X1.1 Introduction—A schematic of a mercury constant
volume-falling head hydraulic system is given in Fig. 4. In this
figure, the falling head is applied by the difference in elevation
between the mercury levels in the tailwater and headwater
tubes of the mercury U-tube manometer. In designing this type
of hydraulic system, the area of the tailwater tube (aout) is made
significantly smaller than that of the headwater tube (ain). This
is done for three reasons: