Siphon

Siphon is a bent tube used to remove liquids from vessels when it is inconvenient to disturb the vessel. One end of the siphon dips into the liquid; the other end is outside the vessel and is lower than the surface of the liquid inside. The tube may first be filled with the liquid, and the ends C and A closed, while it is inverted and placed in the position illustrated in the diagram. On opening the ends, liquid will flow from A as long as C is immersed. As the liquid issues from A, it tends to form a vacuum at B, but the pressure of the atmosphere at the surface N of the liquid forces more fluid up the tube, and so the flow goes on. If M N be the level of the liquid, there is at A the pressure of the atmosphere, together with the head of liquid M A, tending to drive the liquid out, while only the atmospheric pressure is tending to prevent its fall. The difference between the two is the head of liquid M A, and the liquid issues at that pressure. The height of B above N must not be greater than the height of a column of liquid, which is just supported by the pressure of the air, or the tube B A will simply empty itself. If the liquid be mercury, this height must not exceed about 30 inches, and if water the limit is abont 33 feet.