coustics we learned
at high school. But I think I'm on the right track with the muffler and
the cutting down of the noise of the explosions in the cylinders. I'm
working both ends, you see--making a motor that doesn't cause as much
racket as those now in use, and also providing means to take care of
the noise that is made. It isn't possible to make a completely silent
motor of an explosive gas type. The only thing that can be done is to
kill the noise after it is made."

"What about the propeller blades?"

"Oh, they aren't giving me any trouble. The noise they make can't be
heard a hundred feet in the air, but I am also working on improvements
to the blades. Take it altogether, I'll have an almost silent aeroplane
if my plans come out all right."

"Have you said anything to the government yet?"

"No; I want to have it pretty well perfected before I do. Besides, I
don't want any publicity about it until I'm ready. If these Universal
people are after me I'll fool 'em."

"That's right, Tom! Well, I must go. Another week of this Liberty Bond
campaign!"

"I suppose you'll be glad when it's over."

"Well, I don't know," said Ned slowly. "It's part of my small
contribution to Uncle Sam. I'm not like you--I can't invent things."

"But you have an awful smooth line of talk, Ned!" laughed his chum. "I
believe you could sell chloride of sodium to some of the fishes in the
Great Salt Lake--that is if it has fishes."

"I don't know that it has, Tom. And, anyhow, I'm not posing as a salt
salesman," and Ned grinned. "But I must really go. Our bank hasn't
reached its quota in the sale of Liberty Bonds yet, and it's up to me
to see that it doesn't fall down."

"Go to it, Ned! And I'll get busy on my silent motor."

"Getting busy" was Tom Swift's favorite occupation, and when he was
working on a new idea, as was the case now, he was seldom idle, night
or day.

"I have hardly seen you for two weeks," Mary Nestor wrote him one day.
"Aren't you ever coming to see me any more, or take me for a ride?"

"Yes," Tom wrote back. "I'll be over soon. And perhaps on the next ride
we take I won't have to shout at you through a speaking tube because
the motor makes so much noise."

From this it may be gathered that Tom was on the verge of success.
While not altogether satisfied with his progress, the young inventor
felt that he was on the right track. There were certain changes that
needed to be made in the apparatus he was building