ard, today, during the
tests. He will give Mr. Somers whatever orders are necessary in order
to make the tests most successful."

"Why not give the orders to me, sir?" Jack asked.

"Why, you see, Mr. Benson," replied the major, "I plan for you to be on
shore, out on the neck, to make certain observations regarding the work
of your craft. Those observations you will turn in to me."

"Very good, sir. The neck, I take it, is the narrow strip of land that
separates this part of the bay from the ocean?"

"Quite right, Mr. Benson."

It was to be observed that the major, like naval officers, addressed Jack
by the title of "mister," not "captain." This was because, in the
military service, Army and Navy titles are not recognized unless
conferred by government appointment or commission. Hence, though young
Benson was "captain" to his crew and to civilians, officers of the
United Service could address, him only as "mister."

"The neck, Mr. Benson," continued Major Woodruff, "is the land best
suited for watching our work from to-day. And now, I will state what
the object of to-day's tests is. This morning our tug will be engaged
in planting certain submarine mines. Mr. Somers will watch our work of
planting. Of course the mines will contain no explosives. You young
men have, I understand, solved the problem of leaving a submarine boat
while it lies on the bottom? You are also able to enter the submarine
again from the surface?"

"Quite right, Major," Jack nodded.

"Then, if Mr. Somers watches the planting of the dummy mines, he will
have the same advantage as would the commander of an enemy's submarine
in knowing where our mines are planted. We shall plant four of them,
this morning, and Mr. Somers, after seeing each mine planted, will mark
down its position on a chart of the bay. He will then take the boat
outside, enter under water, and, without touching any of our mines,
while handling the boat, will see if he can stop close by and cut the
connecting wires."

"If your mines contain no explosive, Major," Eph inquired, "how are you
going to be able to tell whether I collide gently with one of your
submarine mines?"

"We shall know at once," smiled Major Woodruff. "If you should collide
with one, you will cause, a bell to be rung in the camera obscura room
over at the fort. The bell that rings will show us which one of the
mines you touched against."

The "camera obscura," as used at a modern fort, is