esistance and the internal force of the tuning fork restraining its
movements.

Finally, the stylus, _s_, which touches the glass plate directly above
_y_, is for registering the moments when by the falling ball the sound
is produced and when the observer presses the key. This is brought
about by the rod, _i_, to which _s_ is firmly screwed, being jerked
upward a short distance at each of these instants, so that the
horizontal lines which the stylus, _s_, marks upon the screen passing
in front of it are broken at both places.

The mechanism which jerks the rod, _i_, upward is thus arranged: The
inclined plate, _p_, on which the ball drops, is carried by the upper
horizontal arm of an angular lever turning on the axis, _x_, and
counterpoised by the balancing weight, _x'_. By the falling ball this
arm is pressed downward, and the lower horizontal arm, _w_, of the
lever is also moved. On a second horizontal axis the lever, _v_,
partly concealed, moves, restricted as to its length of swing by the
screws, _n_. As long as the concealed arm is not moved, _v_ is lightly
pressed by the small spring, _e_, against _w_. The projection, _z_, at
the upper end of _v_ holds the rod, _i_, which the strong spring, _h_,
is continually pressing upward. When the ball falls upon the plate,
_p_, the arm, _w_, presses against the lower end of _v_, the
projection, _z_, sets free the rod, and it springs upward. This
movement is soon arrested, as the projection, _z'_, engages with a
stud situated on the right side of the rod, _i_. This projection is
situated on the vertical arm of an angular lever whose other arm is
the key, _t_. When the observer presses the key, the rod, _i_, again
is jerked upward by the spring, _h_. The screw, _o_, tapped into the
rod, _i_, prevents the rod going higher than necessary, by striking a
plate, which also serves as guide for _i_.

To determine the interval between the falling of the ball and pressing
of the key, one has finally to count the waves inscribed by the tuning
fork, which come under the portion of the line inscribed by _s_, which
is bounded by the two breaks produced by the successive movements of
the rod.

To make the glass plate carried by the frame available for more
observations, which plate can be used as a photographic negative, the
frame, T, is adjustable up and down upon the pillars, N. This frame
carries the tuning fork, mercury cup, _n_, and the electro-magnet, M.
The spring, _s_, can also