over which he had spent so many precious
moments.

Through the little porthole he cast a peculiar disk, heavy, such as I
had seen him place so carefully aboard the _Uncas_.

It sank in the water with a splash and trailed along beside the yacht,
held by a wire, submerged, perhaps, ten or twelve feet.

He made a final inspection of the thing as well as he could by the light
of a match, then pressed a key which seemed to close a circuit.

I could feel a dull, metallic vibration, as it were.

"What are you doing?" I asked, looking curiously also at an arrangement,
like a microphone, which he had placed over his ears.

"It works!" he cried excitedly.

"What works?" I reiterated.

"This Fessenden oscillator," he explained. "It's a system for the
employment of sound for submarine signals. I don't know whether you
realize it, but great advance has been made recently since it was
suggested to use water instead of air as the medium for transmitting
signals. I can't stop to explain this apparatus just now, but it is
composed of a ring magnet, a copper tube which lies in an air gap of a
magnetic field, and a stationary central armature. The magnetic field is
much stronger than that in the ordinary dynamo.

"The copper tube, which has an alternating current induced in it, is
attached to solid disks of steel which in turn are attached to a steel
diaphragm an inch thick. In the _Uncas_ I had a chance to make that
diaphragm practically a part of the side of the ship. Here I have had to
hang it overboard, with a large water-tight diaphragm attached to the
oscillator."

I listened eagerly, even if I were not an electrical engineer.

"The same oscillator," he went on, "is used for sending and receiving,
for, like the ordinary electric motor it is also capable of acting as a
generator, and a very efficient one, too. All I have to do is to throw a
switch in one direction when I want to telegraph or telephone under
water, and in the other direction when I want to listen in."

I could scarcely credit what I heard. Craig had circumvented even the
spectacular wireless. He was actually talking through water. Craig had
virtually endowed himself with a sixth sense!

I watched him spellbound. Would he succeed in whatever it was that he
was planning? I waited anxiously.

"There's the answer!" he exclaimed in sudden exultation. "Burke is on
the _Uncas_. He tells me that he went to see Mrs. Petzka and she is with
him--insisted on goi