What does it mean that the glass tube can slide stably under pressure?

Does the subject want to ask, "What is the principle that the glass tube can slide smoothly under pressure?" Stress. With the increase of atmospheric pressure, the mercury column in the glass tube keeps decreasing, which means that the atmospheric pressure increases with the increase of altitude, and the pressure also increases. Mercury column drops under the action of external atmospheric pressure, which decreases with the elevation and increases with the elevation.