▁gravity ▁or ▁the ▁motion ▁of ▁a ▁ship . ▁His ▁first ▁two ▁sea ▁time pie ces ▁H 1 ▁and ▁H 2 ▁( comple ted ▁in ▁ 1 7 4 1 ) ▁used ▁this ▁system , ▁but ▁he ▁real ised ▁that ▁they ▁had ▁a ▁fundamental ▁sens itivity ▁to ▁cent rif ug al ▁force , ▁which ▁meant ▁that ▁they ▁could ▁never ▁be ▁accurate ▁enough ▁at ▁sea . ▁Harrison ▁solved ▁the ▁precision ▁problems ▁with ▁his ▁much ▁smaller ▁H 4 ▁chron ometer ▁design ▁in ▁ 1 7 6 1 . ▁H 4 ▁looked ▁much ▁like ▁a ▁large ▁five - inch ▁( 1 2   cm ) ▁diameter ▁pocket ▁watch . ▁In ▁ 1 7 6 1 , ▁Harrison ▁submitted ▁H 4 ▁for ▁the ▁£ 2 0 , 0 0 0 ▁longitude ▁prize . ▁ ▁His ▁design ▁used ▁a ▁fast - be ating ▁balance ▁wheel ▁controlled ▁by ▁a ▁temperature - comp ens ated ▁spir al ▁spring . ▁ ▁These ▁features ▁remained ▁in ▁use ▁until ▁stable ▁electronic ▁oscill ators ▁allowed ▁very ▁accurate ▁port able ▁time pie ces ▁to ▁be ▁made ▁at ▁afford able ▁cost . ▁In ▁ 1 7 6 7 , ▁the ▁Board ▁of ▁Long itude ▁published ▁a ▁description ▁of ▁his ▁work ▁in ▁The ▁Princi ples ▁of ▁Mr . ▁Harrison ' s ▁time - keeper . ▁ ▁In ▁ 1 7 5 7 , ▁John ▁Bird ▁invent ed ▁the ▁first ▁se xt ant . This ▁replaced ▁the ▁Davis ▁quadr ant ▁and ▁the ▁oct ant ▁as ▁the ▁main ▁instrument ▁for ▁navigation . ▁The ▁se xt ant ▁was ▁derived ▁from ▁the