ss of such a sort as to tend to set other magnets in it in new
positions. If at a distance of ten feet there were a delicately-poised
magnet needle, every change in the position of the magnet held in the
hand would bring about a change in the position of the needle. If the
position of the hand magnet were completely reversed, so the south pole
faced where the north pole faced before, the field would have been
completely reversed, and the poised needle would have been pushed by the
field into an opposite position. If the needle were a hundred feet away,
the change would have been the same except in amount. The same might be
said if the two were a mile apart, or the distance of the moon or any
other distance, for there is no limit to an ether magnetic field.
Suppose the hand magnet to have its direction completely reversed once
in a second. The whole field, and the direction of the stress, would
necessarily be reversed as often. But this kind of change in stress is
known by experiment to travel with the speed of light, 186,000 miles a
second; the disturbance due to the change of position of the magnet will
therefore be felt in some degree throughout space. In a second and a
third of a second it will have reached the moon, and a magnet there will
be in some measure affected by it. If there were an observer there with
a delicate-enough magnet, he could be witness to its changes once a
second for the same reason one in the room could. The only difference
would be one of amount of swing. It is therefore theoretically possible
to signal to the moon with a swinging magnet. Suppose again that the
magnet should be swung twice a second, there would be formed two waves,
each one half as long as the first. If it should swing ten times a
second, then the waves would be one-tenth of 186,000 miles long. If in
some mechanical way it could be rotated 186,000 times a second, the wave
would be but one mile long. Artificial ways have been invented for
changing this magnet field as many as 100 million times a second, and
the corresponding wave is less than a foot long. The shape of a magnet
does not necessarily make it weaker or stronger as a magnet, but if the
poles are near together the magnetic field is denser between them than
when they are separated. The ether stress is differently distributed for
every change in the relative positions of the poles.

A common U-magnet, if struck, will vibrate like a tuning-fork, and gives
out a definite