A submarine torpedo boat, coming into a harbor underneath the surface,
is not pictured on the white table under the camera obscura. So it was
desired to see whether Eph could come in, knowing the exact locations of
each of the four dummy mines, and quickly cut the firing electric wires.
If this could be done, the Army would have to revise its method of firing
such submarine mines by means of the camera obscura detection.
As Eph listened to the explanation his mind began to revolve plans
rapidly whereby he hoped to succeed in cutting the mine wires.
"You will keep sufficiently below the surface, too, Mr. Somers,"
continued Major Woodruff. "We do not want you so close to the surface
of the water that a ripple would show on the camera obscura table. You
cannot, of course, rise and use your periscope to see where you are.
Even the periscope would betray you."
The "periscope" is a device also of the nature of a camera obscura. In
the case of the periscope a narrow metallic tube is thrust above the
water and the shutter turned about, reflecting all the scene about on a
white-covered table in the boat's cabin.
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