lk of the dam. No one looked to see that the men
assembled in the workings all belonged there. At midnight four men who did
not belong there entered the excavation which leads from the bottom of the
lower lock to the sea-level channel into Limon Bay, which is a child of
the Caribbean Sea.

These four men moved about as if accustomed to the situation, only now and
then they halted and whispered together. Other men, workmen, were doing
that, however, and so these four passed on up to the foot of the spillway
without attracting attention.

Here they separated, one to the west, one to the east, where the locks
are, and one to a position half way between the spillway and the west side
of the locks. The fourth man remained near the foot of the spillway.

Due primarily to its size, Gatun dam has received, perhaps, more attention
in the United States than is its due. There is nothing especially
difficult or complicated about this dam, and many dams have been
successfully built in this country to withstand much larger pressures and
greater heads of water than the Gatun dam without being given one-quarter
of the attention.

Gatun dam fills the opening between the hills at Gatun through which the
Chagres river flows to the Caribbean Sea. It consists, if it may be
regarded in the light of a finished production, of a water-tight center or
core composed of sand and clay mixed in proper proportion and deposited
hydraulically; that is by being pumped in.

On each side a wall of rock confines this core. The bulk of the dam rests
on impermeable material of sufficient supporting power. The locks and
spillway are considered a part of the dam.

The locks are built in an excavation at the east end of the dam, in rock,
and will lift vessels from the Atlantic level to the level of the Lake of
Gatun. The spillway is a concrete-lined opening cut through a hill of rock
near the center of the dam. When supplied with suitable gates, it will
regulate the level of the lake.

The dam proper is about 9,000 feet long over all, measured on its crest,
including locks and spill way, but for only five hundred feet of this
great distance will it be subjected to great pressure. During this space
there is, or will be, a weight of about eighty-five feet depth against the
barrier. For only about half its length will the head of water on the dam
be over fifty feet.

It will be seen from the above description that the point of attack on the
dam would natu