▁some ▁of ▁whose ▁instruments ▁they ▁used . ▁There ▁was ▁keen ▁interest ▁in ▁their ▁work ▁and ▁much ▁communication ▁between ▁the ▁survey ors , ▁M ask ely ne ▁and ▁other ▁members ▁of ▁the ▁British ▁Scient ific ▁establishment ▁in ▁the ▁Royal ▁Society ▁in ▁Britain , ▁not ably ▁Henry ▁Cav end ish . ▁During ▁such ▁survey ▁work , ▁it ▁is ▁normal ▁to ▁survey ▁from ▁point ▁to ▁point ▁along ▁the ▁line ▁and ▁then ▁survey ▁back ▁to ▁the ▁starting ▁point , ▁where ▁if ▁there ▁were ▁no ▁errors ▁the ▁origin ▁and ▁re - sur vey ed ▁position ▁would ▁co inci de . ▁Norm ally ▁the ▁return ▁errors ▁would ▁be ▁random ▁– ▁i . e . ▁the ▁return ▁survey ▁errors ▁compared ▁to ▁the ▁intermediate ▁points ▁back ▁to ▁the ▁start ▁point ▁would ▁be ▁spatial ly ▁randomly ▁distributed ▁around ▁the ▁start ▁point . ▁Mason ▁and ▁D ixon ▁found ▁that ▁there ▁were ▁larger ▁than ▁expected ▁system atic ▁errors , ▁i . e . ▁non - random ▁errors , ▁that ▁led ▁the ▁return ▁survey ▁consist ently ▁being ▁in ▁one ▁direction ▁away ▁from ▁the ▁starting ▁point . ▁When ▁this ▁information ▁got ▁back ▁to ▁the ▁Royal ▁Society ▁members , ▁Henry ▁Cav end ish ▁real ised ▁that ▁this ▁may ▁have ▁been ▁due ▁to ▁the ▁gravit ational ▁pull ▁of ▁the ▁Al leg hen y ▁Mountains ▁def lect ing ▁the ▁the od ol ite ▁pl umb - b obs ▁and ▁spirit ▁levels . ▁M ask ely ne ▁then ▁proposed ▁meas uring ▁the ▁gravit ational ▁force ▁causing ▁this ▁def lection ▁induced ▁by ▁the ▁pull ▁of ▁a ▁nearby ▁mountain ▁upon ▁a ▁pl