DART. 'Guard-pin' is the alternative name for this safety device in the lever escapement. The dart is located on the end of the lever nearest the balance. It is also known as the 'safety pin', and its purpose is to prevent the escape wheel being unlocked except by the impulse pin.

DEAD-BEAT ESCAPEMENT. An escapement in which the escape wheel does not 'recoil' (q.v.). The cylinder escapement is a dead-beat one.

DEAD-BEAT VERGE. The escape wheel is like that in the ordinary verge. Two bevelled-edge pallets are located on the balance staff. These receive impulse in each direction from the tip of an escape wheel tooth successively. This escapement would appear to have been derived from the Debaufre escapement (q.v.).

DEBAUFRE ESCAPEMENT. This escapement, like the cylinder, is a dead-beat, frictional rest escapement. It was invented by Peter Debaufre in 1704. There are two escape wheels, on the same axis, the teeth being saw-cut, and each set alternately in relation to each other, or 'staggered', and facing inwards towards the balance staff, this passing between the two. Fixed to the balance staff is a pallet, a semicircular disc with an inclined plane cut on its edge. One tooth of the escape wheels alternately rests on the flat surface of the pallet while the balance oscillates, until the slope of the inclined plane is presented to it when it escapes, giving impulse as it slips down the inclined plane. Thus a succession of teeth, first one wheel and then its fellow, rest upon the pallet and then give impulse to the balance. There is no recoil.
Escapements known as the 'club-foot verge', the 'chaff-cutter' escapement (from the shape of the escape wheel teeth) or the Ormskirk escapement are derivatives. During the early 19th century the last enjoyed a mild popularity and a number were made at Ormskirk in Lancashire. Henry Sully invented a similar escapement, and Paul Garnier's carriage clock escapement has similarity to the Debaufre form.

DEAF-PIECE. See PULSE-PIECE.

DECK WATCH. A large-sized and accurate watch used on board ship when making observations to find the ship's position.

DEPTH. If the meshing of two gears is excessive, the depth is said to be too great; if it is insufficient, the depth is too shallow. Bad gearing can have very adverse effects on timekeeping. In general, depth is a term used by watchmakers for the degree of intersection or penetration between two parts. See also END SHAKE.

DETACHED ESCAPEMENT. An escapement in which the controller is free (or nearly so) from interference by the train. The lever escapement was often called specifically the 'detached escapement' or the 'detached lever' in its early history to distinguish it from the rack lever.

DETENT. A mechanism, or part, which prevents another part from operating at certain times or at one point in a cycle of operations. Two most common forms are those detents which cause a movement to stop in one direction only, and those which cause a movement to stop in both directions. It is, therefore, a locking device.

DETENT ESCAPEMENT. A detached escapement in which the escape wheel is locked on a stone (jewel) carried in a detent. Impulse is given by the teeth of the escape wheel - when a tooth is unlocked - to a pallet on the balance staff in every alternate swing of the balance. The detent is a blade spring or alternatively a pivoted lever. The detent or chronometer escapement is the most delicate and accurate of escape-ments used in portable timekeepers. The reader is referred to The Marine Chronometer, by R. T. Gould, for this specialised branch of watchmaking. It is also thoroughly investigated in Watches, by Clutton & Daniels.

DIFFERENTIAL DIAL. The centre of the dial is a revolving disc, with the hour numerals I to XII. This disc revolves 13/12 of a full circle in an hour. An ordinary minute hand, centrally placed, revolves once an hour, and is thus always passing over the current hour. A few were made circa 1700, but later examples are known.

DIVIDED LIFT. If the impulse angle is divided between the impulse faces of the pallets and the teeth of the escape wheel, the lift is said to be divided. In the English form of lever escapement, all the lift is on the pallets. With the Swiss or French club-toothed escape wheel the lift is divided. An example of all the lift being on the teeth is S. Mairet's form where the pallets are pointed.

DOME. The second or inner cover of a later 19th-century watch case. This cover is hinged. The dome was a substitute for the earlier dust-cap or the double-bottom case.

DOUBLE-BOTTOM CASE. A form of case in which the inner section is in one piece with the band of the case, the movement being attached to the case by joint and bolt. With the bezel opened, and the bolt pushed back, the movement hinges out of the case. The double bottom has a hole in which the key is inserted to wind the watch when the back is opened. A common form of watch case in England from the early 1800s to well past 1850. See CONSULAR CASE.

DOUBLE ROLLER. A lever escapement in which a second roller is used for the guard action.

DRAW. The very slight recoil action of the lever escapement during unlocking to ensure that the lever is drawn to the banking pins while the balance is turning through its supplementary arc (q.v.). It is a safety action which counteracts the tendency of the lever to leave the banking before being impelled to do so by the impulse pin. In an escapement without draw, a jolt may cause the fork of the lever to move away from the banking pin during the supplementary arc so that the guard pin comes into contact with the roller's edge, thus creating friction. Draw is achieved by the relative angles between the escape wheel teeth and the locking faces of the pallet stones. These are so formed that the pressure of a tooth on the locking face of the pallet produces a drawing-in motion of the pallet towards the wheel.

DROP. The free travel of the escape wheel after impulse and before locking.