se, three hundred feet through at the
waterline, and on top a third of that."

"How high is it?" asked Joe, who always liked to know just how big
or how little an object was. He had a great head for figures.

"It's one hundred and five feet high," the captain informed him,
"and it contains enough concrete so that if it were loaded into
two-horse wagons it would make a procession over three times
around the earth."

"Catch me! I'm going to faint!" cried Blake, staggered at the
immensity of the figure.

"That dam is indeed the key to the whole lock," murmured Mr.
Alcando, as he looked at the wonderful piece of engineering. "If
it were to break--the Canal would be ruined."

"Yes, ruined, or at least destroyed for many years," said Captain
Watson solemnly. "But it is impossible for the dam to break of
itself. No waters that could come into the lake could tear it
away, for every provision has been made for floods. They would be
harmless."

"What about an earthquake?" asked Joe. "I've read that the
engineers feared them."

"They don't now," said the captain. "There was some talk, at
first, of an earthquake, or a volcanic eruption, destroying the
dam, but Panama has not been visited by a destructive earthquake
in so long that the danger need not be considered. And there are
no volcanoes near enough to do any harm. It is true, there might
be a slight earthquake shock, but the dam would stand that. The
only thing that might endanger it would be a blast of dynamite."

"Dynamite!" quickly exclaimed Mr. Alcando. "And who would dare to
explode dynamite at the dam?"

"I don't know who would do it, but some of the enemies of the
United States might. Or someone who fancied the Canal had damaged
him," the captain went on.

"And who would that be?" asked Blake in a low tone.

"Oh, someone, or some firm, who might fancy that the Canal took
business away from them. It will greatly shorten certain traffic
and trade routes, you know."

"Hardly enough to cause anyone to commit such a crime as that, do
you think?" asked the Spaniard.

"That is hard to answer," went on the tug commander. "I know that
we are taking great precautions, though, to prevent the dam, or
the locks, from being damaged. Uncle Sam is taking no chances.
Well, have you pictures enough?"

"I think so," answered Blake. "When we come back we'll stop off
here and get some views from below the dam, showing the spillway."

"Yes, that ought to be interesting,