A good watch may serve to keep a recconing at Sea for some days and to know the time of a Celestial Observ[at]ion: and for this end a good Jewel watch may suffice till a better sort of Watch can be found out. But when the Longitude at sea is once lost, it cannot be found again by any watch.

Letter to Josiah Burchett (1721), quoted in Longitude (1995) by Dava Sobel, p. 60 - Board of Longitude