▁Royal ▁Custom s ▁and ▁quar antine . ▁In ▁ 1 7 8 0 , ▁when ▁pl ague ▁was ▁in ▁Poland , ▁even ▁vessels ▁with ▁gra in ▁from ▁the ▁Balt ic ▁had ▁to ▁spend ▁forty ▁days ▁in ▁quar antine , ▁and ▁un pack ▁and ▁air ▁their ▁cargo es , ▁but ▁due ▁to ▁compla ints ▁mainly ▁from ▁Edinburgh ▁and ▁Le ith , ▁an ▁exception ▁was ▁made ▁for ▁gra in ▁after ▁that ▁date . ▁About ▁ 1 7 8 8 ▁an ▁order ▁of ▁the ▁council ▁required ▁every ▁ship ▁li able ▁to ▁quar antine ▁to ▁ho ist ▁a ▁yellow ▁flag ▁in ▁the ▁day time ▁and ▁show ▁a ▁light ▁at ▁the ▁main ▁top m ast ▁head ▁at ▁night , ▁in ▁case ▁of ▁meeting ▁any ▁vessel ▁at ▁sea , ▁or ▁upon ▁arriv ing ▁within ▁four ▁le agues ▁of ▁the ▁coast ▁of ▁Great ▁Britain ▁or ▁Ireland . ▁ ▁After ▁ 1 8 0 0 , ▁ships ▁from ▁pl ague - aff ected ▁countries ▁( or ▁with ▁f oul ▁b ills ) ▁were ▁permitted ▁to ▁complete ▁their ▁quar antine ▁in ▁the ▁Med way ▁instead ▁of ▁at ▁a ▁Mediter rane an ▁port ▁on ▁the ▁way , ▁and ▁an ▁extensive ▁la z aret ▁was ▁built ▁on ▁Ch et ney ▁Hill ▁near ▁Ch ath am ▁( although ▁it ▁was ▁later ▁demol ished ). ▁The ▁use ▁of ▁floating ▁h ul ks ▁as ▁la z are ts ▁continued ▁as ▁before . ▁In ▁ 1 8 0 0 ▁two ▁ships ▁with ▁h ides ▁from ▁M og ador ▁in ▁Mor oc co ▁were ▁ordered ▁to ▁be ▁s unk