"When the submarine is submerged," explained Lieutenant Stein,
"the periscope is the eye of the vessel. Peering over the waves,
it reflects what it sees into the watching human eye in the
conning tower. Destroy it, and the submarine is a blind thing,
plunging to destruction."

"Then the periscope is the one weak spot in a submarine?" asked
Frank.

"Exactly," was the reply. "Of course, if it were destroyed, the
vessel might rise immediately to the surface and so gain its
bearings. But in the midst of battle it would probably mean
certain destruction; for when it rose the submarine would
naturally be so close to the enemy that a single big shell would
put it out of business."

The boys looked long at this strange mechanical eye. Shaped like
a small pipe, it ran up from the conning tower and protruded
above the vessel. A large lens at the top turned off as does an
elbow in a stove pipe. This portion, when necessary, moved in
all directions. When raised to its maximum height everything
within a radius of ten miles is reflected in it.

"The shaft can be lowered to within a few inches of the top of
the water," the lieutenant explained, "thus guarding against the
danger of being hit. The officer in the conning tower peers into
the binoculars and sees just what the periscope sees."

"Will you explain just how it works?" asked Jack. I