Lincoln's patent, No. 6,469, was granted on May 22, 1849, for a device for "Buoying Vessels Over Shoals," when he was back in Springfield practicing law after one term as an Illinois congressman in Washington. His idea, to equip boats with inflatable bellows of "india-rubber cloth, or other suitable water-proof fabric" levered alongside the hull, came as a result of river and lake expeditions he made as a young man, ferrying people and produce on the Mississippi and the Great Lakes. At least twice his boats ran aground on sandbars or hung up on other obstacles; given the Big River's ever-shifting shallows, such potentially dangerous misadventures happened often. Freeing a beached vessel usually involved the laborious unloading of cargo until the boat rode high enough to clear the snag. According to Harry R. Rubenstein, chair of the Division of Politics and Reform at NMAH, Lincoln "was keenly interested in water transportation and canal building, and enthusiastically promoted both when he served in the Illinois legislature." He was also an admirer of patent law, famously declaring that it "added the fuel of interest to the fire of genius."