end of the shaft. The anvils, the parts and attachments
of which are shown in the smaller objects lying on
the table at the base of the apparatus, consist of
a cylinder of steel partly immersed in a shallow brass
cup and made fast to it by means of a thumb-screw.
This cup carries a threaded bolt, by which it may be
attached to the main shaft at any position on its circumference
by screwing through a hole drilled in the collar.
The adjustment of the anvils about the shaft may be
changed in a moment by the simple movement of loosening
and tightening the thumb-screw constituted by the
anvil and its bolt. The device by which the extent
of the hammer-fall is controlled consists of cam-shaped
sheets of thin wood mounted within parallel grooves
on opposite sides of the loose collars and clamped
to the anvils by the resistance of two wedge-shaped
flanges of metal carried on the anvil bolt and acting
against the sides of slots cut into the sheets of
wood at opposite sides. The periphery of these
sheets of wood—as exhibited by that one
lying beside the loose anvils on the table—is
in the form of a spiral which unfolds in every case
from a point on the uniform level of the anvils, and
which, by variations in the grade of ascent, rises
in the course of a revolution about its center to
the different altitudes required for the fall of the
hammers. These heights were scaled in inches and
fractions, and the series employed in these experiments
was as follows: 1/8, 2/8, 3/8, 5/8, 7/8, 15/8,
24/8 inch. Upon a corresponding pair of standards,
seen at the left of the illustration, is mounted a
slender steel shaft bearing a series of sections of
brass tubing, on which, in rigid sockets, are carried
the shafts of a set of hammers corresponding in number
and position to the anvils of the main axis.
By means of a second shaft borne upon two connected
arms and pivoted at the summit of the standards the
whole group of hammers may at any moment be raised
from contact with the cams of the main shaft and the
series of sounds be brought to a close without interrupting
the action of the motor or of the remainder of the
apparatus. By this means phases of acceleration
and retardation in the series, due to initial increase
in velocity and its final decrease as the movement
ceases, are avoided. The pairs of vertical guides
which appear on this gearing-shaft and enclose the
handles of the several hammers are designed to prevent
injury to the insertions of the hammer shafts in their
sockets in case of accidental dislocations of the
heads in arranging the apparatus. This mechanism
was driven by an electrical motor with an interposed
reducing gear.
[Illustration: PSYCHOLOGICAL REVIEW. MONOGRAPH
SUPPLEMENT, 17. PLATE VIII.
Opposite
p. 314.]