one part to another, and thus to make a drawing of what is seen. If
necessary the experiment must be capable of repetition, with an exactly
similar drop falling from exactly the same height, and illuminated at
exactly the same stage. Then, when this stage has been sufficiently
studied, we must be able to arrange with another similar drop to
illuminate it at a rather later stage, say 1/1000 second later, and in
this way to follow step by step the whole course of the phenomenon.

The apparatus by which this has been accomplished is on the table before
you. Time will not suffice to explain how it grew out of earlier
arrangements very different in appearance, but its action is very simple
and easy to follow by reference to the diagram (Fig. 1).

AA' is a light wooden rod rather longer and thicker than an ordinary
lead pencil, and pivoted on a horizontal axle O. The rod bears at the
end A a small deep watch-glass, or segment of a watch-glass, whose
surface has been smoked, so that a drop even of water will lie on it
without adhesion. The end A' carries a small strip of tinned iron, which
can be pressed against and held down by an electro-magnet CC'. When the
current of the electro-magnet is cut off the iron is released, and the
end A' of the rod is tossed up by the action of a piece of india-rubber
stretched catapult-wise across two pegs at E, and by this means the drop
resting on the watch-glass is left in mid-air free to fall from rest.

[Illustration: FIG. 1.]

BB' is a precisely similar rod worked in just the same way, but carrying
at B a small horizontal metal ring, on which an ivory timing sphere of
the size of a child's marble can be supported. On cutting off the
current of the electro-magnet the ends A' and B' of the two levers are
simultaneously tossed up by the catapults, and thus drop and sphere
begin to fall at the same moment. Before, however, the drop reaches the
surface on which it is to impinge, the timing sphere strikes a plate D
attached to one end of a third lever pivoted at Q, and thus breaks the
contact between a platinum wire bound to the underside of this lever and
another wire crossing the first at right angles. This action breaks an
electric current which has traversed a second electro-magnet F (Fig. 2),
and releases the iron armature N of the lever NP, pivoted at P, thus
enabling a strong spiral spring G to lift a stout brass wire L out of
mercury, and to break at the surface of the mercury a str