(Manometers(pressure Measurement

That is, keep adding on pressure increments as you move down through the layered fluid. A different application is a manometer, which involves both “up” and “down”calculations.
Figure 2.9 shows a simple open manometer for measuring pA in a closed chamber
relative to atmospheric pressure pa, in other words, measuring the gage pressure. The chamber fluid ?1 is combined with a second fluid ?2, perhaps for two reasons: (1) to protect the environment from a corrosive chamber fluid or (2) because a heavier fluid ?2 will keep z2 small and the open tube can be shorter. One can, of course, apply the basic hydrostatic formula (2.20). Or, more simply, one can begin at A, apply Eq. (2.32) “down” to z1, jump across fluid 2 (see Fig. 2.9) to the same pressure p1, and then use Eq. (2.32) “up” to level z2:
pA +?1?zA - z1???2?z1 - z2??= p2 ? patm (2.33)
The physical reason that we can “jump across” at section 1 in that a continuous length of the same fluid connects these two equal elevations. The hydrostatic relation (2.20) requires this equality as a form of Pascal’s law:
Any two points at the same elevation in a continuous mass of the same static fluid
will be at the same pressure.