sound, and the waves carrying wireless messages are
all of a similar type, differing only in their relative rates of
vibration. If unobstructed, and moving through free ether, all of them
travel at practically the same velocity, that is about one hundred
eighty-six thousand miles a second. When, however, they encounter
other substances, as they are continually bound to do, this rate of
velocity changes. The waves of sound, for example, sent out by the
wireless telephone are very slow compared with the high-rate
vibrations that produce waves resulting in light."

Again the youthful teacher paused.

"Now this constant turmoil in the ether which creates the magnetic
area explains why the magnetized needle of a compass unfailingly
points north and south. This one simple fact is a certain proof of its
existence. And once granting a magnetic field to be there it is less
difficult to understand how wireless waves are produced in this
congenial medium and find their way through it, following in their
journey the curve of the earth's surface."

Bob smiled at his audience encouragingly.

"If you can once get this wave law through your heads the rest is not
hard," asserted he, "for the whole wireless system is based on wave
motion."

"With an ocean spread out before us we ought to be able to understand
waves," interpolated Nancy.

"We ought," nodded Bob. "And yet better than using the ocean as an
illustration imagine a small pond. Think, instead, of a nice quiet
little round pond if you can. Now when you chuck a stick or a pebble
into that still water you know how the ripples will at once go out.
There will be rings of them, and the bigger they get the fainter they
will be. In other words, as the area widens the strength of the waves
decreases; and as this same principle applies to radio you can see
that it takes a lot of energy from a wireless station to reach a
receiver a great distance away."

"I've got that!" cried Dick with such spontaneity that every one
laughed.

"Wave lengths, however, have nothing to do with actual distance," went
on Bob quickly. "Of course we think of the wave length as the distance
between one ridge of water and another. There is, though, no law that
would make it possible to translate these spaces into our scale of
miles, for sometimes they are near together, sometimes far apart.
Distance, therefore, depends on the speed with which the wave travels
and the frequency with which the water is d