What is "Archimedes' screw"?

Archimedes' screw is a simple mechanical device for raising water up an inclined surface. Unlike a bucket or a pump, it cannot raise water vertically, but it is an early example of a device that makes use of rotary motion.

It is far easier to understand the device from an illustration such as at http://www.animatedsoftware.com/pics/pumps/archscr5.gif than through words alone, but here goes.

Visualize a central shaft around which a blade is "wound." It would look like the main shaft of a modern day wood screw. Now position that screw so that it is tilted up slightly (like 15 - 30 degrees) from the horizontal. Place it into a trough, like a soda straw split in half lengthwise. Position the bottom end of the screw and trough in water. (The lowest end of the shaft should be at or below the level of the water.) Now when the shaft is turned the water will be "carried" up the trough until it spills out at the top end.

The screw works because the lowest edge of the blade as it rotates "moves" continuously upward. As long as the blade forms something of a seal with the trough, each turn of the screw will push another batch of water up the trough. A perfect seal is not needed, though, because water leaking from one turn of the screw is merely added to the amount in the next turn, and thus the effects of leakage are distributed across the entire length of the trough.

An animated Flash movie of the screw in action can be seen at http://www.animatedsoftware.com/elearning/All%20About%20Pumps/glossary/aap_glossary.html (You'll have to copy and paste the entire link, because the AnswerBag software doesn't map it correctly.)

It is possible to solve the problem of leakage by enclosing the screw completely, as by wrapping a hose around a broomstick....

An Archimedes screw is a water lifting device used for raising water. It was an early version of a water pump, used in irrigation or other areas. There are two construction types, a screw turning inside a tube or a tube wound around a central shaft. It works on the principle that water seeks its own level and that turning the shaft takes advantage of it and raises the water. The shaft can be turned by hand, with a motor or with wind or water power.