The Yalu is the longest river in Corea. Its source is found near the 40th parallel. Flowing northwardly, for about eighty miles, the stream forms the boundary between Ping-an and Ham-kiung. Then, turning to the westward, it receives on the Manchurian side twelve tributaries, which run down the gorges of the Ever-White Mountains. Each of these streams is named, beginning westwardly, after the numerals of arithmetic. The waters of so many valleys on the west, as well as on the north and east, emptying into the Yalu, make it, in spring and fall, a turbulent stream, which sinuates like the writhing of a dragon; whence its name. In the summer, its waters are beautifully clear, and blue or green—the Coreans having no word to distinguish between these two colors. It empties by three mouths into the Yellow Sea, its deltas, or islands, being completely submerged during the melting of the snows. It is easily navigable for junks to the town of Chan-son, a noted trading place, sixty miles from the sea. The valley of the Yalu is extremely fertile, and well wooded, and the scenery is superb. Its navigation was long interdicted to the Chinese, but steamers and gunboats have entered it, and access to the fertile valley and the trade of the region will be gained by other nations. The Tong-kia River drains the neutral strip. The town nearest the frontier, and the gateway of the kingdom, is Ai-chiu. It is situated on a hill overlooking the river, and surrounded by a wall of light-colored stone. The annual embassy always departed for its overland journey to China through its gates. Here also are the custom-house and vigilant guards, whose chief business it was to scrutinize all persons entering or leaving Corea by the high road, which traverses the town. A line of patrols and guard-houses picketed the river along a length of over a hundred miles. Nevertheless, most of the French missionaries have entered the mysterious peninsula through this loophole, disguising themselves as wood-cutters, crossing the Yalu River on the ice, creeping through the water-drains in the granite wall, and passing through this town. Or they have been met by friends at appointed places along the border, and thence have travelled to the capital. Through this exit also, Corea sent to Peking or Mukden the waifs and sailors cast on her shores. A number of shipwrecked Americans, after kind treatment at the hands of the Coreans, have thus reached their homes by way of Mukden. This prosperous city, having a population of over two hundred thousand souls, and noted for its manufactures, especially in metal, is the capital of the Chinese province of Shing-king, formerly Liao Tong. It is surrounded by a long wall pierced with eight gates, one of which—that to the northeast—is called “the Corean Gate.” Niu-chwang has also a “Corean Gate.”