In order to see how powerful the sun’s rays
are, you have only to take a magnifying glass and
gather them to a point on a piece of brown paper,
for they will set the paper alight. Sir John
Herschel tells us that at the Cape of Good Hope the
heat was even so great that he cooked a beefsteak
and roasted some eggs by merely putting them in the
sun, in a box with a glass lid! Indeed, just
as we should all be frozen to death if the sun were
sold, so we should all be burnt up with intolerable
heat if his fierce rays fell with all their might
upon us. But we have an invisible veil protecting
us, made — of what do you think? Of those
tiny particles of water which the sunbeams draw up
and scatter in the air, and which, as we shall see
in Lecture IV, cut off part of the intense heat and
make the air cool and pleasant for us.
Week 4
We have now learnt something of the distance, the
size, the light, and the heat of the sun — the
great source of the sunbeams. But we are as
yet no nearer the answer to the question, What is
a sunbeam? how does the sun touch our earth?
Now suppose I with to touch you from this platform
where I stand, I can do it in two ways. Firstly,
I can throw something at you and hit you — in
this case a thing will have passed across the space
from me to you. Or, secondly, if I could make
a violent movement so as to shake the floor of the
room, you would feel a quivering motion; and so I
should touch you across the whole distance of the
room. But in this case no thing would have passed
from me to you but a movement or wave, which passed
along the boards of the floor. Again, if I speak
to you, how does the sound reach you ear? Not
by anything being thrown from my mouth to your ear,
but by the motion of the air. When I speak I
agitate the air near my mouth, and that makes a wave
in the air beyond, and that one, another, and another
(as we shall see more fully in Lecture VI) till the
last wave hits the drum of your ear.
Thus we see there are two ways of touching anything
at a distance; 1st, by throwing some thing at it and
hitting it; 2nd, by sending a movement of wave across
to it, as in the case of the quivering boards and
the air.
Now the great natural philosopher Newton thought that
the sun touched us in the first of these ways, and
that sunbeams were made of very minute atoms of matter
thrown out by the sun, and making a perpetual cannonade
on our eyes. It is easy to understand that this
would make us see light and feel heat, just as a blow
in the eye makes us see starts, or on the body makes
it feel hot: and for a long time this explanation
was supposed to be the true one. But we know
now that there are many facts which cannot be explained
on this theory, though we cannot go into them here.
What we will do, is to try and understand what now
seems to be the true explanation of the sunbeam.
