of View in Telescopes,
105, 113
Fixed Stars, 267, 268
Flatness of Clusters, 283
Fog in Space, 265
Foucault, 29, 32
Galileo, 7-9, 12, 24-29, 43-49,
77,81, 123,179, 196.197,295
Galle, 163
Gauss, 155
Geneva, Lake (near Chicago),
no
George III, King, 95
George V, King, 115
Georgium Sidus, 95
Gilbert, W. S., 220, 268
Gill, Sir D., 261, 269
Glacier, movement of, 294
Glaisher, James, 74, 75
Golf balls, hitting, 296
Grahame-White, 72, 73
Grating, 129
Gravity, 5, 10-13, 21, 45, 153,
278, 284, 288
Gray, 32
Great Bear, 282
Green, Mr. (of Royal Inst.), 174
Green, N. E., 138, 140
Greenwich, 57, 161, 162, 180,
195. 230
Gregory, 82-85
Griffith, George, 4, 164
Groups of Stars, 278-284
Grubb, Sir H., 116
Gulliver's Travels, 106
Gyro compass, 32-34
Gyroscope, 32, 33, 181, 182
302
INDEX
Hagen, J. G., 29
Haggard, Rider, Sir, 165, 166
Hale, G. E., 99, 121126, 137,
221, 222, 230
Halley, E., 27, 28, 3640, 162,
267, 268
Halley's Comet, 19, 38-40,
201 202
Halos round Star images, 251
Hamburg, 107
Hansky, 214-218
Harvard Observatory, 97, 178
Heath, J. (of Royal Inst.), 64,
175, I? 6
Heat of Sun, 206-211
Height of Mountains in Moon,
184-187
Helium, 79
Hencke, 155
Herschel, Caroline, 95, 107
Herschel, William, 93-96, 107,
156, 198
Hertz, 169
Hevelius, 91
Hodgson, R., 198, 199, 215, 224
Hollis, H. P., 39
Home Rule, 146, 147
Honeymoon in Spact, 4, 164
Hooke, R., 2528
Horrox, J., 62-64
Huggins, Sir W., 212, 218, 271
Hydrogen on Sun, 220-223
Icaromenippus, I
Illustrated London News, 204
Image in Telescope, 111-113
Iron filings and magnet, 220
apanese Eclipse, 119, 237
erusalem, 40
ockey, in, 112
osephus, 39
uno, 155
upiter, 147-149, 152, 157,
180, 202, 231
Jupiter's Satellites, 109, 164-
169, 179, 231
Kapteyn, J. C., 289, 291
Kelvin, Lord, 210, 211
Kepler, 16-21, 27, 153
Kinematograph, 287
King Solomon's Mines, 165
Kish-wau-ke-toc, no
Kites, 67, 75, 76
Ladies of Mount Wilson Staff,
124
Layers of Air, 67
Lens, 102
Leonid Meteors, 203-206
Leverrier, U. J., 159-164, 178
Lick, James, 107-109
Lick Observatory, 70, 107, 116,
180
Life on Planets, 140-146, 187
Light, Velocity of, 89, 168, 250
Light-years, 262, 282, 283
Liquid Air, 140, 188-191, 266
Los Angeles, 99, 127
Lowell, Percival, 142-144
Lucian, i
Maelstrom, 222
Magdeburg Hemispheres, 64, 65
Magnetic Storms, 228, 229
Magnetism of Sun, 219-223
Magnifying power, in
Magnitudes of Stars, 257, 258
Mars, 69, 70, 106, 138-148,
152, 157, 180, 187
Mars, Moons of, 106
Matilda, Queen, 40
Maunder, E. W., 224-230
Maundeville, Sir J., 54
Maxwell, J. Clerk, 149, 150, 169
McClean, Dr. F., 115
Melotte, P. J., 1 80, 181
Mercury, 157, 295
Meteorological Office, 67, 78
Meteors, 79-85, 200-206
Meteors causing Sunspots, 200-
206
Migrating, 284-294
Milky Way, 251, 291, 292
Minor planets, 152-156
Mirror, 92-98, 103
Momentum, Angular, 176-177
Mont Blanc, 88
Moon, 2-4, 13, 21, 47-56, 86,
87, 157, 165, 173-174, 182-
188, 225
INDEX
303
Moon, " First Men in," 3
Mountains in Moon, 184-187
Mount Whitney, 69, 70
Mount Wilson, 69, 97-99, 120-
124, 221
Motor-car, 271
Murray, Sir James, 71
Nasmyth, James, 213, 218
Nasmyth and Carpenter, 185
Nautical Almanac, 186
Nebula in Cygnus, 256
Nebula, Great, in Andromeda,
II3-H5
Nebula round Nova Persei, 297
Nebula, Split.in Andromeda, 255
Nelson, Horatio, 62
Neon, 79
Neptune, 160-164, 180, 295
Newton, Sir Isaac, 12, 20-29,
34-37. 153. 2 7 8
New Zealand, Solar Observa-
tory, 231
Nicholson, S. B., 180, 181
Nile floods, 142
Normans, 40
North Pole, 34
Nova Persei, 296-298
Object glass, 123
Obscuring patches, 252-257
Oil in alcohol and water,
174-175
Origin of planets, 1 70
Oxford, 27, 37, 65,71, 268, 292
Oxford Radcliffe Observatory,
183
Oxford University Observa-
tory, 119
Pallas, 155
Pan, Peter, 31
Parabola, 35-38
Parallax (see distance)
Paris Observatory, 164
Parsons, Hon. C. A., 83, 96
Pasadena, 99, 127
Patience (operetta), 220
Pearson's Magazine, 47, 48
Pendulum, 29-31, 43, 44, 285,
290, 295
Perrine, C. D., 180, 181
Perturbations, 158-164, 202
Petrograd, 107
Phoebe, 177-182
Photography of Stars, 117, 118
Piazzi, 152, 154, 178
Pickering, W. H., 178
Pisa, 7-9, 26, 43
Pleiades, movements of, 279
Plumb-line, 33
Polarization of Corona, 244-
248
Poles of Mars, 139, 145
Police, Astronomical, 152, 156
Pressure of Air, 64-71
Prism, 101, 129
Proctor, R. A., 280, 281
Prominences, 216, 217
Ptolemy, 267
Pulkovo, 107
Punch, 133
Puppets, 260, 261
Radium, 210, 211
Ramsay, Sir W., 79
Range-finder, 55
Rapid Stars, distances of, 269
Rayleigh, Lord, 72, 78
Reed, E. J., 133
Refraction, 99-102
Repsolds, 107
Retrograde Satellites, 180-182
Ribbons, coloured, 130
Riffelberg, 88
Roemer, O., 168, 169
Rosse, Lord, 96, 104, 115
Rotation, effects of, 170-177
Rotation of Earth, 29-34, IJ 6
Rotation of Sun, 195-198, 228
230
Royal Astronomical Society,
21, 184
Royal Institution, 169
Royal Society, 26, 28
Russell, John, R.A., 183
St. Andrews, 296
St. John, C. E., 222, 224
San Francisco Earthquake, 108
Satellites, formation of, 175,
208
304
INDEX
Saturn, 149, 157, 178-180,
202-206, 275-277
Saturn, Ring of, 275-277
Saturn, Satellites of, 165, 178,
181
Saunder, S. A., 183-184
Scale and colour compared, 273
Selenium, 134
Shadows of Lunar Mountains,
185
Shakespeare, 24
Shrinking of Sun, 174177,
208-211
Sights for observing, 15
Sirius, 128, 262, 267, 282
Sizes of Stars, 260-264
Snow Horizontal Telescope,
120
Sodium lines, 128
Somersaults of Planets, 181,182
Sound, 85, 86, 167, 249
Southern Cross, 251
Spectroheliograph, 131, 216
223
Spectroscope, 126-130,275-277
Spider for balloons, 69, 77, 78
Spreading of light, 264, 265
Squint, 49-57, 262, 270
Stars, distance of, 89
Stars, streaming of, 284291
Stebbins, Joel, 134-137
Stonehenge, 193, 194
Stratton, F. J. M., 182
Sun, 157, 192-248, 292-295
Sun, distance of, 57-63, 88, 211
Sun, movement of, 292-295
Sun, photograph of, 195-196
Sunrise, 193
Sunspots, 195-198, 200-206
Sun worship, 194
Surface of Sun, 212-216
Swift, Dean, 106
Sykes, G., 144
Taurus, Stream in, 280
Telescope, 87-137
Tennyson, A., 106, 268, 269,
297
Theoria Motus, 155
Tides, 226, 227
Titius, 151
Totem posts, 65, 66
Tower Telescope, 121-126, 137
Tracks of Eclipses, 233
Train whistling, 271
Tramp Abroad, 88, 294
Transit of Venus, 57-63
Trinity College, Cambridge, 26
Trouvelot, 114, 115
Turn-table, 177
Twain, Mark, 88, 294
Uraniborg, 15
Uranus, 95, 156, 158164, 180
Vanishing Point, 281
Vatican Observatory, 29
Venus, 157
Venus, Transit of, 57-63
Verne, Jules, 2, 3, 11-13, 86 >
87, 138, 183
Vesta, 155
Vesuvius, 185
Victoria Telescope, 115
Voyage in Space, I, 266, 294
Wallace, A. R., 141
Waltzing of Stars, 277279
Washington Observatory, 106
Water on Planets, 140-146,
188-189
Watts, Dr. I., 116
Week, Days of, 156
Wells, H. G., 3, 87, 183, 188, 189
Westminster Abbey, 164
Whitworth Planes, 65
Windmills, 72
Wireless Telegraphy, 170
Wireless Vibrations, 250, 293
Wolf's Sunspot numbers, 201
Xenon, 79
Yerkes, C. T., 109
Yerkes Observatory, 42, 104,
109, no, 116
Zodiac, 117, 153, 250
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