Sol. This would be impossible if there were air above the surface of the water since the pressure of the air would then have been equal in the first case but not equal in the second, and the water could not have remained on the same level. Therefore, the U-tube was evacuated before being soldered.

As to the pressure of water-vapour, part of the water-vapour in one limb is condensed when the tube is tilted, while a corresponding amount of water in the other limb turns to vapour, and the pressure of water-vapour above the level of the liquid remains the same in both limbs. Therefore, the level of the liquid in each limb remains at the same height.