On March 10, 1849, Abraham Lincoln filed a patent for a device for "buoying vessels over shoals" with the US Patent Office. Patent No. 6,469 was approved two months later, giving Abraham Lincoln the honor of being the only US president to hold a patent. During his brief experience as a ferryman on the Mississippi River, Lincoln was stranded twice on riverboats that had run aground. His invention, "adjustable buoyant air chambers," would be attached to the sides of a boat. They could be lowered into the water and inflated to lift the boat over obstructions in the water.