Quicksilver is a bright and pretty metal, and, unlike every other
metal, it is a liquid under ordinary circumstances. If you spill
quicksilver, it is a difficult task to gather the liquid up again. It
breaks into little drops, and you cannot easily lift them with your
fingers; they slip away and escape your grasp. Quicksilver will run
easily through a hole so small that water would hardly pass, and it
is so heavy that an iron nail or a bunch of keys will float upon it.
Now, this heavy, bright, nimble metal is known by another name besides
quicksilver; a chemist would call it mercury, and the astronomers use
exactly the same word to denote a pretty, bright, nimble, and heavy
planet which seems to try to elude our vision. Though Mercury is so
hard to see, yet it was discovered so long ago, that all record is lost
of who the discoverer was.
You must take special pains if you want to see the planet Mercury, for
during the greater part of the year it is not to be seen at all. Every
now and then a glimpse is to be had, but you must be on the alert to
look out just after sunset, or you must be up very early in the morning
so as to see it just before sunrise. Mercury is always found to be in
attendance on the sun, so that you must search for him near the sun;
that is, low down in the west in the evenings, or low down in the east
in the mornings. To ascertain the proper time of the year at which to
look for him you must refer to the almanac.
We have seen how Mercury revolves in a path inside that of Venus, and
it is therefore nearer to the sun. Indeed, Mercury is so close to the
sun that it is generally overpowered by his brilliance and cannot
be seen at all. Like every other planet, Mercury is lighted by the
sun’s rays, and shows phases in the telescope just as the moon does
(Fig. 48). In this figure the different apparent sizes of the planet at
different parts of its path are shown. Of course the nearer Mercury is
to the earth the larger does it seem.
If we can only see Mercury so rarely, and if even then it is a
very long way off, does it not seem strange that we can tell how
heavy it is? Even if we had a pair of scales big enough to hold a
planet, what, it may be asked, would be the use of the scales when
the body to be weighed was about a hundred millions of miles away?
Of course the weighing of a planet must be conducted in some manner
totally different from the kind of weighing that we ordinarily use.
Astronomers have, however, various methods for weighing these big
globes, even though they can never touch them. We do not, of course,
want to know how many pounds, or how many millions of tons they
contain; there is but little use in trying to express the weight in
that way. It gives no conception of a planet’s true importance. One
world must be compared with another world, and we therefore estimate
the weights of the other worlds by comparing them with that of our own.
We accordingly have to consider Mercury placed beside the earth, and
to see which of the two bodies is the bigger and the heavier, or what
is the proportion between them. It so happens that Mercury, viewed as
a world, is a very small body. It is a good deal less in size than our
earth, and it is not nearly so massive. To show you how we found out
the mass of Mercury I shall venture on a little story. It will explain
one of the strange devices that astronomers have to use when they want
to weigh a distant body in space.
There was once, and there is still, a little comet which flits about
the sky; we shall call it after the name of its discoverer, Encke.
There are sometimes splendid comets which everybody can see--we will
talk about these afterwards--but Encke is not such a one. It is very
faint and delicate, but astronomers are interested in it, and they
always look out for it with their telescopes; indeed, they could not
see the poor little thing without them. Encke goes for long journeys
through space--so far that it becomes quite invisible, and remains
out of sight for two or three years. All this time it is tearing along
at a tremendous speed. If you were to take a ride on the comet, it
would whirl you along far more swiftly than if you were sitting on a
cannon-ball. When the comet has reached the end of its journey, then
it turns round and returns by a different road, until at last it comes
near enough to show itself. Astronomers give it all the welcome they
can, but it won’t remain; sometimes it will hardly stay long enough for
us to observe that it has come at all, and sometimes it is so thin and
worn after all its wanderings that we are hardly able to see it. The
comet never takes any rest; even during its brief visit to us it is
scampering along all the time, and then again it darts off, gradually
to sink into the depths of space, whither even our best telescopes
cannot follow it. No more is there to be seen of Encke for another
three years, when again it will come back for a while. Encke is like
the cuckoo, which only comes for a brief visit every spring, and even
then is often not heard by many who dearly love his welcome note;
but Encke is a greater stranger than the cuckoo, for the comet never
repeats his visit of a few weeks more than once in three years; and he
is then so shy that usually very few catch a glimpse of him.
An astronomer and a mathematician were great friends, and they used to
help each other in their work. The astronomer watched Encke’s comet,
noted exactly where it was, on each night it was visible, and then told
the mathematician all he had seen. Provided with this information the
mathematician sharpens his pencil, sits down at his desk, and begins
to work long columns of figures, until at length he discovers how to
make a time table which shall set forth the wanderings of Encke. He is
able to verify the accuracy of his table in a very unmistakable way by
venturing upon prophecies. The mathematician predicts to the astronomer
the very day and the very hour at which the comet will reappear. He
even indicates the very part of the heavens to which the telescope
must be directed, in order to greet the wanderer on his return. When
the time comes the astronomer finds that his friend has been a true
prophet; there is the comet on the expected day, and in the expected
constellation.
This happens again and again, so that the mathematician, with his
pencil and his figures, marks stage by stage the progress of Encke
through the years of his invisible voyage. At each moment he knows
where the comet is situated, though utterly unable to see it.
The joint labors of the two friends having thus discovered law and
order in the movements of the comet, you may judge of their dismay
when on one occasion Encke disappointed them. He appeared, it is
true, but then he was a little late, and he was also not in the spot
where he was expected. There was nearly being a serious difference
between the two friends. The astronomer accused the mathematician of
having made mistakes in his figures, the mathematician retorted that
the astronomer must have made some blunder in his observations. A
quarrel was imminent, when finally it was suggested to interrogate
Encke himself, and see whether he could offer any explanation. The
mathematician employed peculiar methods that I could not explain, so I
shall transform his processes into a dialogue between himself and the
offending comet.
“You are late,” said he to the comet. “You have not turned up at the
time I expected you, nor are you exactly in the right place; nor,
indeed, for that matter, are you now moving exactly as you ought to do.
In fact, you are entirely out of order, and what explanation have you
to give of this irregularity?”
You see the questioner felt quite confident that there must have been
some cause at work that he did not know of. Mathematicians have one
great privilege; they are the only people in the world who never make
any mistakes. If they knew accurately all the various influences that
were at work on the comet, they could, by working out the figures, have
found exactly where the comet would be placed. If the comet was not
there, it is inevitable that there must have been something or other
acting upon the comet, of which the mathematician was in ignorance.
The comet, like every other transgressor, immediately began to make
excuses, and to shuffle off the blame on somebody else. “I was,” said
Encke, “going quietly on my rounds as usual. I was following out stage
by stage the track that you know so well, and I would certainly have
completed my journey and have arrived here in good time and in the
spot where you expected me had I been let alone, but unfortunately I
was not let alone. In the course of my long travels--but at a time
when you could not have seen me--I had the misfortune to come very
close to a planet, of which I dare say you have heard--it is called
Mercury. I did not want to interfere with Mercury; I was only anxious
to hurry past and keep on my journey, but he was meddlesome, and began
to pull me about, and I had a great deal of trouble to get free from
him, but at last I did shake him off. I kept my pace as well as I could
afterwards, but I could not make up the lost time, and consequently I
am here a little late. I know I am not just where I ought to be, nor am
I now moving quite as you expect me to do; the fact is, I have not yet
quite recovered from the bad treatment I have experienced.”
The astronomer and the mathematician proceeded to test this story. They
found out what Mercury was doing; they knew where he was at the time,
and they ascertained that what the comet had said was true, and that
it had come very close indeed to the planet. The astronomer was quite
satisfied, and was proposing to turn to some other matter, when the
mathematician said:--
“Tarry a moment, my friend. It is the part of a wise man to extract
special benefit from mishaps and disasters. Let us see whether the
tribulations of poor Encke cannot be made to afford some very valuable
information. We expected to find Encke here. Well, he is not here--he
is there, a little way off. Let us measure the distance between the
place where Encke is, and the place where he ought to have been.”
This the astronomer did. “Well,” he said, “what will this tell you? It
merely expresses the amount of delinquency on the part of Encke.”
“No doubt,” said the mathematician, “that is so; but we must remember
that the delinquency, as you call it, was caused by Mercury. The bigger
and the heavier Mercury was, the greater would be his power of doing
mischief, the more would he have troubled poor Encke, and the larger
would be the derangement of the comet in consequence of the unfortunate
incident. We have measured how much Encke has actually been led astray.
Had Mercury been heavier than he is, that distance would have been
larger; and if Mercury had been lighter than he is, you would not, of
course, have found so large an error in the comet.”
We may illustrate what is meant in this way. A steamer sails from
Liverpool to New York, and in favorable circumstances the voyage across
the Atlantic should be accomplished within a week. But supposing that
in the middle of the ocean a storm is encountered, by which the ship is
driven from her course. She will, of course, be delayed, and her voyage
will be lengthened. A trifling storm, perhaps, she will not mind, but a
heavy storm might delay her six hours; a still greater storm might keep
her back half a day; while cases are not infrequent in which the delay
has amounted to one day, or two days, or even more.
The delay which the ship has experienced may be taken as a measure of
the vehemence of the storm. I am not supposing that her machinery has
broken down; of course, that sometimes happens at sea, as do calamities
of a far more tragic nature. I am merely supposing the ship to be
exposed to very heavy weather, from which she emerges just as sound as
she was when the storm began. In such cases as this we may reasonably
measure the intensity of the storm by the number of hours’ delay to
which the passengers were subjected. “The weather we had was much worse
than the weather you had,” one traveller may say to another. “Our ship
was two days late, while you escaped with a loss of one day.”
When the comet at last returned to the earth after a cruise of three
years through space, the number of hours by which it was late expressed
the vehemence of the storm it experienced. The only storm that the
comet would have met with, at least in so far as our present object
is concerned, was the trouble that it had with Mercury. The mass of
Mercury was, therefore, involved in the delay of the comet. In fact,
the delay was a measure of the mass of the planet. I do not attempt to
describe to you all the long work through which the mathematician had
to plod before he could ascertain the mass of Mercury. It was a very
tedious and a very hard sum, but at last his calculations arrived at
the answer, and showed that Mercury must be a light globe compared to
the earth. In fact, it would take twenty-five globes, each equal to
Mercury, to weigh as much as the earth.
I dare say you will think that this was a very long and roundabout way
of weighing. Supposing, however, we had to weigh a mountain, or rather
a body which was bigger than fifty thousand mountains, and which was
also many millions of miles away, all sorts of expedients would have
to be resorted to. I have told you one of them. If you feel any doubts
as to the accuracy with which such weighings can be made, then I must
tell you that there are many other methods, and that these all agree in
giving concordant results.
[Illustration: FIG. 49.--Relative Weights of Mercury and the Earth.]
We hardly know anything as to what the globe of Mercury may be like.
We can see little or nothing of the nature of its surface. We only
perceive the planet to be a ball, brightly lighted by the sun, and we
cannot satisfactorily discern permanent features thereon, as we are
able to do on some of the other planets.