sācōma — Lewis & Short
sācōma, ătis, n., = sh/kwma,
I a counterpoise, a weight precisely balancing something in the opposite scale:
pendet ex altera parte aequo pondere phelli sacoma saburrale,Vitr. 9, 8, 8.—Hence, ad sacoma appendĕre, precisely, exactly, Vitr. 9, praef. § 9.