At the level of the water surface A (Fig. 1), the upward pressure in the short arm of the siphon is the atmospheric pressure, while the downward pressure is the weight of the water column AB. The upward pressure at the point D is also the atmospheric pressure, and the downward pressure is the weight of the water column DC. The resulting upward pressure at any point will be the atmospheric pressure minus the pressure of the water column at that point, and as the column CD is longer than the column AB, the resulting upward pressure at A is greater than at D, and the water is forced from A toward D.
It is evident that unless the pressure due to the liquid column AB is less than the pressure of the atmosphere, there will be no flow of the liquid. Hence a siphon cannot raise water more than 34 ft., nor mercury more than 30 in., when the barometer stands at the normal height.
The final resultant of the pressures depends upon the difference in the heights of the liquid columns; hence the greater the difference in height, the faster will be the flow.
It is evident that the size of the tube used as a siphon does not affect the height to which a liquid can be raised, since both the downward pressure of the liquid column due to its weight and the total upward pressure of the atmosphere at the end of the tube are directly proportional to its area of cross section .
