We consider a volume of air trapped over a glycerol column in a eudiometer. We demonstrate that there is an approximately linear relationship between the volume of trapped air and the height of the glycerol column. Simply by moving the eudiometer up and down, we cause the glycerol-column height and trapped-air volume to vary. The plot of volume versus height has a slope that depends on atmospheric pressure. We show how to determine atmospheric pressure from the plot of trapped-air volume versus glycerol-column height. This exercise allows students to use a familiar instrument in an unfamiliar way, while improving their understanding of pressure measurements and Boyle's law. (Contains 2 figures.)