Normal atmospheric pressure is 1 atm = 760 mm Hg. However, it is also 14.7 lbs/in.² (14.7 pounds per square inch). Given that liquid mercury has a density of 13.6 g/mL, explain (show a calculation) why 1 atm also equals 14.7 lbs/in.². Then, go back to the introduction of this chapter and calculate that the atmospheric force holding the Magdeburg spheres together is indeed close to the stated 4.5 tons (the surface area of a sphere is given by 4πr²).