▁hum our ▁value . ▁ ▁These ▁units ▁may ▁or ▁may ▁not ▁have ▁precise ▁object ively ▁meas urable ▁values , ▁but ▁all ▁of ▁them ▁measure ▁quantities ▁that ▁have ▁been ▁defined ▁within ▁the ▁International ▁System ▁of ▁Units . ▁ ▁Systems ▁ ▁F FF ▁units ▁ ▁Most ▁countries ▁use ▁the ▁International ▁System ▁of ▁Units ▁( SI ). ▁In ▁contrast , ▁the ▁f url ong / f irk in / fort night ▁system ▁of ▁units ▁of ▁measurement ▁draw s ▁attention ▁by ▁being ▁extremely ▁old ▁fashion ed ▁and ▁off - be at ▁at ▁the ▁same ▁time . ▁ ▁One ▁f url ong ▁per ▁fort night ▁is ▁very ▁nearly ▁ 1 ▁cent imet re ▁per ▁minute ▁( to ▁within ▁ 1 ▁part ▁in ▁ 4 0 0 ). ▁Indeed , ▁if ▁the ▁in ch ▁were ▁defined ▁as ▁ 2 . ▁cm ▁rather ▁than ▁ 2 . 5 4   cm ▁exactly , ▁it ▁would ▁be ▁ 1   cm / min . ▁Besides ▁having ▁the ▁meaning ▁of ▁" any ▁obsc ure ▁unit ", ▁f url ongs ▁per ▁fort night ▁have ▁also ▁served ▁frequently ▁in ▁the ▁class room ▁as ▁an ▁example ▁on ▁how ▁to ▁reduce ▁a ▁unit ' s ▁fraction . ▁The ▁speed ▁of ▁light ▁may ▁be ▁expressed ▁as ▁being ▁roughly ▁ 1 . 8 ▁t era f url ongs ▁per ▁fort night ▁( or ▁meg af url ongs ▁per ▁micro fort night ). ▁ ▁Great ▁Under ground ▁Empire ▁( Z ork ) ▁In ▁the ▁Z ork ▁series ▁of ▁games , ▁the ▁Great ▁Under ground ▁Empire ▁had ▁its ▁own ▁system