[Illustration:  Fig. 107.—­Plan of water turbine, showing arrangement of nozzle.]

The nozzle should be a straight, tapered tube of some kind—­the nose of a large oil can will serve the purpose.  The exit must be small enough to allow the water to leave it at high velocity; if too large, the efficiency of the wheel will be diminished.  To the rear end of the nozzle should be soldered a piece of brass tubing, which will make a tight fit with the hose pipe leading from the water supply.  A few small brass rings soldered round this piece will prevent the hose blowing off if well wired on the outside.

Now comes the boring of the hole for the nozzle.  Fig. 106 shows the line it should take horizontally, so that the water shall strike the uppermost bucket just below the centre; while Fig. 107 indicates the obliquity needed to make the stream miss the intervening bucket.  A tapered broach should be used to enlarge the hole gradually till the nozzle projects sufficiently.  If the line is not quite correct, the tip should be bent carefully in the direction required.  One must avoid distorting the orifice, which should be perfectly circular; clean it out with a small twist drill of the proper size.

A brass elbow, which may be purchased for a few pence, should be driven into the waste hole, and a small shield be nailed under the air hole.  A couple of screwed-on cross pieces are required to steady the motor sideways and raise the elbow clear of the ground.

The motor may be geared direct to a very small dynamo, if the latter is designed to run at high speeds.  If a geared-down drive is needed, a small pulley—­such as is used for blinds, and may be bought for a penny—­should be attached to the shaft, and a bootlace be employed as belt.  Avoid overloading the wheel, for if it is unable to run at a high speed it will prove inefficient.

[Illustration:  Fig. l08.-Water motor working a photographic dish-rocker.]

Lubrication.—­The water will keep the bearings cool, but the bearings should be well lubricated.  The most convenient method of effecting this is to bore holes in the bearings, and from them run small pipes to an oil reservoir on the top of the case (as in Fig. 70), where they are fed on the siphon principle through strands of worsted.

Alternative Construction.—­If an all-metal case is preferred, the reader might utilize the description given of a steam turbine on pp. 170-178.  The details there given will apply to water as well as steam, the one exception being that a nozzle of the kind described above must be substituted for the steam pipe and small ports.

XXII.  MODEL PUMPS.