It is a law, or rule, that, in the world around us, "the same causes
always produce the same effects." If you think a little about this, it will
become plain to you that it is so, and if you observe carefully you will
see that this rule is the same in connection with the smallest as well as
the greatest things; if it ever seems that it is not so, be sure that this
is only because you do not yet know all about what you have been observing.
And now learn a little about the beautiful rule by which the planets are
kept in their places.
Two hundred years ago, Sir Isaac Newton discovered that everything in the
universe attracts or draws every other thing to itself, and this power or
attraction he called "the force of gravitation." I cannot do much more than
tell you the name of this "law," but you will learn more about it one day I
hope, and see how simple and yet how wonderful it is. An astronomer of our
own day says, in his _Story of the Heavens_, that there are "grounds for
believing that the law of gravitation is obeyed throughout the length, the
breadth, the depth, and the height of the entire universe," and a little
observation and thought will enable you to see something of its working in
the world around us.
Do you remember my telling you how fond I was of swimming boats long ago?
When my brother and I used to launch our paper boats--not on the river,
but in that big tub in the yard--our great difficulty was to keep them
from running each other down, and becoming dismal wrecks before they had
completed their first voyage.
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