Aglass tube 100 cm long and 3.8cm in diameter is mounted in a ‘Y’ on a metal base. Arod 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. The ‘Y’ supporting the tube may be adjusted vertically and laterally until the metal disc will not touch the glass. Coiled spring attached to the ‘Y’ holds the glass tube in position.