a circular cross section when it is filled with water under  pressure.  The  outward  push  of  the  water is  equal  in  every  direction. So   far   we   have   explained   the   effects   of atmospheric  pressure  on  liquids  and  how  external forces are distributed through liquids. Let us now focus  our  attention  on  forces  generated  by  the weight of liquids themselves. To do this, we must first discuss density, specific gravity, and Pascal’s law. Density and Specific Gravity The density of a substance is its weight per unit volume.  The  unit  volume  in  the  English  system of  measurement  is  1  cubic  foot.  In  the  metric system it is the cubic centimeter; therefore, density is expressed in pounds per cubic foot or in grams per  cubic  centimeter. To find the density of a substance, you must know its weight and volume. You then divide its weight by its volume to find the weight per unit volume.  In  equation  form,  this  is  written  as Equation  2-4. EXAMPLE: The liquid that fills a certain container   weighs   1,497.6   pounds.   The container  is  4  feet  long,  3  feet  wide,  and 2  feet  deep.  Its  volume  is  24  cubic  feet (4 ft x 3 ft x 2 ft). If 24 cubic feet of this liquid weighs 1,497.6 pounds, then 1 cubic foot  weighs or  62.4  pounds.  Therefore,  the  density  of the  liquid  is  62.4  pounds  per  cubic  foot. This  is  the  density  of  water  at  4°C  and  is usually  used  as  the  standard  for  comparing densities of other substances. The temperature of 4°C was selected because water has its maximum density  at  this  temperature.  In  the  metric  system, the   density   of   water   is   1   gram   per   cubic centimeter.  The  standard  temperature  of  4°C  is used  whenever  the  density  of  liquids  and  solids is  measured.  Changes  in  temperature  will  not change the weight of a substance but will change the  volume  of  the  substance  by  expansion  or contraction,  thus  changing  the  weight  per  unit volume. In physics, the word specific  implies  a  ratio. Weight  is  the  measure  of  the  earth’s  attraction  for a body. The earth’s attraction for a body is called gravity.  Thus,  the  ratio  of  the  weight  of  a  unit volume  of  some  substance  to  the  weight  of  an equal volume of a standard substance, measured under  standard  pressure  and  temperature  con- ditions,  is  called  specific  gravity.  The  terms specific weight and specific density are sometimes used to express this ratio. The  following  formulas  are  used  to  find  the specific gravity (sp gr) of solids and liquids, with water used as the standard substance. or, The same formulas are used to find the specific gravity  of  gases  by  substituting  air,  oxygen,  or hydrogen  for  water. If  a  cubic  foot  of  a  certain  liquid  weighs  68.64 pounds,  then  its  specific  gravity  is  1.1, Thus, the specific gravity of the liquid is the ratio of its density to the density of water. If the specific gravity of a liquid or solid is known, the density of the liquid or solid maybe obtained by multiplying its specific gravity by the density of water. For example, if a certain hydraulic liquid has  a  specific  gravity  of  0.8,  1  cubic  foot  of  the liquid  weighs  0.8  times  as  much  as  a  cubic  foot of water—0.8 times 62.4, or 49.92 pounds. In the metric system, 1 cubic centimeter of a substance with a specific gravity of 0.8 weighs 1 times 0.8, or 0.8 grams. (Note that in the metric system the specific gravity of a liquid or solid has the same numerical  value  as  its  density,  because  water weighs  1  gram  per  cubic  centimeter.) Specific gravity and density are independent of the size of the sample under consideration and depend  only  on  the  substance  of  which  it  is  made. A   device   called   a   hydrometer   is   used   for measuring  the  specific  gravity  of  liquids. 2-4