The amount of this force may be computed as follows: First, the card stays on the end of the tube until the _weight_ of water from above equals the force of the water from below, and second, the card remains until the water is at the same _height_ inside the tube as it is outside. Now if we find the weight of water at a given depth in the tube, we can determine the force of the water from below. If for instance the chimney has an area of cross-section of 12 sq. cm. and is filled with water to a depth of 10 cm., the volume of the water contained will be 120 ccm. This volume of water will weigh 120 g. This represents then, not only the weight of the water in the tube, but also the force of the water against the bottom. In a similar way one may measure the force of water against any horizontal surface. =37. Force and Pressure.=--We should now distinguish between _force_ and _pressure_. Pressure refers to the force acting against _unit area_, while force refers to the action against the whole surface. Thus for example, the atmospheric _pressure_ is often given as 15 pounds to the square inch or as one kilogram to the square centimeter. On the other hand, the air may exert a _force_ of more than 300 pounds upon each side of the hand of a man; or a large ship may be supported by the _force_ of thousands of tons exerted by water against the bottom of the ship. In the illustration, given in Art. 36, the upward _force_ of the water against the end of the tube at a depth of 10 cm. is computed as 120 grams. The _pressure_ at the _same_ depth will be 10 grams per sq. cm. What will be the pressure at a depth of 20 cm.? at a depth of 50 cm.? of 100 cm.? Compare these answers with the law of liquid pressure in Art. 36. =38. Density.=--If other liquids, as alcohol, mercury, etc., were in the jar, the chimney would need filling to the same level outside, with the _same_ liquid, before the card would fall off. This brings in a factor that was not considered before, _that of the mass[B] of a cubic centimeter of the liquid_. This is called the _density_ of the liquid. Alcohol has a density of 0.8 g. per cubic centimeter, mercury of 13.6 g. per cubic centimeter, while water has a density of 1 g. per cubic centimeter. [B] The _mass_ of a body is the _amount of matter in it_, the _weight_ is the _pull of the earth upon it_. =39. Liquid Force against Any Surface.=--To find the force exerted by a liquid against a surface we must take into consideration the area of the surface, and the height and the =density= of the liquid above the surface. The following law, and the formula representing it, which concisely expresses the principle by which the force exerted by a liquid against any surface may be computed, should be memorized: _The force which a liquid exerts against any surface, equals the area of the surface, times its average depth below the surface of the liquid, times the weight of unit volume of the liquid._