With this instrument, we can observe the trajectory of a liquid flowing out of a nozzle at different angles.

The vertical glass tube is filled with mercury. The lower part of the tube is fitted with a faucet, whose angle can be set from 0 to 90 degrees. When the faucet is opened, the mercury spurts out, describing a parabolic trajectory that stands out in bright contrast on the vertical wall of the apparatus. The opening of the parabolas described is a function of the angle of the jet. The maximum range of the liquid is obtained by setting the nozzle at 45 degrees.

In his description of the instrument, the French abbot Jean-Antoine Nollet suggested the possibility of replacing mercury with water. The problem with mercury was that, while it produced a more spectacular effect, it was very expensive and could also corrode the faucet. Nollet stressed that the parabolic trajectory of liquids and that of projectiles obeyed the same physical principles. Indeed, the apparatus was used to demonstrate the Galilean discovery of the parabolic trajectory of projectiles.