Many towboats have all of the latest inventions for quick and safe travelling in water that is often more tricky than the open sea. There’s a lot of traffic to watch out for on the Mississippi, and the river sweeps around in many bends. Mud collects on the river bottom, so the captain can’t always know how deep the water is going to be. Uprooted trees and other big things that could damage vessels often come floating downstream. And when it’s pitch dark, or when a thick fog hangs over the water, all these problems get much worse. Radar is one of the inventions that help towboats avoid danger. Radar sends out radio waves which bounce back to the towboat from anything they hit. In the towboat’s pilothouse is a radarscope, which is a little like a television screen. The returning radio waves show up as spots of light called pips on the radarscope. By looking at the pips, the pilot can locate the shores of the river, other vessels, floating trees and anything else that’s dangerous. Another wonderful invention, called a depth recorder, tells the pilot how deep the water is under the head barge in his tow. If the river seems to be getting shallow, he can steer the whole tow into safer water. The depth recorder works by sending out sound waves and making a record of them when they bounce back from the river bottom. [Illustration] In the old days, river craft had a leadman who measured depth with a line tied to a lead weight. Knots and pieces of leather marked the line. Even at night the leadman could tell by feel how deep the water was. For instance, if his fingers felt that the line was wet up to a place where there were two strips of leather, he would know that two fathoms (twelve feet) of water lay underneath. Two markers at two fathoms. “By the mark twain,” the leadman would call out to the captain. There was once a Mississippi River pilot named Samuel Clemens who, like all pilots, loved to hear that call. It meant that there was enough water to keep his vessel afloat. Later, when he began to write books, he signed them with the name Mark Twain. In Mark Twain’s time, the Mississippi River boats were driven by huge paddle wheels. As the wood-burning steam engine turned the wheels, the paddles pushed against the water and shoved the boat forward. Steam engines began working in rivers very quickly after the first successful paddle boat, the Clermont, proved that she could push upstream. River boatmen needed engines more than seafaring men did, because winds seldom blow upstream as they do on the Nile. Before there were paddleboats, men took cargo down the Mississippi in keelboats. Then they had to get the boats up-river again almost entirely by muscle-power. Pushing against the bottom with poles, or pulling with ropes from the shore, river boatmen worked the whole way up from New Orleans to Pittsburgh. [Illustration]