't take any chances," said Job Titus. "Our time is too nearly
up. In fact I'm afraid we won't finish on time as it is. There is too
much of that hard rock to cut through."

"There's only one thing to do," said Tom, after an investigation. "As
you say, there is more of that hard rock than we calculated on. To try
to blast and take it out in the ordinary way will be useless. We must
try desperate means."

"What is that?" asked Walter Titus.

"We must set off the biggest blast we can with safety. We'll bore a
lot of extra holes, and put in double charges of the explosive. I'll
add some ingredients to it that will make it stronger. It's our last
chance. Either we'll blow the tunnel all to pieces, or we'll loosen
enough rock to make sufficient progress so we can finish on time. What
do you say? Shall we take the chance?"

The Titus brothers looked at one another. Failure stared them in the
face. Unless they completed the tunnel very soon they would lose all
the money they had sunk in it.

"Take the chance!" exclaimed Job. "It's sink or swim anyhow. Set off
the big blast, Tom."

"All right. We'll get ready for it as soon as we can."

That day preparations were made for setting off a great charge of the
powerful explosive. The work was hurried as fast as was consistent with
safety, but even then progress was rather slow. Precautions had to be
taken, and the guards about the tunnel were doubled. For it was feared
that some word of what was about to be done would reach the rival firm,
who might try desperate means to prevent the completion of the work.

There was plenty of the explosive on hand, for Mr. Swift had sent Tom a
large shipment. All this while no word had come from Mr. Nestor, and
Tom was beginning to think that his prospective father-in-law was very
angry with him. Nor had Mary written.

Professor Bumper came and went as he pleased, but his quest was
regarded as hopeless now. Tom and his friends had little time for the
bald-headed scientist, for they were too much interested in the success
of the big blast.

"Well, we'll set her off to-morrow," Tom said one night, after a hard
day's work. "The rocky wall is honeycombed with explosive. If all goes
well we ought to bring down enough rock to keep the gangs busy night
and day."

Everything was in readiness. What would the morrow bring--success or
failure?

Chapter XXIV

The Hidden City

Gathered beyond the mouth of the tunnel, far enough away