entirely by mechanism, and, therefore, the graphic results attained by
it are free from all sources of error, which errors other methods
always introduce to a greater or less extent. Thus its results are
quite unexceptionable.

[Illustration: REACTION PERIOD OF HEARING.]

The apparatus shown in the cut rests on three feet, two of them
consisting of strong screws, so that by aid of the circular level,
_l_, on the base plate, it can be adjusted perfectly level. On a
little shelf attached to a square rod, seen on the left of the
instrument, rising from the base plate, and near its top, is a
horizontal tube, through which, by a bulb not shown in the cut, a
blast of air can be blown. In front of the other opening of the tube
is a horizontal fork of ebonite, whose arms carry on the side opposite
the tube a metallic ball. Through the arms of the fork pass the wires
of the circuit of an electric battery. These terminate in two rounded
ends, which, when the arms approach each other, are touched by the
metallic ball, so that the latter also closes the metallic circuit. By
the blast of air a wooden wedge contained in the tube is driven
between the arms of the fork, the ball falls from them, and the
electric stream is cut off. The ball drops upon the inclined metallic
plate, _p_, bounces off it, and is received in a little sack, S. When
the observer hears the ball strike the plate, he presses on the key,
_t_, and the interval between the two instants, namely, the falling of
the ball upon the plate and the pressing of the key, _t_, is what is
to be mechanically fixed and measured.

The electric current, which is closed by the ball as long as it lies
on the jaws of the fork, flows around the arms of the electro-magnet,