Cut out along the line with a keyhole saw.  Then saw out the square of wood.  A 5/8-inch hole is now bored edgeways through the wood into the “bulge” for the escape, and in what will be the top edge is drilled a 1/4-inch hole to allow air to enter.

[Illustration:  Fig. l05.—­Simple water turbine.]

Cut out the sides, and screw them on to the centre at the four corners, taking care that the grain runs the same way in all three pieces, so that they may all expand or contract in the same direction.  Plane off the edges of the sides flush with the centre.

The parts should now be separated, after being marked so that they can be reassembled correctly, and laid for a quarter of an hour in a pan of melted paraffin wax, or, failing this, of vaseline, until the wood is thoroughly impregnated.  Reassemble the parts, and put in the rest of the holding screws, which should have their heads countersunk flush with the wood.

[Illustration:  Fig. 106.—­Water turbine, with pulley side of casing removed.]

For the shaft select a piece of steel rod 5/32 inch in diameter, and 3 or 4 inches long; for the bearings use two pieces, 3/4 inch long each, of close-fitting brass tube.  Now take a drill, very slightly smaller in diameter than the bearings, and run holes right through the centres of, and square to, the sides.  Both holes should be drilled at one operation, so that they may be in line.

With a wooden mallet drive the bearings, which should be tapered slightly at the entering end, through the sides.  Push the shaft through them.  If it refuses to pass, or, if passed, turns very unwillingly, the bearings must be out of line; in which case the following operation will put things right.  Remove the bearing on the pulley side, and enlarge the hole slightly.  Then bore a hole in the centre of a metal disc, 1 inch in diameter, to fit the bearing; and drill three holes for screws to hold the disc against the case.  Rub disc and bearing bright all over.

Replace the bearing in its hole, slip the disc over it, and push the shaft through both bearings.  Move the disc about until the shaft turns easily, mark the screw holes, and insert the screws.  Finally, solder the bearing to the disc while the shaft is still in place.

The wheel is a flat brass disc 4 inches in diameter.  Polish this, and scratch on one side twelve equally spaced radii.  At the end of each radius a small cup, made by bending a piece of strip brass 1/4 inch wide and 1/2 inch long into an arc of a circle, is soldered with its extremities on the scratch.  A little “Tinol” soldering lamp (price 1s. 6d.) comes in very handy here.

To fix the wheel of the shaft requires the use of a third small piece of tubing, which should be turned off quite square at both ends.  Slip this and the wheel on the shaft, and make a good, firm, soldered joint.  Note.—­ Consult Fig. 107 for a general idea of the position of the wheel, which must be kept just clear of the case by the near bearing.