op with India-rubber Surface._

A sheet of thin india-rubber, about the thickness of that used in
air-balls, as it appears _before_ they have been blown out, must be
stretched over a ring of wood or metal eighteen inches in diameter, and
securely wired round the edge. The wire will hold the india-rubber
better if the edge is grooved. This does not succeed if tried on a
smaller scale. This experiment was shown by Sir W. Thomson at the Royal
Institution.

_Jumping Frame._

This is easily made by taking a light glass globe about two inches in
diameter, such, for instance, as a silvered ball used to ornament a
Christmas-tree or the bulb of a pipette, which is what I used. Pass
through the open necks of the bulb a piece of wire about one-twentieth
of an inch in diameter, and fix it permanently and water-tight upon the
wire by working into the necks melted sealing-wax. An inch or two above
the globe, fasten a flat frame of thin wire by soldering, or if this is
too difficult, by tying and sealing-wax. A lump of lead must then be
fastened or hung on to the lower end, and gradually scraped away until
the wire frame will just be unable to force its way through the surface
of the water. None of the dimensions or materials mentioned are of
importance.

_Paraffined Sieve._

Obtain a piece of copper wire gauze with about twenty wires to the inch,
and cut out from it a round piece about eight inches in diameter. Lay it
on a round block, of such a size that it projects about one inch all
round. Then gently go round and round with the hands pressing the edge
down and keeping it flat above, until the sides are evenly turned down
all round. This is quite easy, because the wires can allow of the kind
of distortion necessary. Then wind round the turned-up edge a few turns
of thick wire to make the sides stiff. This ought to be soldered in
position, but probably careful wiring will be good enough.

Melt some paraffin wax or one or two paraffin candles of the best
quality in a clean flat dish, not over the fire, which would be
dangerous, but on a hot plate. When melted and clear like water, dip the
sieve in, and when all is hot quickly take it out and knock it once or
twice on the table to shake the paraffin out of the holes. Leave upside
down until cold, and then be careful not to scratch or rub off the
paraffin. This had best be done in a place where a mess is of no
consequence.

There is no difficulty in filling it or in setti