copper tubing, furnished with a
union for connection with a boiler.

[Illustration: FIG. 77.---Large steam top and base.]

The copper tubing must be well annealed, so as to bend quite easily. Bevel
off one end, and solder this to the plate. Bend a couple of inches to the
curve of the plate, clamp it in position, and solder; and so on until the
circle is completed, bringing the tube snugly against the bevelled end. A
hole should now be drilled through the tube into this end--so that steam
may enter the ring in both directions-and plugged externally.

By preference, the ring should be below the plate, as this gives a greater
thickness of metal for drilling, and also makes it easy to jacket the tube
by sinking the plate into a wooden disc of somewhat greater diameter.

Under 50 lbs. of steam, a top of this kind attains a tremendous velocity.
Also, it flings the condensed steam about so indiscriminately that a ring
of zinc 3 inches high and 18 inches in diameter should be made wherewith to
surround it while it is running.

If a little bowl with edges turned over be accurately centred on the wheel,
a demonstration of the effects of centrifugal force may be made with water,
quicksilver, or shot, which fly up into the rim and disappear as the top
attains high speed, and come into sight again when its velocity decreases
to a certain figure. A perforated metal globe threaded on the spindle gives
the familiar humming sound.

A spectrum disc of the seven primary colours--violet, indigo, blue,
green, yellow, orange, red--revolved by the top, will appear more or less
white, the purity of which depends on the accuracy of the tints used.

XVIII. MODEL BOILERS.

A chapter devoted to the construction of model boilers may well open with a
few cautionary words, as the dangers connected with steam-raisers are very
real; and though model-boiler explosions are fortunately rare, if they do
occur they may be extremely disastrous.

Therefore the following warnings:--

(1.) Do not use tins or thin sheet iron for boilers. One cannot tell how
far internal corrosion has gone. The scaling of 1/100 inch of metal off a
"tin" is obviously vastly more serious than the same diminution in the
thickness of, say, a 1/4-inch plate. Brass and copper are the metals to
employ, as they do not deteriorate at all provided a proper water supply be
maintained.

(2.) If in doubt, make the boiler much more solid than is needed, rather
than run any ri