In a U-tube manometer, the height of one leg of fluid changed by altering the density of some of the fluid in the other leg. In a well with drillpipe in the hole, the string of drillpipe is one leg and the annulus between the drillpipe and the wellbore is the other. If a denser mud goes into the drillpipe, mud flows up the annulus, and vice versa. The practice of putting a dense slugging pill in the drillpipe in order to pull a dry string makes use of the U-tube effect.