"In here we listen for lost ships just as you listen for the voice of a friend over the telephone. How does that strike you?"
While in the Navy Jay and Dick had come to know only too well how the microphone was used to hear other vessels, and how it had been a powerful means in the overthrow of the U-boats and the safeguarding of American troops bound for Europe. The microphone listened for moving vessels and was acquainted with their movements because the swish of the propeller blades was borne into the listening device of the Yankee craft.
But how could a salvage ship "listen" for a helpless wreck lying foundered on the bottom of the sea? They were soon to know. Captain Austin conducted them first into the forward hold and showed them another compartment with a massive winch used to raise or lower an object in the water under the keel. Taking them aft he showed another compartment equipped as was the one forward.
"We use the so-called Hughes balance," explained the skipper as the boys gathered close to him in order to hear above the whirr of the throbbing engines. "They are two massive rings suspended by cables and raised or lowered at will by the winches. These rings or cups are wound with copper wire. The lower windings connect with an ordinary telephone receiver while other spools are in series with a microphone and three dry cells. This makes a sensitive instrument."
Dick, who was somewhat of a mechanic, was beginning to see light.
"When these induction coils are trailed through the water from underneath the Nemo the telephone receiver in the control station gives no sound as long as the two balances move through the water," continued the captain. "But the minute one of them comes within the vicinity of a wreck, the electrical balance will be disturbed and the telephone will sound its warning to the operator. The nearer the balances come to the wreck the louder the sound. All you have to do is cruise back and forth near the spot where the sunken vessel is supposed to lie, and sooner or later the faithful induction balance will find the wreck."
"How do you judge for the depth?" asked Dick.
"The depth of the ocean naturally varies more or less," the captain explained further. "If a deeper strata is encountered the induction balances must be lowered further in the water than in cruising in shallow water. Not only will the induction balance give the exact spot where the ship is located, but it will give the precise location even though the lost ship is covered with sand or silt."
"But how do you determine the depth? Do you drop a plumb line, or have you a new method of depth sounding?" persisted Dick, who was taking an engineering course at Brighton preparatory to studying electrical engineering at college. Naturally he was interested in every engineering problem.
Captain Austin smiled whimsically.