O

Octant :
To find his position at sea a navigator needs to know his longitude and latitude. For both determinations he must measure the altitude of the sun or a star above the horizon, and from the 15th to the early 18th centuries he employed simple instruments known as the 'cross-staff' and the 'backstaff'. A much more accurate instrument, the octant, was invented by John Hadley in London and, independently, by Thomas Godfrey of Philadelphia in the 1730s. In this instrument the horizon was sighted directly through a glass half silvered and half clear, and the image of the sun brought on to the silvered portion from the observer's side by reflection in a rotatable plane mirror. When the sun is on the horizon the two mirrors are parallel; under other conditions the angle through which the rotatable mirror must be turned to bring the two views of the sun into coincidence is half its altitude above the horizon. The sextant is a modified form of octant.

OG (Ogee) Clock :
A double, continuous S-like curve used as a molding on certain straight rectangular clocks of the early 1800s.

Ogee Moulding :
The name given to the cyma recta moulding, used architecturally and in furniture; the reversed ogee (or O.G., O-gee, etc.) is known as the cyma reversa. The section of the moulding is simply a double, concave-convex curve, or shallow S. It givees its name to a popular type of American clock.

Oil :
To lubricate clocks effectively is one of the horologist's greatest problems. In the past, mixtures of animal and vegetable oils were used. Great care is necessary in refining oils, to prevent their decomposition into a corrosive varnish-like substance after a few months of running. Only the best-quality oil, which must be of the correct grade for the size of the parts to be lubricated, should be used. The mainspring and escapement require different grades. Many beginners make the mistake of over-oiling clocks, even to the extent of lubricating the wheel and pinion teeth, which are designed to run dry. Only sufficient oil to form a small meniscus in the bottom of the oil sink should be applied to the pivot holes. Frictional rest and recoil escapements should have the pallet faces lightly oiled, although some clock repairers maintain that clocks fitted with the verge escapement suffer less wear if the pallets run dry. The chronometer escapement should never be oiled, except in the pivots of the balance and escape wheel. The mainspring should be lubricated with a suitable heavy oil or grease, and the levers of the striking work should be lightly oiled where they come into contact, as should the crutch's contact points with the pendulum.

Oil Lamp Clock :
Known also simply as a lamp clock, this fulfilled the dual purpose of providing light, like an ordinary oil lamp, and telling the time. The principle is simple: a conventional clock movement revolves a lampshade marked with the hours and quarters and read against a fixed pointer. Another version has a conventional dial and hands on one side of the opal or frosted-glass shade, illuminated by the flame of the lamp behind. The idea never seems to have been exploited widely; the form originated in the 19th century and several designs were produced in France. Another, earlier type of oil lamp clock, datable to the 18th century, consisted of a pewter stand, not unlike a candlestick, at the top of which a glass container of oil fed a wick supported by a holder projecting at right angles to the stand. The glass container was marked with hour divisions which corresponded to the time it took for the oil level to drop in the container as it burned at the wick.

Oil Sink :
The depression surrounding a pivot hole. Early in the 18th century, Henry Sully discovered that a small basin around the pivot hole reduced the tendency for oil to spread across the plate and dry up. The idea was eventually perfected by Julien Le Roy, and was generally adopted by the mid 18th century. Most clocks made before that time, without sinks, have subsequently been modified. Narrow deep sinks are more effective than wide shallow ones.

Oilstone :
An abrasive stone such as Arkansas, India or Turkey, used with a lubricant for sharpening tools to a fine edge. Fine oil is normally used. Oilstones can be obtained in grades such as fine, medium and coarse, and in different shapes and sizes. Dirty or clogged stones can be cleaned by soaking in paraffin or benzine. Oilstone dust is oilstone ground to powder and combined with oil for grinding or polishing steel.

Open Escapement :
The wheel and pallet movement that can be seen on some clock dials.

Organ Clock :
An unusual form of clock in which the chiming of the hour is preceded or replaced by a short tune played on a miniature pipe organ. The sound made by these clocks is comparable to that of the hurdy-gurdy, a portable organ formerly carried by itinerant street musicians. Sometimes the organ clock only plays every third hour, probably to conserve power. The organ clock appears to have had its origin in the 16th century.

Ormolu :
Ornamental metalwork made from cast bronze, worked and chased before being gilded. It originated in France in the 17th century for decorating cabinetwork and the cases of clocks.

Orpheus Clock :
A group of nine mid 16th-century table clocks bear decoration in the form of Orpheus charming the beasts with music, and Eurydice and Cerberus at the entrance to Hades. The cases are believed to be the work of a Nuremberg goldsmith, and the association between the Orpheus scene and the clocks is that the scene is an allegorical presentation of musical harmony, closely linked with the theories of cosmic harmony and the music of the spheres propounded by Pythagoras. Some of the clocks have round cases and others square ones, but while the decoration has the same theme the movements differ widely. Except that the clocks are believed to be south German, made 1560-80, no definite conclusions about them can be reached until more is known.

Orrery :
This term is usually applied to a small-scale three-dimensional representation of the movements of the heavenly bodies of the solar system, the Earth, moon, planets and their satellites, about the sun, operable by hand or clock-driven. The instrument shows in considerable detail the rotation of the Earth on its own axis and in orbit around the sun, also the rotation of the moon around the Earth, and the rotation of the sun about its own axis.

Orrery Clock :
A clock mounted with a mechanism of rotating spheres designed to show the relative sizes, positions and motions (but not distances) of the heavenly bodies. In the late 18th and early 19th centuries the scientific clockmakers of France experimented with such devices. Antide Janvier produced several, and those made a few years later by his pupil Z. Raingo were so successful that the term 'Raingo clock' became synonymous with a type of orrery clock.

Oyster-shell Veneer :
A type of veneer used on the cabinetwork of longcase clocks, which was produced by cutting the smaller boughs of walnut and laburnum trees in thin slices across the grain. When these are arranged in a pattern, sometimes edged with boxwood stringing, the effect is like that of the irregular layers on a oyster shell; hence the name. The style reached its greatest popularity during the last quarter of the 17th century.