ed at Jimmy and Joe in triumph.

"What did I tell you?" he said. "Perhaps now you'll believe that my idea
wasn't such a fool one after all."

"But what did it do, Bob?" asked Joe, mystified.

"It increased the sensitivity of that old audion tube, that's what it
did," replied Bob, absently, his mind already busy with inventive
thoughts. "I can't see yet just how it accomplished it, but the
connection with the station was certainly clearer and more distinct than
usual."

"But how can a magnet increase the sensitivity of a vacuum tube?" asked
Jimmy, not yet wholly convinced. "It doesn't make sense."

"Well, I don't see why not," contradicted Joe slowly. "I suppose the
improvement is due to the magnetic effect of the magnet upon the
electrons flowing from the filament to the plate. I don't exactly see
why it should be an improvement, but if it is, then there must be some
reason for it."

"I wish we could find the reason!" cried Bob excitedly. "If we could
make some improvement upon the vacuum tube----"

"Don't wake him up, he is dreaming!" cried Herb. "If you don't look out,
old boy, you'll have us all millionaires."

"Well, there are worse things," retorted Bob, taking the magnet from
Herb's hand and placing it near the tube. "This has given us something
to think about, anyway."

For a while they puzzled over the mystery, trying to find some way in
which the discovery might be made to serve a practical purpose--all
except Herb, who retired to one corner of the "lab" to fuss with some
chemicals which he fondly hoped might be used in the construction of a
battery.

So engrossed were the boys in the problem of the magnet and vacuum tube
that they forgot all about Herb and his experiments. So what happened
took them completely off their guard.

There was a sudden cry from Herb, followed closely by an explosion that
knocked them off their feet. For a moment they lay there, a bit dazed by
the shock. Then they scrambled to their feet and looked about them.
Herb, being the nearest to the explosion, had got the worst of it. His
face and hands were black and he was shaking a little from the shock. He
gazed at the boys sheepishly.

"Wh-what happened?" asked Jimmy dazedly.

"An earthquake, I guess," replied Bob, as he looked about him to see
what damage had been done.

Some doughnuts, which their namesake had recently fetched from the
store, lay scattered upon the floor, together with some rather
dilapidated-looking