Damaged, corroded device may hold clues to submarine's final minutes
BY BRIAN HICKS
Of The Post and Courier (Charleston SC)
It looks like a contraption right out of Jules Verne, more fanciful than functional. But the depth gauge found in the H. L. Hunley may prove more than useful to scientists trying to figure out what happened to the Confederate submarine in its final minutes on a cold February night in 1864.
Archaeologists say evidence suggests the sub's depth gauge was either broken on Feb. 17, 1864 - the night it sank the USS Housatonic - or a short time later.
"When we found George Dixon, literally at his feet was a pool of mercury," said Maria Jacobsen, senior archaeologist on the Hunley project.
The mercury came from the depth gauge, a device made of iron tubing and glass that allowed the Hunley crew to measure water pressure, and thus depth, outside the submarine.
Although the depth gauge was probably fragile, finding mercury below the sediment could indicate the device broke because of some trauma to the sub on the night it sank.
Mercury, which can make people sick if they come in contact with it, would not have been allowed to stand in the sub, which means in all likelihood the depth gauge was intact when the sub pulled away from Battery Marshall on its final cruise.
The depth gauge, which looks a little like a trombone, was a complex device. Connected to the hull on the top port side by a seacock, its operation was simple: An open valve allowed water into the pipe. The water moved the heavier mercury only as pressure increased. As the Hunley went deeper, the mercury rose in the glass tube.
Most likely, the depth gauge, which was nearly 3 feet tall with piping running down the portside hull and halfway back up, was mounted to a board with markings indicating the sub's depth.
Hunley senior conservator Paul Mardikian said the technology could have been inspired by or copied from any number of pressure gauges in use with boilers or the like in the mid-19th century.
The depth gauge idea was at least a century-old by the time the Hunley sailed. Contemporary reports of David Bushnell's Turtle say the Revolutionary War-era submersible carried some sort of depth gauge.
Jacobsen said the way the gauge fits into the sub makes it appear that it was built specially for the Hunley. Originally, the Hunley was designed to tow a contact mine behind it, dive under ships and pull the mine into the enemy ship's hull. The depth gauge would have been an important tool for that sort of navigation.
When it sank the USS Housatonic off Sullivan's Island, the Hunley delivered its charge with a spar mounted on the bow. The depth of the submarine may not have mattered much then, and it could be the gauge rarely was used.Overall, scientists say the Hunley's depth gauge is in worse condition than most artifacts they've found in the submarine.
Besides the broken glass and spilled mercury, the iron piping is
heavily corroded. That could have been caused by scouring of
sediment pouring into the sub, or could be some reaction to holding the caustic liquid metal, scientists say.
"Getting it out of the submarine was a scary ordeal; it is very
fragile," Mardikian said.
State Sen. Glenn McConnell, chairman of the Hunley Commission, said the depth gauge shows the thought and innovation that went into the sub's construction.
"This is more evidence that this was no suicide mission," according to McConnell, R-Charleston. "They did anything they could to have eyes or awareness of their surround-ings when they were in the vessel."
FACES OF HISTORY
In honor of the anniversary of the Hunley crew burial on April 17, 2004, the Warren Lasch Conservation Center has put on display the facial reconstructions of the sub's final crew. The reproductions can be seen during regular tours of the Hunley. The center is open to the public from 10 a.m. to 5 p.m. on Saturdays and noon to 5 p.m. Sundays.