arry very sensitive compasses. Explain why such ships are
made entirely of wood and brass.

290. Thunder rolls; that is, after the first peal there is a
reverberating sound that becomes less and less distinct.

SECTION 32. _Static electricity._

What is electricity?

What makes thunder and lightning?

Why does the barrel or cap of a fountain pen pick up small
bits of paper after it has been rubbed on your coat sleeve?

Why do sparks fly from the fur of a cat when you stroke it in
the dark?

The Greeks, 2000 years ago, knew that there was such a thing as
electricity, and they used to get it by rubbing amber with silk. In
the past century men have learned how to make electricity do all sorts
of useful work: making boats and cars and automobiles go, ringing
bells, furnishing light, and, in the telephone and telegraph, carrying
messages. But no one knew what electricity really was until, within
the last 25 years, scientists found out.

ATOMS AND ELECTRONS. When we talked about molecules, we said that they
were as much smaller than a germ as a germ is smaller than a mountain.
Well, a molecule is made up, probably, of some things that are much
smaller still,--so small that people thought that nothing could be
smaller. Those unthinkably tiny things are called _atoms_; you will
hear more about them when you come to the parts of this book that tell
about chemistry.

But if you took the smallest atom in the world and divided it into
1700 pieces, each one of these would be about the size of a piece of
electricity.

Electricity is made up of the tiniest things known to man--things so
small that nobody really can think of their smallness. These little
pieces of electricity are called _electrons_, and for all their
smallness, scientists have been able to find out a good deal about
them. They have managed to get one electron all by itself on a droplet
of oil and they have seen how it made the oil behave. Of course
they could not see the electron, but they could tell from various
experiments that they had just one. Scientists know how many trillions
of electrons flow through an incandescent electric lamp in a second
and how many quadrillions of them it would take to weigh as much as a
feather. They know what the electrons do when they move, how fast they
can move, and what substances let electrons move through them easily
and what substances hold them back; and they know perfectly well