rally be where the pressure is greatest, also at the locks,
which would make a mighty channel for the flood of water, and which would
be difficult to repair. The spillway, too, if enlarged by explosives,
would make a nasty hole to build up.

Now another point which Ned had considered when he looked over the crude
drawings he had discovered. Hard rock underlies the dam near the surface
of the ground except for about one-fifth of its entire length. Here the
rock dips down to a minimum depth below sea-level of from 195 feet in the
depression east of the spillway to 255 feet in that west of the spillway.
Here, of course, would be another point of attack by one designing
permanent mischief.

These depressions or valleys have been slowly filled during past ages.
Measured from sea-level down, the first 80 feet consists of sand and clay;
the next 100 feet or so is stiff blue clay, while the last 20 to 60 feet
is a conglomerate, composed of sand, shells and stone. It will be readily
seen that great damage might be done by a raging torrent boring into the
sand and clay of the first strata.

Now, the outer walls of rock are 1,200 feet apart, the interval being
filled with spoil from the canal and lock excavations. The south "toe," as
it is called, has a height of 60 feet, while the north or down-stream
"toe" is 30 feet high. Spoil from the excavations will be dumped outside
the "toes" until the dam is 2,000 feet in width at the bottom. The top of
the dam is, or will be, 30 feet above water level and have a width of 100
feet. The channel of the spillway is 300 feet wide.

Ned had figured it out that one attacking the dam would naturally seek to
enlarge the locks and the spillway and also to burrow in under the bulk of
the dam where the sand and clay had been washed in below sea-level by
countless years of flood and storm. The locks and spillway, enlarged,
would require years of active work for repair; the sand and clay, if
subjected to high explosives, would cause the crest of the dam to drop in
on the north side and so enfeeble the entire structure, requiring the
gigantic work of constructing new foundations.

Therefore, when Ned saw the four men moving toward the spillway, saw them
part and seek the vulnerable points which have been described above, he
knew that the time he had been waiting for had come. The treacherous
rascals were there to do their wicked work that night--to carry out plans
long formed and well considered--