We can all feel that the sun is very hot, and we know that it is very
big and a long way off. Let us first talk about the heat from the sun.
On a cold day it is pleasant to go into a room with a good fire, and
everybody knows that the nearer we go to the fire, the more strongly
we feel the heat. The boy who is at the far end of the room may be
shivering with cold, while those close to the fire are as hot as they
find to be pleasant. If we could draw much nearer to the sun than we
actually are, we should find the heat greatly increased. Indeed, if we
went close enough, the temperature would rise so much that we could
not endure it; we should be roasted. On the other hand, we should
certainly be frozen to death if we were transported much further away
from the sun than we are now. We are able to live comfortably, because
our bodies are just arranged to suit the warmth which the sun sends to
that distance from it at which the earth is actually placed.
Suppose you were able to endure any degree of heat, and that you had
some way of setting out on a voyage to the sun. Take with you a wax
candle, a leaden bullet, a penny, a poker, and a flint. Soon after
you have started you find the warmth from the sun increasing, and the
candle begins to get soft and melt away. Still, on you go, and you
notice that the leaden bullet gets hotter and hotter, until it becomes
too hot to touch, until at last the lead has melted, as the wax had
previously done. However, you are still a very long way from the sun,
and you have the penny, the poker, and the flint remaining. As you
approach closer to the luminary the heat is ever increasing, and at
last you notice that the penny is beginning to get red-hot; go still
nearer, and it melts away, and follows the example of the bullet and
the candle. If you still press onwards, you find that the iron poker,
which was red-hot when the penny melted, begins to get brighter and
brighter, till at last it is brilliantly white, and becomes so dazzling
that you can hardly bear to look at it; then melting commences, and the
poker is changed into liquid like the penny, the lead, and the wax.
Yet a little nearer you may carry the flint, which is now glowing with
the same fervor which fused the poker, but even the flint itself will
have to yield at last and become, not merely a liquid like water, but
a vapor like steam.
You will ask, how do we learn all this? As nobody could ever make such
a journey, how can we feel certain that the sun is so excessively
hot? I know that what I say is true for various reasons, but I will
only mention one, which is derived from an experiment with the
burning-glass, that most boys have often tried.
[Illustration: FIG. 1.--How to use the Burning-glass.]
[Illustration: FIG. 2.--The Noonday Gun.]
We may use one of those large lenses that are intended for magnifying
photographs. But almost any kind of lens will do, except it be too
flat, as those in spectacles generally are. On a fine sunny day in
summer, you turn the burning-glass to the sun, and by holding a
piece of paper at the proper distance a bright spot will be obtained
(Fig. 1). At that spot there is intense heat, by which a match can be
lighted, gunpowder exploded, or the paper itself kindled. The broad
lens collects together the rays from the sun that fall upon it, and
concentrates them in one spot, which consequently becomes hot and
bright. If we merely used a flat piece of glass the sunbeams would go
straight through; they would not be gathered together, and they would
not be strong enough to burn the paper. The lens, you see, is not
flat; its faces are curved, and they thus acquire the power of bending
in rays of light or heat, so as to unite their effect on that one
point which we call the _focus_. When a great number of rays are thus
collected on the same spot, each of them contributes a little warmth.
[Illustration: FIG. 3.--A Tell-tale for the Sun.]
Some ingenious person has turned this principle to an odd use, by
arranging a burning-glass over a cannon in such a way that just when
noon arrived the spot of light should reach the touch-hole of the
cannon and fire it off. Thus the sun itself is made to announce the
middle of the day (Fig. 2).
Another application of the burning-glass is to obtain a record of the
number of hours of sunshine in each day. You will understand the
apparatus from Fig. 3; the lens is here replaced by a glass globe,
which acts as a burning-glass. As the sun moves over the sky the bright
spot of light also moves, and therefore burns its track on a sheet of
paper marked with lines corresponding to the hours. When the sun is
hidden by clouds the burning ceases, so by preserving each day the
piece of paper, we have an unerring tell-tale, which shows us during
what hours the sun was shining brightly, and the hours during which he
was hidden. You see, the burning-glass is not merely a toy, it can be
made useful in helping us to learn something about the weather.
Another experiment with the burning-glass will also teach us something.
Take a candle, and from its flame you can get a bright point at the
focus. It may fall upon your hand, but you can hardly feel it, and
you will readily believe that the focus is not nearly so hot as the
candle. Even when a burning-glass is held in front of a bright fire
there is comparatively little heat in the focus. By using a lens to
condense the beams from an electric lamp, Professor Tyndall has shown
how to light a piece of paper, and to produce many other effects.
But, nevertheless, the focus is not nearly so hot as the arc between
the two glowing carbons. You might move your finger through the focus
without much inconvenience, but I would not recommend you to trust your
finger between the poles of the electric light itself. The temperature
obtained at the focus of a burning-glass seems thus to be always less
than that prevailing at the source of heat itself. This principle
will be equally true when we turn a burning-glass to the sun, and
hence we know that the sun must be hotter than any heat which can be
obtained by the biggest burning-glass on the brightest of summer days.
But burning-glasses a yard wide have been made, and astonishing heat
effects have been produced. Steel has thus been melted by the sunbeams,
and so have other substances which even our greatest furnaces cannot
fuse. Therefore the sun must have a higher temperature than that of
molten steel; higher, indeed, than any temperature we can produce on
the earth.
I have tried to prove to you that the sun is very hot; but it would be
well to see what arguments might be used on the other side. Indeed, it
is by considering objections that we often learn. So I shall tell you
of a difficulty that was once raised when I was endeavoring to explain
the heat of the sun to an intelligent man. “I am sure,” said my friend,
“that you must be quite wrong. You said that the nearer you got to
the sun the hotter it would be; but I know this to be a mistake. When
tourists go to Switzerland, they sometimes climb very high mountains.
But the top of a mountain, of course, is nearer the sun than below; and
so, if the sun were really hot, the climber should have found it much
warmer on the top of the mountain than at its base. But every one knows
that there is abundant ice and snow on lofty Alpine summits, while down
below in the valleys there may be at the same time excessively warm
weather. Does it not therefore seem that the nearer we go to the sun
the colder it is, and the further we are from the sun the warmer it
is?”
But my friend was quite wrong in his argument. The coldness of the
mountain tops depends upon something which he had not taken into
account. There is something else besides the sun which helps to make
us so warm and comfortable. This other essential thing is more or less
deficient at great heights. You know that we live by breathing air, and
we find air wherever we go, over land and sea, all round the earth.
Those who ascend in balloons are borne upwards by the air, and thus we
can show that air extends for miles and miles over our heads, though it
becomes lighter and thinner the loftier the elevation.
We not only utilize the air for breathing, but it is also of
indispensable service to us in another way. It acts as a blanket to
keep the earth warm; indeed, we ought rather to describe the air as
a pile of blankets one over the other. These air blankets enable the
earth to preserve the heat received from the sunbeams by preventing it
from escaping back again into space. Thus warmth is maintained, and
our globe is rendered habitable. You see then, that for our comfort
we require not only the sun to give us the heat, but also the set of
blankets to keep it when we have got it. If we threw off the blankets
we should be uncomfortable, though the sun were as bright as before.
A man who goes to the top of a mountain at mid-day does approach the
sun to some extent, and, so far as this goes, he ought no doubt to
feel warmer, but the gain is far too small to be thought of. Even at
the top of Mont Blanc the increase in heat due to the approach to the
sun would be only one ten-millionth part of the whole. This would be
utterly inappreciable; even a thermometer would not be delicate enough
to show it. On the other hand, by ascending to the top of the mountain,
the climber has got above the lower regions of the air; he has not, it
is true, reached even halfway to the upper surface--that is still very
far over his head--but the higher layers of the atmosphere are so very
thin that they form most indifferent blankets. The Alpine climber on
the top of the mountain has thus thrown off the best portion of his
blankets, and receives a chill; while the gain of heat arising from his
closer approach to the sun is imperceptible. Perhaps you will now be
able to understand why eternal snow rests on the summits of the great
mountains. They are chilled because they have not so many air blankets
as the snug valleys beneath.
The brightness of the sun is among the most wonderful things in nature,
and there are three points that I ask you to remember, and then indeed
you will agree with Milton, that the sun is “with surpassing glory
crowned.” First think of the beauty and brilliancy of a lovely day in
June. Then remember that all this flood of light comes from a single
lamp at a most tremendous distance; and thirdly, recollect that the
sun is not like a bull’s-eye lantern, concentrating all his light
_specially_ for our benefit, but that he diffuses it equally around,
and that we do not get on this earth the two-thousand-millionth part
of what he gives out so plenteously! When we think of the brightness
of day, of the distance from which the light has come, though Nature
has not adjusted any vast lenses to direct the light specially in our
direction, we begin to comprehend the sun’s true magnificence.