Hammering is another kind of muscle work that
machines can do quickly and easily. Suppose the water pipes under your street need mending. Repair men have to tear up the pavement in order to reach the pipes. So they bring in jack hammers to do the pounding. Strong blasts of air run the hammers, and, in no time, the pavement is broken up.
Crushed rock was used for making the paved street in the first place. It came from a big machine called a rock crusher, which breaks up chunks of stone into small pieces. Strong jaws inside the crusher chew at the stone until they have made it into bits that are just the right size.
An even bigger pounding machine is the pile driver. It can hammer a great thick log down into the ground almost as easily as a man can hammer a nail through a board. One kind of pile driver does its pounding job with a steam piston. Another kind lifts a heavy weight and lets it bang down on top of the log, called a pile. The one in the picture works in a harbor. It drives piles deep into the earth that lies under water. A whole group of piles make the foundation for a pier in the harbor, for ships to tie up alongside.
Harbors and rivers must be kept safe for ships. If mud and sand pile up in a thick layer on the bottom, ships may get stuck. So dredges go to work clearing the mud and sand away. Often a clean-up job takes a long time. The men who run the machinery live on board the dredge, just as sailors live on a ship.
Some dredges have scoops that dig under water. Others, like the one in the picture, use giant suction pumps. The mud or sand they suck up is called spoil.