Good Math, Bad Math : Manual Calculation: Using a Slide Rule (part

Back when Men were Men, they used to get excellent approximations for mathematical problems by manipulating sliding bits of wood or plastic with numbers written on....

(Hmmm, somehow that didn't come out quite the way I intended... ;->)

http://scienceblogs.com/goodmath/2006/09/manual_calculation_using_a_sli_1.php