ong current
that has circulated round the primary circuit of a Ruhmkorff's induction
coil; this produces at the surface of the mercury a bright
self-induction spark in the neighbourhood of the splash, and it is by
this flash that the splash is viewed. The illumination is greatly helped
by surrounding the place where the splash and flash are produced by a
white cardboard enclosure, seen in Fig. 2, from whose walls the light is
diffused.

[Illustration: FIG. 2.]

It will be observed that the time at which the spark is made will depend
upon the distance that the sphere has to fall before striking the plate
D, for the subsequent action of demagnetizing F and pulling the wire L
out of the mercury in the cup H is the same on each occasion. The modus
operandi is consequently as follows:--The observer, sitting in
comparative but by no means complete darkness, faces the apparatus as it
appears in Fig. 2, presses down the ends A'B' of the levers first
described, so that they are held by the electro-magnet C (Fig. 1); then
he presses the lever NP down on the electro-magnet F, sets the timing
sphere and drop in place, and then by means of a bridge between two
mercury cups, short-circuits and thus cuts off the current of the
electro-magnet C. This lets off drop and sphere, and produces the flash.
The stage of the phenomenon that is thus revealed having been
sufficiently studied by repetition of the experiment as often as may be
necessary, he lowers the plate D a fraction of an inch and thus obtains
a later stage. Not only is any desired stage of the phenomenon thus
easily brought under examination, but the apparatus also affords the
means of measuring the time interval between any two stages. All that
is necessary is to know the distance that the timing sphere falls in the
two cases. Elementary dynamics then give us the interval required. Thus,
if the sphere falls one foot and we then lower D 1/4 inch, the interval
between the corresponding stages will be about .0026 second.

Having thus described the apparatus, which I hope shortly to show you in
action, I pass to the information that has been obtained by it.

This is contained in a long series of drawings, of which a selection
will be presented on the screen. The First Series that I have to show
represents the splash of a drop of mercury 0.15 inch in diameter that
has fallen 3 inches on to a smooth glass plate. It will be noticed that
very soon after the first moment of impact