Thousands of years ago, mechanical irrigation first made it possible to grow food in times of low rainfall. Without irrigation, the great civilizations of ancient Egypt, China, and Mesopotamia could not have risen. There were many types of ancient irrigation technologies, including dams, canals, and human- or animal-powered pumps. One of the first really useful pumps was called the water screw.

Re-creating the Invention That Made the Desert Bloom

Archimedes, the great mathematician of ancient Syracuse, was said to be something of an oddball. Apparently, he would become so involved with whatever project he was working on that he simply would forgo everything else, including eating and drinking. Even worse, at least for his assistants, was that he would forget to bathe and clean himself. On occasion, his coworkers would get so fed up with Archimedes that they would lift him up and bodily drag him to the public bath and wash away some of the stink.

Despite his lack of hygiene, so great was his genius, and so prodigious was his intellect, that the rich and powerful of the city would come to him, beseeching him to solve their most pressing technological problems.

A prime example of an important Archimedean invention was the water screw, a simple but effective device that provided ancient farmers with a better way to grow crops. By dipping one end of the machine into a river or stream and rotating its auger-like conveyor, farmers could irrigate large tracts of otherwise arid farmland (see Figure 4-1).

Archimedes’ water screw was a technological breakthrough. The device was later adapted throughout the ancient world and also put to use removing water from the bilges of ships and pumping water and muck out of mines.

The Genius of Archimedes’ Water Screw

Water screws are still quite common. They transport liquids in sewage treatment plants, fish hatcheries, and farmlands; they move solids in coal mines, grain elevators, snowblowers, and a host of other devices.

At first glance, the manner in which Archimedes’ water screw works is something of a mystery. Just how does turning a crank cause water to rise and move?

A mechanical engineer would classify Archimedes’ water screw as a simple type of progressive cavity pump. In this sort of pump, a water-holding cavity progresses up a twisting path from the water inlet at the bottom to the discharge spout at the top of the pump (see Figure 4-2).

If you’re still having trouble imagining exactly how Archimedes’ water screw works, try to picture what happens to a small ball if you place it in the screw-auger mouth at the bottom of the device. The ball rests in the depression defined by the curve of the screw. As the crank is turned, the instantaneous location of the ball-holding cavity moves up the centerline of the screw, and so, too, does the ball.

Old Greek-style water screws like this have been examined by scientists, who have found them to be about 30 efficient. This means that about a third of the energy you put into cranking the spiral around and around actually goes into lifting the water.

Replacing the ball with water makes visualizing what’s going on with the screw a bit harder. And, if you want to get mathematical about it, you’ll need a lot of pencil lead and time, because in order to calculate the volume of water you will move with one turn of the screw, you need to manipulate many different parameters, including the radius and angle of the screw, the ratio of the screw’s outer and inner cylinders, and the pitch of the blades.

But that’s likely more detail than you are interested in. Suffice it to say that if you understand that a screw pump works because it simply creates a cavity that is formed by the screw blade and the side of the tube, and that the cavity moves upward with each turn of the screw, you have a rough idea of how a water screw pump works.

Making a Water Screw Pump

Building a traditional screw pump takes a lot of woodworking skill. But luckily for us, we can greatly simplify re-creating Archimedes’ water screw by using a great invention of the 20th century—inexpensive plastic tubing. Building a model screw pump this way is quick and simple, and such a pump has the advantage of being easy to reconfigure, so many experiments are possible.

Here’s what you’ll need and how to build a water pump that works on the same principle as Archimedes’ original screw.

Assembling the Water Screw

To make the water screw pump, follow these steps:

1. Fasten the bottom end of the first hose piece, as shown in Figure 4-3, to the area near the bottom of the incline tube using a deck screw.
2. Screw through the hose’s inner wall only (the wall that’s touching the PVC pipe) because you want its mouth to remain open and unobstructed.
3. Wrap the tube around the incline tube using a 4-to-1 pitch (see Figure 4-3) and fasten the upper end to the incline tube with another deck screw in the same fashion.
4. Cut off any excess tubing with a pair of scissors.
5. If you desire, add a second, third, or even fourth hose to the incline tube next to the first. Again, fasten each to the incline tube using deck screws.

   Now you should build a base for the screw pump.

6. Assemble the 45-degree elbow, a 1-inch-long pipe, the pipe union (not screwed too tight), another 1-inch-long pipe, the flange, and the weight, as shown in Figure 4-3. (You may have to improvise a bit to attach the flange to the weight.)

   

7. Cement the pipe fittings and place the 15-degree shim under the flange so the screw is held at an angle of about 30 degrees from horizontal.
8. Build a crank for the upper end of the screw from the two 6-inch lengths of pipe, the two elbow fittings, and the cap.
9. To operate the screw pump, place its bottom end in the water.
10. Turn the crank so that as the screw rotates, the open end of the tubing scoops up water. With each subsequent turn, the scoop of water will rise one pitch-length up the screw shaft, finally emptying the water out at the top.

Explorimenting with Your Water Screw

In the first century CE, the Roman architect Marcus Vitruvius studied the work of Archimedes and figured out ways to make the original water screw work a bit better.

You can do the same. Although the water screw is simple, it lends itself to a host of science experiments. For example, you can easily experiment with a number of parameters, including these:

• Finding the best angle between the PVC shaft bearing the spiral tube and the vertical
• Determining the number of turns of tubing around the shaft and documenting the volume of water moved and the effort it takes to move it
• Discovering the effect that the diameter of tubing has on the volume of water moved and the time it takes to move it