ib ▁tam ams ▁have ▁their ▁si xty ▁divisions ▁gathered ▁into ▁groups ▁of ▁five ▁that ▁are ▁number ed ▁in ▁both ▁directions ▁to ▁and ▁from ▁the ▁a pe x . ▁ ▁The ▁double ▁number ing ▁of ▁the ▁arc ▁means ▁that ▁the ▁“ J a ib s ” ▁and ▁“ J a ib ▁tam ams ” ▁labels ▁are ▁relative ▁to ▁the ▁measurement ▁being ▁taken ▁or ▁to ▁the ▁calculation ▁being ▁performed ▁at ▁the ▁time ▁and ▁the ▁terms ▁are ▁not ▁attached ▁to ▁one ▁or ▁the ▁other ▁of ▁the ▁graduated ▁scales ▁on ▁the ▁instrument . ▁ ▁Me as uring ▁a ▁cel est ial ▁alt itude ▁with ▁the ▁quadr ant ▁ ▁On ▁one ▁of ▁the ▁straight ▁edges ▁of ▁the ▁non ▁mar itime ▁quadr ant ▁( sol id ▁sheet ▁form ) ▁there ▁are ▁two ▁sight ing ▁pl ates ▁which ▁are ▁called ▁“ H ada f at ani ”. ▁ ▁Each ▁of ▁the ▁alignment ▁pl ates ▁having ▁a ▁small , ▁cent r ally ▁placed ▁a pert ure ▁or ▁" pin hole ", ▁the ▁two ▁a pert ures ▁( front ▁and ▁back ) ▁forming ▁the ▁optical ▁axis ▁through ▁which ▁one ▁s ights ▁an ▁incl ine ▁object ▁or ▁Sun . ▁ ▁The ▁light ▁ray s ▁from ▁the ▁Sun ▁passing ▁through ▁both ▁a pert ures , ▁the ▁spot ▁image ▁of ▁the ▁Sun ▁being ▁concent ric ▁with ▁the ▁center ▁of ▁the ▁rear ▁plate ▁pin hole , ▁imag ing ▁on ▁to ▁a ▁finger ▁( project ▁screen ) ▁if ▁desired ▁but ▁not ▁necessary ▁or ▁less ▁frequent ▁the ▁eye ▁at ▁night . ▁The ▁h anging ▁pl um ▁line ▁serving