It sounds like you're describing a specific setup used in experiments to compare the densities of liquids. The apparatus you're referring to likely consists of a fine polished wooden stand with a meter rule attached vertically. The meter rule has markings from 0 at the bottom to 45 cm at the top.

To compare the densities of liquids using this setup, two columns of liquid are produced by applying suction to the short limb of the apparatus. By measuring the heights of these two columns of liquid on the meter rule, you can compare their densities.