possible--1/40 to 1/50 inch--and inclined at an angle of not more than
45 degrees to the top plate. The best drills for the purpose are tiny Morse
twists, sold at from 2d. to 3d. each, held in a pin vice rotated by the
fingers. The points for drilling should be marked with a punch, to give the
drills a hold. Commence drilling almost vertically, and as the drill enters
tilt it gradually over till the correct angle is attained.

If a little extra trouble is not objected to, a better job will be made of
this operation if three little bits of brass, filed to a triangular section
(Fig. 76 a), are soldered to the top plate at the proper places, so that
the drilling can be done squarely to one face and a perfectly clear hole
obtained. The one drawback to these additions is that the vanes of the
turbine may strike them. As an alternative, patches may be soldered to the
under side of the plate (Fig. 76, b) before it is joined to the barrel;
this will give longer holes and a truer direction to the steam ports.

[Illustration: FIG. 76. Steam port details.]

Note that it is important that the ports should be all of the same diameter
and tangential to the circle on which they are placed, and all equally
inclined to the plate. Differences in size or direction affect the running
of the top.

Solder the spindle to the wheel in such a position that the vanes clear the
boiler by an eighth of an inch or so. If tests show that the top runs quite
vertically, the distance might be reduced to half, as the smaller it is the
more effect will the steam jets have.

A small brass filler should be affixed to the boiler halfway up. A filler
with ground joints costs about 6d.

A wick spirit lamp will serve to raise steam. Solder to the boiler three
legs of such a length as to give an inch clearance between the lamp wick
and the boiler. If the wick is arranged to turn up and down, the speed of
the top can be regulated.

A Large Top.--The top just described must be light, as the steam driving
it is low-pressure, having free egress from the boiler, and small, as the
steam has comparatively low velocity. The possessor of a high-pressure
boiler may be inclined to make something rather more ambitious--larger,
heavier, and useful for displaying spectrum discs, etc.

The top shown in Fig. 77 is 3 inches in diameter, weighs 1 oz., and was cut
out of sheet-zinc. It stands on a brass disc, round the circumference of
which is soldered a ring of 5/32-inch