When the Potomac River in Alexandria is at low tide, a chunk of brown stone covered with moss and trash rises above the waterline. In Southeast, at Naylor Road and Southern Avenue, there is another brown stone nub. A similar piece of stone is hidden beneath a growth of weeds and vines at 54th and D streets SE.

In all, 40 sandstone pillars were set in place by surveyors 190 years ago to mark the 40-mile-long border of the original federal city. Over the years, some of the original boundary markers have been uprooted and others have been damaged by vandalism, traffic and other hazards.

Sen. Charles McC. Mathias Jr. (R-Md.) recently introduced a bill asking Congress to provide an estimated $360,000 to protect and maintain the markers as monuments to the project's chief surveyor, Maj. Andrew Ellicott of Philadelphia, and his assistant, Benjamin Banneker, a free black man who made the mathematical calculations to determine where the first marker would be placed.

"Too often we find ourselves mourning the loss of some significant historical feature of our nation," Mathias told the Senate Committee on Energy and Natural Resources when he introduced his bill last month. "The boundary markers of the nation's capital are such endangered monuments of the history of the federal city. With just a little bit of care, they can be assured preservation for decades to come."

Congress agreed in 1790 to create a 100-mile-square federal city with parcels of real estate donated by Maryland and Virginia. The federal government returned Virginia's portion of the planned city in 1846, but the boundary markers have remained in their original locations. Fourteen of the markers are in Virginia and 26 are located along the Maryland-District boundary line.

Mathias, who has presented the same bill unsuccessfully for the last two terms of Congress, was asked in 1976 to support legislation to protect the boundary stones after a report detailing the declining condition of the markers was published by the National Capital Planning Commission.

That report, developed by the commission, the Daughters of the American Revolution and the American Society of Civil Engineers, told of the efforts of the DAR since 1914 to protect the markers.

"The DAR, while as committed as ever to continuing its stewardship role, can no longer assume the financial and personal burden necessary to assure the maintenance of these stones," Mathias told the Energy and Natural Resources Committee.

Ralph Shaffer, chairman of the local history and heritage committee of the American Society of Civil Engineers, said that although the 40 boundary markers have been designated as Category II landmarks by the Joint Committee on Landmarks of the Nation's Capital, no agency has clear jurisdiction over the markers.

"We know that markers located in parklands fall in the jurisdiction of the National Park Service," Shaffer said. "But there is a question of who controls markers located in the back yards of private homes."

Shaffer, who is also the acting surveyor of the D.C. government, said the original markers are no longer used as markers for surveying. Every section of the city has been divided into squares, with each square having its own point of reference, Shaffer said.

"But those original markers have an invaluable historical significance," he added. "They tell us that one of the earliest civil engineering projects in this country involved a free black man."