This experimental apparatus illustrates several properties of the pendulum and of falling bodies—properties discovered by Galileo and described in his writings.

A wooden ring, fixed to a large triangular base, carries a vertical bar representing the diameter of the circle. On the bar is inserted a brass peg to which we can attach a pendulum. This consists of a plumb line whose length is equal to the radius of the circle. When we make the pendulum oscillate, we observe that it swings back to the same height as its starting point. There are holes along the vertical bar for inserting pegs. When the oscillating pendulum encounters the pegs, the length of the oscillation is shortened. Despite this interruption, however, the pendulum always swings back to its initial height.

The device is also fitted with two small brass tracks on the wooden ring. Thanks to a set of pins and clamps, the tracks can be adjusted to represent any two chords of the circle. When we drop two identical balls down the tracks, we can observe that their travel time is always the same. This time is equal to the time taken by the ball to descend the vertical diameter.

Let us drop another ball from the top of the bar. To determine the ball's downward acceleration, we use a vertical rod four times as long as the circle diameter. We drop the ball from the top of the rod at the same time as we swing the pendulum. We observe that, in the time taken by the pendulum to complete a half-oscillation, the falling ball covers a distance equal to the circle diameter. The symmetry between the two movements, however, changes during the second half-oscillation of the pendulum. The pendulum swings back to its initial height in a period of time equal to the first half-oscillation. But the ball, because of its acceleration, travels three times the previous distance. The apparatus thus provides an experimental confirmation of the Galilean law of uniformly accelerated motion, which states that the distances traveled in equal times from rest increase in the same sequence as odd numbers from 1 to 3 to 5 and so on.