This apparatus, described by Jean-Antoine Nollet in Leçons de physique expérimentale (Paris, 1743-1748), allows observations of phenomena connected with the fall of bodies. Two vertical supports are fastened to a wooden base, open at the top, which carries a sliding tray. A beam with two holes slides between the supports. Two clips (one of them now split) are fastened opposite the holes. Two balls of equal diameter but of different weight are fastened to the clips and dropped simultaneously into the tray, filled beforehand with soft earth. One can also drop balls of equal weight from different heights. The force of the balls' impact can be measured by their penetration into the earth. This arrangement allowed experiments to determine the degree to which the impact of different free-falling bodies depended on their velocity, volume, or weight.

The instrument comes with a second profiled board that can be inserted in the supports instead of the first. The second board has an opening above which a moving hammer is attached. The hammer's angle of fall is adjusted by means of a toothed brass arc. Two balls are placed in the opening and held together with wax. When the hammer is released, it strikes only one of the balls. They begin to fall simultaneously (one under the combined effect of the impact and of natural motion, the other under the effect of natural motion alone), but with different velocities. They thus reach the ground in different times. Provenance: Lorraine collections.