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 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.