e a look," replied the young
inventor. "It won't be safe to go in for a while yet, though. That
smoke will hang in there a long time. I didn't think there'd be a back
draft."

"There is, for we've often had the same trouble with our shots," Walter
Titus said. "I can't account for it unless there is some opening in the
shaft, connecting with the outer air, which admits a wind that drives
the smoke out of the mouth, instead of forward into the blast hole.
It's a queer thing and we haven't been able to get at the bottom of it."

"That's right," agreed his brother. "We've looked for some opening, or
natural shaft, but haven't been able to find it. Sometimes we shoot
off a charge and everything goes well, the smoke disappears in a few
minutes. Again it will all blow out this way and we lose half a day
waiting for the air to clear. There's a hidden shaft, or natural
chimney, I'm sure, but we can't find it."

"Thot blast didn't make much racket," commented Tim Sullivan. "I doubt
thot much rock come down. An' thot's not sayin' anythin' ag'in yer
powder, lad," he went on to Tom.

"Oh, that's all right," Tom Swift replied, with a laugh. "My explosive
doesn't work by sound. It has lots of power, but it doesn't produce
much concussion."

"We've often made more noise with our blasts," confirmed Job Titus,
"but I can't say much for our results."

They were all anxious, Tom included, to hurry into the tunnel to see
how much rock had been loosened by the blast, but it was not safe to
venture in until the fumes had been allowed to disperse. In about an
hour, however, Tim Sullivan, venturing part way in, sniffed the air and
called:

"It's all right, byes! Air's clear. Now come on!"

They all hurried eagerly into the shaft, Mr. Damon stumbling along at
Tom's side, as anxious as the lad himself. Before they reached the face
of the cliff against which the bore had been driven, and which was as a
solid wall of rock to further progress, they began to tread on
fragments of stone.

"Well, it blew some as far back as here," said Walter Titus. "That's a
good sign."

"I hope so," Tom remarked.

There were still some fumes noticeable in the tunnel, and Mr. Damon
complained of a slight feeling of illness, while Koku, who kept at
Tom's side, murmured that it made his eyes smart. But the sensations
soon passed.

They came to a stop as the face of the cliff loomed into view in the
glare of a searchlight which Job Titus switched on. T