s done, but with little better effect. The explosive, powerful
as it was, ate only a little way into the rock. Blast after blast had
the same poor effect.

"This won't do," said Job Titus, despairingly, one day. "We aren't
making any progress at all. There's a half mile of this rock, according
to my calculations, and at this rate we'll be six months getting
through it. By that time our limit will be up, and we'll be forced to
give up the contract What can we do, Tom Swift?"

Chapter XXI

A New Explosive

The young inventor was idly handling some pieces of the very hard rock
that had cropped out in the tunnel cut Tom had tested it, he had
pulverized it (as well as he was able), he had examined it under the
microscope, and he had taken great slabs of it and set off under it, or
on top of it, charges of explosive of various power to note the effect.
But the results had not been at all what he had hoped for.

"What's to be done, Tom?" repeated the contractor.

"Well, Mr. Titus," was the answer, "the only thing I see to do is to
make a new explosive."

"Can you do it, Tom?"

The reply was characteristic.

"I can try."

And in the days that followed, Tom began work on a new line. He had
brought from Shopton with him much of the needful apparatus, and he
found he could obtain in Lima what he lacked.

A message to his father brought the reply that the new ingredients Tom
needed would be shipped.

"The kind of explosive we need to rend that very hard rock," the young
inventor explained to the Titus brothers, "is one that works slowly."

"I thought all explosions had to be as quick as a flash," said Walter.

"Well, in a sense, they do. Yet we have quick burning and slow-burning
powders, the same as we have fuses. A quick-burning explosive is all
right in soft rock, or in soil with rock and earth mingled. But in rock
that is harder than flint if you use a quick explosive, only the outer
surface of the rock will be scaled off.

"If you take a hammer and bring it down with all your force on a hard
rock you may chip off a lot of little pieces, or you may crack the
rock, but you won't, under ordinary circumstances, pulverize it as we
want to do in the tunnel.

"On the other hand, if you take a smaller hammer, and keep tapping the
rock with comparatively gentle blows, you will set up a series of
vibrations, that, in time, will cause the hard rock to break up into
any number of small pieces.

"Now that is