If now a cubic centimeter of water be poured upon _AB_ it will raise the level 1 cm., or the head of water exerting pressure upon _CD_ becomes 11 cm., or the total force in _CD_ is 16×11 g., _i.e._, each square centimeter of _CD_ receives an additional force of 1 g. _Hence the force exerted on a unit area at_ _AB_ _is transmitted to every unit area within the vessel._ The usual form in which this law is expressed is as follows: _Pressure applied to any part of a confined liquid is transmitted unchanged, in all directions, and adds the same force to all equal surfaces in contact with the liquid_. [Illustration: FIG. 19.--The force is proportional to the area.] The importance of this principle, as Pascal himself pointed out, lies in the fact that by its aid we are able to exert a great force upon a large area by applying a small force upon a small area of a confined liquid, both areas being in contact with the same liquid. Thus in Fig. 19 if the area of the surface _CD_ is 2000 times the area of the surface _AB_, then 1 lb. applied to the liquid on _AB_ will exert or sustain a force of 2000 lbs. on _CD_. =41. Hydraulic Press.=--An important application of Pascal's principle is the _hydraulic press_. See Fig. 20. It is used for many purposes where great force is required, as in pressing paper or cloth, extracting oil from seeds, lifting heavy objects, etc. Many high school pupils have been seated in a _hydraulic chair_ used by a dentist or barber. This chair is a modified hydraulic press. [Illustration: FIG. 20.--Cross-section of a hydraulic press.] The hydraulic press contains two movable pistons, _P_ and _p_ (see Fig. 20). The larger of these, _P_, has a cross-sectional area that may be 100 or 1000 times that of the smaller. The smaller one is moved up and down by a lever; on each upstroke, liquid is drawn in from a reservoir, while each down-stroke forces some of the liquid into the space about the large piston. Valves at _V_ and _V´_ prevent the return of the liquid. If the area of _P_ is 1,000 times that of _p_, then the force exerted by _P_ is 1000 times the force employed in moving _p_. On the other hand, since the liquid moved by the small piston is distributed over the area of the large one, the latter will move only 1/1000 as far as does the small piston. The relation between the motions of the two pistons and the forces exerted by them may be stated concisely as follows: The _motions_ of the two _pistons_ of the hydraulic press are inversely proportional to the forces exerted by them. The _cross-sectional areas_ of the two pistons are, on the other hand, directly proportional to the forces exerted by them.