Figure 2-1.—Device for determining the arrangement of theforce, pressure, and area formula.Figure 2-1 illustrates a memory device forrecalling the different variations of this formula.Any letter in the triangle may be expressed as theproduct or quotient of the other two, dependingon its position within the triangle.For example, to find area, consider the letterA as being set off to itself, followed by an equalsign. Now look at the other two letters. The letterF is above the letter P; therefore,NOTE: Sometimes the area may not beexpressed in square units. If the surface isrectangular, you can determine its area bymultiplying its length (say, in inches) by its width(also in inches). The majority of areas you willconsider in these calculations are circular in shape.Either the radius or the diameter may be given,but you must know the radius in inches to findthe area. The radius is one-half the diameter. Todetermine the area, use the formula for findingthe area of a circle. This is written A = whereA is the area, is 3.1416 (3.14 or 3 1/7 for mostcalculations), and r^{2 }indicates the radius squared.Atmospheric PressureThe atmosphere is the entire mass of air thatsurrounds the earth. While it extends upward forabout 500 miles, the section of primary interestis the portion that rests on the earth’s surface andextends upward for about 7 1/2 miles. This layeris called the troposphere.If a column of air 1-inch square extending allthe way to the “top” of the atmosphere couldbe weighed, this column of air would weighapproximately 14.7 pounds at sea level. Thus,atmospheric pressure at sea level is approximately14.7 psi.As one ascends, the atmospheric pressuredecreases by approximately 1.0 psi for every 2,343feet. However, below sea level, in excavations anddepressions,atmospheric pressure increases.Pressures under water differ from those under aironly because the weight of the water must beadded to the pressure of the air.Atmospheric pressure can be measured by anyof several methods. The common laboratorymethod uses the mercury column barometer. Theheight of the mercury column serves as anindicator of atmospheric pressure. At sea level andat a temperature of 0° Celsius (C), the height ofthe mercury column is approximately 30 inches,or 76 centimeters. This represents a pressure ofapproximately 14.7 psi. The 30-inch column isused as a reference standard.Another device used to measure atmosphericpressure is the aneroid barometer. The aneroidbarometer uses the change in shape of anevacuated metal cell to measure variations inatmospheric pressure (fig. 2-2). The thin metal ofthe aneroid cell moves in or out with the variationof pressure on its external surface. This movementis transmitted through a system of levers to apointer, which indicates the pressure.The atmospheric pressure does not varyuniformly with altitude. It changes more rapidlyat lower altitudes because of the compressibilityof the air, which causes the air layers close to theearth’s surface to be compressed by the air massesabove them. This effect, however, is partiallycounteracted by the contraction of the upperFigure 2-2.—Simple diagram of the aneroid barometer.2-2