Gearing.—­The gear wheels should be of good width, not less than 3/16 inch, and the smaller of steel, to withstand prolonged wear.  Constant lubrication is needed, and to this end the cover should make an oil-tight fit with the casing, so that the bottom of the big pinion may run in oil.  To prevent overfilling, make a plug-hole at the limit level, and fit a draw-off cock in the bottom of the cover.  If oil ducts are bored in the bearing inside the cover, the splashed oil will lubricate the big pinion spindle automatically.

[Illustration:  Fig. 73.—­Perspective view of completed turbine.]

General—­The sides of the casing are held against the drum by six screw bolts on the outside of the drum.  The bottom of the sides is flattened as shown (Fig. 70), and the supports, S1 S2, made of such a length that when they are screwed down the flattened part is pressed hard against the bed.  The oil box on top of the casing has a pad of cotton wool at the bottom to regulate the flow of oil to the bearings.  Fit a drain pipe to the bottom of the wheel-case.

Testing.—­If your boiler will make steam above its working pressure faster than the turbine can use it, the nozzle may be enlarged with a broach until it passes all the steam that can be raised; or a second nozzle may be fitted on the other end of the diameter on which the first lies.  This second nozzle should have a separate valve, so that it can be shut off.

XVIL.  STEAM TOPS.

A very interesting and novel application of the steam turbine principle is to substitute for a wheel running in fixed bearings a “free” wheel pivoted on a vertical spindle, the point of which takes the weight, so that the turbine becomes a top which can be kept spinning as long as the steam supply lasts.

These toys, for such they must be considered, are very easy to make, and are “warranted to give satisfaction” if the following instructions are carried out.

A Small Top.—­Fig. 74 shows a small specimen, which is of the self-contained order, the boiler serving as support for the top.

[Illustration:  Fig. 74.-Simplest form of steam top.] [1]

[Footnote 1:  Spirit lamp shown for heating boiler.]

For the boiler use a piece of brass tubing 4 inches or so in diameter and 3 inches long. (The case of an old brass “drum” clock, which may be bought for a few pence at a watchmaker’s, serves very well if the small screw holes are soldered over.) The ends should be of brass or zinc, the one which will be uppermost being at least 1/16 inch thick.  If you do not possess a lathe, lay the tube on the sheet metal, and with a very sharp steel point scratch round the angle between tube and plate on the inside.  Cut out with cold chisel or shears to within 1/16 inch of the mark, and finish off carefully—­testing by the tube now and then—­to the mark.  Make a dent with a centre punch in the centre of the top plate for the top to spin in.