"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. "That is another of our new processes," he added after a moment's reflection. "Echo--that's the answer in a nutshell." The captain led the way to the ship's marimeter, a cylindrical contrivance that looked as though it might house a compass or a binnacle lamp. "The marimeter works on the principle of electricity controlled by sound vibration," the captain expounded in his competent fashion. "A sound wave is sent out from the bottom of the vessel by mechanical means and the instant this sound is started it is picked up electrically and relayed to the recording instrument and the dial of the latter begins to register. The sound wave travels to the bottom of the ocean and returns in the form of an echo, and this echo is also picked up by the diaphragm in the bottom of the boat and is also relayed by electricity to the recording instrument, causing the pointer to stop immediately. Sound travels at practically a uniform rate in the water, at about 4000 feet a second. The depth is measured by accurately taking and recording mechanically the time for sound to travel down and back. The depth is shown on the dial in fathoms, and four soundings may be made per minute." It all sounded so simple, and yet what a wonderful contrivance as against the old-fashioned method of taking deep-sea soundings. To demonstrate Captain Austin took an electrical sounding for his new protegées and in a few seconds the "echo" had returned from the bottom of the Sound, showing a depth of ten fathoms. For some hours, under the guidance of the ship's skipper, the trio of newcomers thoroughly inspected the _Nemo_. This plainly was the "prospecting" boat of the salvage company's fleet. It went out and staked the claim and then called on the full facilities of the fleet for completion of the job. Captain Austin, completely won by the honesty and candor of his new friends, and acting under instructions of superintendent Brown, took the boys entirely into his confidence.