Chronometers
Accurate time was critically essential not just for finding longitude, but for timing speeds and distances, so, in 1714, the British Admiralty offered the astronomical (no pun intended) prize of £20,000 that was awarded to Yorkshire clockmaker John Harrison for his marine chronometer. It took another hundred years to perfect, but by the early 1800s, chronometers were aboard all ships in the Royal Navy and British Merchant Marine.

Once a prized possession of every ship's captain, (these now antique) chronometers are fairly common, with 56-hour movements, usually mounted on gimbals inside a case similar to the ones that house box compasses. Moore and Arnold are two British makers of early chronometers, while Seth Thomas and Chelsea Clock Co. are prominent among American clockmakers. An American 1918 Waltham boxed ship's chronometer is on the market for about $2,500, and chronometers from the 1800s can easily run $3,000 to $5,000. For shopping tips, log onto La Timonerie Antiquities Marine (France) at www.la-timonerie-antiquites.com.