If there’s hard-caked mud on the bottom, cutter heads break it up. Then it’s ready to be pumped out through huge steel pipes that stretch away from the dredge like a great snake and pour the spoil out on land.
Of course, a dredge must stay in one place while it is working. So it carries along two huge spikes called spuds. These move straight up and down at the stern of the dredge. When they ram into the earth underwater, they keep the dredge from drifting.
A spud is so heavy that it pokes its own hole in the muddy bottom of a river or harbor. But making holes on dry land is a different problem. For instance, you can’t just poke a telephone pole into the hard ground, or pound it in easily with a pile driver, either. So, in many places, a machine bores holes for telephone poles, just the way a carpenter bores a hole with a brace and bit. Then the machine’s long arms reach out, lift a pole into the air and plug it down neatly into place.
Long ago our ancestors discovered how to use simple tools—such as hammers, shovels, crowbars and rollers. These things seem very ordinary to us, but they were really wonderful discoveries. The clever men who invented them were providing ideas, one by one, which scientists and engineers used much later. Our great machines are combinations of many, many things that men discovered from using simple tools.
POWERFUL PUSHERS
The giant shovel digs; the overhead crane lifts; the pile driver pounds. All machines multiply the power that’s in the muscles of men—or of animals. The pushingest animal is an elephant. In some places in the world, elephants are trained to clear land by putting their foreheads against a tree and heaving until the tree topples over.
A tree-dozer can out-push an elephant. The one in the picture has a special forehead built in front. With a slow, steady shove, it clears the way for roads or opens up fields for farms.