so as to let through a stream of water as big as desired up to three inches square. As the flume was more than a foot below the water level in the pond, there would be a pretty good force in the stream flowing out of the flume. To utilize it mechanically they had to build first a water wheel.

There are three main kinds of water wheels, overshot, undershot, and turbines. In the overshot wheel the water is delivered near the top but on the downstream side of the center. Buckets are provided such that the water flows in at the top buckets and its weight carries over the wheel till the top becomes the bottom, and the buckets are emptied. The water gives up practically all of the energy of position which it had by reason of its higher level in the pond than in the stream below. This form of water wheel gives fully seventy-five per cent efficiency under favorable circumstances. With it there is no object in setting the flume low in the dam so as to get a swift rush of water. Overshot wheels are very large in diameter compared to other types and awkward to set up on that account unless the locality exactly suits them.

With the undershot wheel, on the other hand, the water flows with a rush against paddles which it strikes below the center of the wheel on its upstream side. Part of the energy of motion which the water possesses is thus given up to the wheel and the machinery it drives. The efficiency of this type of wheel is usually not very high, although a special type called the Poncelet wheel has such a cleverly designed combination of

the flume and the curved paddle blades that the efficiency of this wheel runs up almost to seventy per cent.

The turbine is much more complex. This water wheel oftentimes has a vertical instead of horizontal axis, though not always. The water descends through the hollow axis and then spreads out to the circumference, striking against very numerous specially formed vanes, or else enters at the circumference and emerges at the center. Nearly all of the energy both of position and of motion is given up by the water to the wheel before it is finally discharged. Though the turbine is as small as or even smaller than the undershot, its efficiency is almost eighty per cent under favorable circumstances.

George and Paul were practically restricted by the circumstances and their lack of tools and experience to make their water wheel very simply on the undershot plan. They made a plain paddle wheel with straight radial paddles. It was about a foot in diameter and six inches wide, and had eight paddles. The axis was a wooden stick with bolts driven into its ends to rest in the bearings. They fastened a six-inch grooved wooden pulley onto the wooden shaft, in order to belt to the machinery they proposed to run with the water wheel.

Of course with only a couple of feet of "head" of water and so small and inefficient a wheel, the machinery could only be play machinery, but they arranged it as a sawmill. First, there was a "board saw." This they made by cutting the end out of a tin can

and filing forward-pointing notches all around its circumference to give it teeth. They "set" these teeth to avoid "pinching" by bending them alternately slightly to one side and then the other. They mounted their "board saw" by screwing two nuts on either side of it onto a quarter-inch bolt, drilling small holes centrally in the ends of the bolt and holding it between pivots fastened underneath a board through which the saw protruded within a saw cut. They belted their saw to the water wheel by means of a loop of string running over the six-inch water wheel pulley and a little half-inch pulley fixed upon the axle of the saw.

When they let on a good flow of water the saw hummed quite excitingly, but it was only possible to saw "logs" that had lain so long in the bark as to grow punky. They found a good many gray birch limbs in this condition and sawed them up with a lot of fun.

The next thing was a stave saw, for the real sawmills near the Anson place almost all made a specialty of sawing pine keg and barrel staves. These must be curved in section like the keg or barrel they are to form, and so must be sawn out with a cylindrical saw. To make their stave saw, George and Paul took a smaller tin can than they had used before and cut off its end truly at right angles to its axis. They then filed and set teeth around the circumference of the curved side surface. As before they mounted the saw on a quarter-inch bolt with nuts, fastened on a little pulley and set pivots in the ends of the shaft, attaching them to a piece of board. In this case they