[Illustration: FIG. 12.--APPARATUS FOR DEMONSTRATING THE PRINCIPLE OF
THE GRAMME MACHINE.]

The apparatus is arranged in such a way that indications may thus be
obtained upon the drum by means of induction sparks jumping between
the style and the surface of the cylinder. To the left of the figure
is seen the apparatus constructed by Lieutenant Ziegler for
experimenting on the duration of combustion of bomb fuses.

[Illustration: FIG. 13.--VAN RYSSELBERGHE'S REGISTERING
THERMOMETROGRAPH.]

Shortly after the drum has commenced revolving, the contact, K, opens
a current which supports the heavy armature, P, of an electro-magnet,
M. This weight, P, falls upon the rod, d, and inflames the fuse, Z, at
that very instant. At this precise moment the electro-magnet, H,
inscribes a point, and renews it only when the cartridge at the
extremity of the fuse explodes.

[Illustration: FIG. 14.--VAN RYSSELBERGHE'S REGISTERING
THERMOMETROGRAPH.]

This apparatus perhaps offers the inconvenience that the drum must be
revolved by hand, and it would certainly be more convenient could it
be put in movement at different velocities by means of a clockwork
movement that would merely have to be thrown into gear at the desired
moment. As it is, however, it presents valuable qualities, and,
although it has already been employed in Germany for some time, it
will be called upon to render still more extensive services.