The turbine just described can hardly be termed an efficient one, as the vanes, owing to their simple formation, are not shaped to give good results.  We therefore offer to our readers a design for a small turbine of a superior character.  This turbine is shown in elevation and section in Fig. 70.  The casing is, as in the preceding instance, made up of flat brass plates and a ring of tubing, and the bearings, BG1, BG2, of brass tube.  But the wheel is built up of a disc 3 inches in diameter, round the circumference of which are 32 equally-spaced buckets, blades, or vanes, projecting 5/8 inch beyond the edge of the disc.  The wheel as a whole is mounted on a spindle 3-1/8 inches long, to which it is secured by three nuts, N1 N2 N3.  One end of the spindle is fined down to take a small pinion, P1, meshing with a large pinion, P2, the latter running in bearings, BG3, in the wheel-case and cover.  The drive of the turbine is transmitted either direct from the axle of P2 or from a pulley mounted on it.

CONSTRUCTION.

[Illustration:  Fig. 71.—­Plate marked out for turbine wheel blades.  B is blade as it appears before being curved.]

The Wheel.—­If you do not possess a lathe, the preparation of the spindle and mounting the wheel disc on it should be entrusted to a mechanic.  Its diameter at the bearings should be 5/32 inch or thereabouts. (Get the tubing for the bearings and for the spindle turned to fit.) The larger portion is about twice as thick as the smaller, to allow room for the screw threads.  The right-hand end is turned down quite small for the pinion, which should be of driving fit.

The Blades.—­Mark out a piece of sheet iron as shown in Fig. 71 to form 32 rectangles, 1 by l/2 inch.  The metal is divided along the lines aaaa, bbbb, and ab, ab, ab, ab, etc.  The piece for each blade then has a central slot 5/16 inch long and as wide as the wheel disc cut very carefully in it.

Bending the Blades.—­In the edge of a piece of hard wood 1 inch thick file a notch 3/8 inch wide and 1/8 inch deep with a 1/2-inch circular file, and procure a metal bar which fits the groove loosely.  Each blade is laid in turn over the groove, and the bar is applied lengthwise on it and driven down with a mallet, to give the blade the curvature of the groove.  When all the blades have been made and shaped, draw 16 diameters through the centre of the wheel disc, and at the 32 ends make nicks 1/16 inch deep in the circumference.

True up the long edges of the blades with a file, and bring them off to a sharp edge, removing the metal from the convex side.

Fixing the Blades.—­Select a piece of wood as thick as half the width of a finished blade, less half the thickness of the wheel disc.  Cut out a circle of this wood 2 inches in diameter, and bore a hole at the centre.  The wheel disc is then screwed to a perfectly flat board or plate, the wooden disc being used as a spacer between them.