Sol. It is evident that on a level with the lower end of the tube d, which passes through the neck of the vessel, that is, on a level with opening b, the pressure of the liquid is equal to that of the atmosphere. Therefore, when opening a is uncorked, the pressure outside will be greater than that inside and bubbles of air will enter the vessel, the level of the water in the vessel will fall and it will enter the tube d. When the level of the water in the tube reaches the level of opening a, the air will cease to enter the vessel. If opening b be uncorked instead of a, equilibrium will be maintained as a result of the equality of external and internal pressures, i.e. air will not enter the vessel, nor will water pour out of it, If opening c be unstopped, then the water will pour out of the vessel and air will enter through tube d. However, the pressure at the lower end of the tube will remain equal to that of the atmosphere, regardless of changes in the level of water in the vessel, and water will pour out at a constant velocity until the level of the water falls to the level of the lower end of tube d. After that, the water will continue to pour out, but the speed of flow will decrease.