▁Han cock . ▁Each ▁weight ▁is ▁a ▁box ▁of ▁steel , ▁filled ▁with ▁lead , ▁ 1 7 ▁feet ▁square ▁by ▁ 3 ▁feet ▁high . ▁Each ▁weight ▁rest s ▁on ▁a ▁steel ▁plate . ▁The ▁plate ▁is ▁covered ▁with ▁lub ric ant ▁so ▁the ▁weight ▁is ▁free ▁to ▁slide . ▁But ▁the ▁weight ▁is ▁attached ▁to ▁the ▁steel ▁frame ▁of ▁the ▁building ▁by ▁means ▁of ▁spr ings ▁and ▁shock ▁absor bers . ▁When ▁the ▁Han cock ▁s ways , ▁the ▁weight ▁tends ▁to ▁remain ▁still , ▁allowing ▁the ▁floor ▁to ▁slide ▁under ne ath ▁it . ▁Then , ▁as ▁the ▁spr ings ▁and ▁shock s ▁take ▁hold , ▁they ▁begin ▁to ▁t ug ▁the ▁building ▁back . ▁The ▁effect ▁is ▁like ▁that ▁of ▁a ▁gy ros cope , ▁stabil izing ▁the ▁tower . ▁The ▁reason ▁there ▁are ▁two ▁weights , ▁instead ▁of ▁one , ▁is ▁so ▁they ▁can ▁t ug ▁in ▁opposite ▁directions ▁when ▁the ▁building ▁tw ists . ▁The ▁cost ▁of ▁the ▁dam per ▁was ▁$ 3 ▁million . ▁The ▁d amp ers ▁are ▁free ▁to ▁move ▁a ▁few ▁feet ▁relative ▁to ▁the ▁floor . ▁ ▁According ▁to ▁Campbell , ▁engine ers ▁discovered ▁that — des p ite ▁the ▁mass ▁dam per — the ▁building ▁could ▁have ▁fallen ▁over ▁under ▁a ▁certain ▁kind ▁of ▁wind ▁loading . ▁The ▁structure ▁was ▁ass essed ▁as ▁more ▁un stable ▁on ▁its ▁narrow ▁sides ▁than ▁on ▁the ▁big ▁flat ▁sides . ▁ ▁Some ▁ 1 , 5 0 0 ▁tons ▁of ▁diagonal ▁steel ▁b rac