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The adjacent figure shows the surface of a liquid in contact with air.
Liquid and air are in a static equilibrium.
Apart from motion at microscopic distances,
at macroscopic distances everything is in perfect rest.
The dark rectangle represents an imaginary box below the surface
of the liquid with a horizontal upper surface of area
S
and an equally large horizontal bottom surface.
The bottom surface of the imaginary box is at height
y in some system of coordinates,
whereas the top surface of the imaginary box is at height
y+Δy.
Hence, the imaginary box is Δy high
and encloses a volume
V=S×Δy
of liquid.
The pressure, which is due to the movement of the molecules,
at the bottom of the box is given by
P(y),
at the top by P(y+Δy).
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