A glass tube 100 cm long and 3.8cm in diameter is mounted in a 'Y' on a metal base. A rod about 60 cm long is held firmly at its mid point in a sturdy clamp at one end of the base. One end of the rod has a metal disc attached to it and projects into the glass tube. The other end of the tube is closed by a close fitting piston mounted on the base such that its position in the tube is adjustable through 20cm. app.