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Douglas's post about chronometer testing at Kew refers to chronometer _watches_ rather than chronometers as the mariner might know them. These were watches with chronometer escapements, escapements which allow the balance to swing free of the rest of the movement for a full rotation of the balance, before once again receiving impulse and releasing the movement, as it were for one tick. Because they swing free, they are said to be detached. They were not a great success in watches because, if given a rotary motion in the plane of the balance wheel at the wrong moment, the watch would "trip" and let off more than one tick.

This accounts for the great success of the lever escapement, which is now pretty universal in all mechanical watches. Watches with lever escapements can be made to keep rate closely, but because the escapement requires to be oiled, unlike the chronometer escapement, and oil changes over time, they never quite rivalled the marine chronometer

A fine watch would be adjusted for temperature and five positions, unlike a marine chronometer, which would be adjusted only for one position, face up, and temperature. It is of course much easier to adjust a timekeeper for only one position.

Bill Morris
Pukenui
New Zealand
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