THE TEACHING OF MECHANICS BY EXPEKIMENT. 259 
deflecting now automatically removes itself. The beam swings, and 
by observing the angle of deflection in the first swing the student can 
determine what the velocity of the body must have have been when it 
fell into the bucket. 
Fig. 5 shows an apparatus which allows the moment of inertia of 
rods and other bodies to be measured experimentally. It consists of 
a brass cylindrical disc hung by three wires,—a tri-filar suspension. 
Its own moment of inertia is calculated from its measured weight and 
dimensions. It is caused to swing in azimuth, and its period is ob¬ 
served. Then the body whose moment of inertia is to be determined 
is placed upon it, and the period is again observed. The stability 
of the system is increased in proportion to the increase of weight. 
But knowing the weight of the added body as well as that of the disc, 
and knowing also the two periods of swing, with and without the add¬ 
ed body, we have all the necessary material for working out the mo¬ 
ment of inertia of the added body. 
Fig. 6 is an 
apparatus meant 
to illustrate the 
important sub¬ 
ject of balancing, 
especially in re¬ 
lation to a loco¬ 
motive or other 
engine. Here we 
have four discs 
which are set on 
a central axis and 
revolve with it. 
They are capable, 
however, of being 
shifted into any 
positions along 
the axis and also 
of being; turned 
round separately 
through any 
angle. Weights 
are attached to 
them near the 
periphery of each 
disc and the 
system is set in 
motion by means 
of an electric 
motor. The whole 
system, motor 
and all, is suspen- ^Apparatus for testing bdld:ic& of 
ded on a frame revilvinj miicMnzry. 
