r six feet away from the spark,
and the photographic plate should be held as close to the stream of
water as is possible without touching. The shadow is then so definite
that the photograph, when taken, may be examined with a powerful lens,
and will still appear sharp. Any rapid dry plate will do. The room, of
course, must be quite dark when the plate is placed in position, and the
spark then made. The regular breaking up of the jet may be effected by
sound produced in almost any way. The straight jet, of which Fig. 41 is
a representation, magnified about three and a quarter times, was
regularly broken up by simply whistling to it with a key. The fountains
were broken up regularly by fastening the nozzle to one end of a long
piece of wood clamped at the end to the stand of a tuning-fork, which
was kept sounding by electrical means. An ordinary tuning-fork, made to
rest when sounding against the wooden support of the nozzle, will
answer quite as well, but is not quite so convenient. The jet will break
up best to certain notes, but it may be tuned to a great extent by
altering the size of the orifice or the pressure of the water, or both.

_Fountain and Sealing-wax._

It is almost impossible to fail over this very striking yet simple
experiment. A fountain of almost any size, at any rate between
one-fiftieth and a quarter of an inch in the smooth part, and up to
eight feet high, will cease to scatter when the sealing-wax is rubbed
with flannel and held a few feet away. A suitable size of fountain is
one about four feet high, coming from an orifice anywhere near
one-sixteenth of an inch in diameter. The nozzle should be inclined so
that the water falls slightly on one side. The sealing-wax may be
electrified by being rubbed on the coat-sleeve, or on a piece of fur or
flannel which is _dry_. It will then make little pieces of paper or cork
dance, but it will still act on the fountain when it has ceased to
produce any visible effect on pieces of paper, or even on a delicate
gold-leaf electroscope.

_Bouncing Water-jets._

This beautiful experiment of Lord Rayleigh's requires a little
management to make it work in a satisfactory manner. Take a piece of
quill-glass tube and draw it out to a very slight extent (see a former
note), so as to make a neck about one-eighth of an inch in diameter at
the narrowest part. Break the tube just at this place, after first
nicking it there with a file. Connect each of these tubes by