Anchor escapement it he showed that the wide pendulum swings of verge clocks caused them to be inaccurate, because the period of oscillation of the pendulum was not isochronous but varied to a small degree due to circular error with changes in the amplitude of the pendulum's swing, which occurred with unavoidable changes in drive force. The realization that only small pendulum swings were nearly isochronous motivated clockmakers to design small swing escapements. The chief advantage of the anchor was that by locating the pallets farther from the pivot, the swing of the pendulum was reduced from around 100° in verge clocks
Escapement patented it in 1782. The early forms had two escape wheels. The duplex escapement was difficult to make but achieved much higher accuracy than the cylinder escapement, and could equal that of the (early) lever escapement and when carefully made was almost as good as a detent escapement. It was used in quality English pocketwatches from about 1790 to 1860, In the duplex, as in the chronometer escapement to which it has similarities, the balance wheel only receives an impulse during one of the two swings in its cycle. The escape wheel has two sets of teeth (hence the name