ij ing ▁via ▁Heb ei ▁in ▁the ▁middle ▁of ▁May , ▁after ▁being ▁held ▁up ▁for ▁two ▁months ▁at ▁Kal gan ▁( mod ern ▁Z hang j ia kou ). ▁Here ▁he ▁was ▁able ▁to ▁communicate ▁in ▁Latin ▁with ▁the ▁Jes uit ▁Ferdinand ▁Ver bi est . ▁His ▁diplom acy ▁proved ▁un success ful , ▁and ▁he ▁returned ▁to ▁Si ber ia ▁by ▁the ▁same ▁route ▁in ▁Spring ▁ 1 6 7 7 . ▁ ▁Ach iev ements ▁ ▁Mil es cu ▁is ▁the ▁author ▁of ▁one ▁of ▁the ▁first ▁Russian ▁works ▁on ▁arithmetic , ▁" Ar ith m olog ion ", ▁which ▁was ▁written ▁in ▁ 1 6 7 2 , ▁based ▁on ▁his ▁own ▁Greek ▁original . ▁The ▁manuscript ▁was ▁preserved ▁in ▁the ▁Ch ud ov ▁Mon aster y , ▁till ▁it ▁was ▁discovered ▁by ▁church ▁historian ▁Nikol ay ▁K ed rov . ▁ ▁In ▁his ▁road ▁journal ▁– ▁later ▁published ▁under ▁the ▁title ▁Tra vel s ▁through ▁Si ber ia ▁to ▁the ▁Chinese ▁borders , ▁Mil es cu ▁correctly ▁described ▁the ▁middle ▁course ▁of ▁the ▁Ob , ▁Ir ty sh , ▁and ▁Ang ara . ▁He ▁assumed ▁the ▁Ob ▁to ▁have ▁its ▁source ▁in ▁Lake ▁Te let sko ye ▁in ▁the ▁Al ta i ▁Mountains . ▁He ▁was ▁also ▁the ▁first ▁person ▁to ▁describe ▁Lake ▁Ba ikal ▁and ▁all ▁the ▁rivers ▁feed ing ▁the ▁lake , ▁and ▁the ▁first ▁to ▁point ▁out ▁Ba ikal ' s ▁unf ath om able ▁depth . ▁ ▁On ▁his ▁way ▁through ▁Si ber ia