A mechanical instrument for measuring time, depending on the uniform flow of sand or other fine-grained material through a narrow aperture between two glass bulbs. In the simplest and commonest form the two bulbs are of equal volume, and the unit of time measured is that taken for the whole of the sand content to flow from one bulb to the other. Occasionally, the assembly consists of one large bulb and four smaller bulbs, the time unit being thus divided into four. The earliest-known records of sand glasses date from the 14th century, and one is shown in a fresco of 1338 by Ambrogio Lorenzetti in the Palazzo Pubblico in Siena. There are records of sand glasses in ships' inventories from about this date, and it seems that their principal early use was for dead reckoning at sea, for measuring the progress of a ship requires a timekeeper. Slow-running glasses, emptying in two or four hours, were also used at sea for timing the duty watches.
Sand glasses were sometimes assembled and mounted in groups of four timed for 1/4-hour, 1/2-hour, and 3/4-hour and one-hour periods. All early sand glasses were made with two glass bulbs separated by a thin metal plate with a central hole, the assembly being waxed together and covered with thread. From the mid 18th century, however, they were blown in one piece, the central constriction being of glass and the opening sealed by a cork. From c. 1800 the opening was sealed after filling by the glass blower, making the whole airtight.
A frame saw, now called a hacksaw, consisting of a frame and handle and detachable metal-cutting saw blade. Sometimes the blade is held in the frame by the tension of the frame itself, while other frames have blade hooks with threaded extensions and wing nuts to tension the blade. With this kind of fitting the blade can be turned in the frame to permit long cuts to be made. The blade is intended to cut in one direction only, and should be fitted so that it saws in the direction away from the handle. Another type of metal-cutting saw is known as the 'backed saw'. It has a wide blade with a stiff, ridged backing and a handle.
A saw consisting of an adjustable frame with jaws for clamping the ends of a fine, wire-like blade. The frame is adjustable so that blades of different lengths can be fitted, normally with the cutting teeth pointing towards the handle, so that the saw cuts as it is pulled. Piercing saws are used to cut decorative work in sheet metal, clock hands, and internal holes and shaped slots. The blade is threaded through a previously drilled hole before it is fixed into its frame.
A general term applied to all sundials in which the hour lines are engraved on a hollowed-out surface. Sundials of ancient Egypt had their lines marked on flat surfaces, but the Greeks and Romans constructed large, fixed stone sundials of scaphe form. The simplest is a hemispherical hollow in a horizontal slab, the gnomon consisting of a vertical spike the shadow of whose tip shows the time. In the so-called 'hemicycle' of Berosus, the useless part of the hemisphere is cut away, leaving an open front on the south side of the dial.
By the mid 16th century screws had begun to be used in clockmaking, and they have increasingly replaced pins and wedges, although pins are still used to secure certain parts. Standardisation of screw threads has been attempted at various times, but a huge variety of sizes, pitches and thread angles remains.
Usually, a steel blade and handle with a thin flat driving element which engages in the slot or other recess of the screw. Recently, specially shaped recesses in screws to fit special screwdrivers have come into use, available in many sizes.
Screwhead, or Screwpoint Polishing Tool
This tool is held in a bench vice and has support bearings for a spindle which is rotated by rolling the hand over its knurled or octagonal body. It is furnished with a range of large and small brass and steel chucks, lanterns and polishing laps. The thread of a screw is first polished by screwing it into and out of a piece of soft wood, charged with oilstone dust and oil. The screwhead is then stoned with Arkansas, and its slit is cleaned with pegweed cut to a chisel shape and charged with oilstone dust and oil. After all traces of the polishing mixture have been removed with soft bread, the screw is mounted in the polishing tool and polished with an iron or bell-metal lap charged with red stuff or diamantine at its head and point. An earlier tool for polishing screws was rotated with the bow.
A flat piece of hardened tool steel with various-sized drilled and tapped holes for producing screw threads on metal rods, up to about 1/4 in. in diameter. The holes are often in pairs or sets of three, one row finished normal size while the others are over-size, which enables this somewhat primitive tool to be used partly to form a thread using an over-size hole, and to finish the thread with the normal size of hole. This is necessary because the tool does not actually cut the thread, but squeezes it into shape, which requires considerable force except for the smallest sizes of rod. To attempt to make a full thread in one operation would risk a fracture of the rod. To cut threads more easily some screwplates have extra holes drilled at the side of, and breaking into the threaded hole to give cutting edges and clearance for swarf.
A tool in the form of nutcracker pliers, with jaws adjusted by means of a set screw and nut, and a number of screw-forming holes between the jaws. Also means a tool in which split-screw dies are held.
The wooden board to which the movement of a longcase clock is attached by two screws through the lower movement pillars in London-made and other good-quality clocks, and by J-bolts in provincial clocks. Seat boards should be secured to the clock case to prevent accidental overbalancing of the movement during dismantling.
A simple proportional measuring gauge consisting of two straight brass or steel arms, pivot-jointed at one end to form an adjustable gap gauge which is set to the required size (or gap) by a slotted quadrant plate secured to one arm at the opposite end from the joint, and locked with a screw. The two arms and quadrant plate are marked with numbered scales and are used primarily to ascertain the correct proportions between a clock wheel and the pinion to be geared with it. As a wheel and pinion gauge it is limited in its accuracy above and below certain dimensions, for it cannot take into account the various forms of pinion-leaf addenda curves. The gauge, however, is invaluable to the clockmaker for obtaining other measurements.
The name given to small clocks which are traditionally associated with the 18th-century sedan chair, though there is no direct evidence that they were used exclusively for this type of conveyance, if at all. They have dials 3 to 4 in. in diameter, mounted in a circular wooden frame turned with mouldings on the front and polished; a pendant bow is fitted at the top for hanging. Their movements are normally those of 30-hour pocket watches, and it is possible that they are derived from the large coach or travelling watches of the late 17th century.
When a fusee clock is being assembled and the line or chain is wound on the barrel, the mainspring is pre-tensioned by winding the mainspring arbor, usually one half to one and a half turns, enabling the fusee to deliver an even amount of power until the end of its run. This is known as the mainspring set-up.
A modification of John Hadley's octant made by Captain John Campbell c. 1771. Whereas the octant is in the form of a sector comprising one-eighth of a circle, the sextant occupies one-sixth of a circle. With the doubling of angle provided by the rotating mirror it enables up to 120º to be measured, as against 90º for the octant.
Sharp Gothic Clock
An American case style, designed by Elias Ingraham c. 1843 for use with spring-driven movements, and first produced by Brewster & Ingrahams, probably for the English export market. The design was soon copied by many other firms, including Birge & Fuller who made a modified case-on-frame for wagon-spring and later fusee movements. Probably the greatest variety of fusee and spring-wound movements, c. 1843-52, appeared in this case. About 1852 a smaller version 16 in. high was introduced for casing 30-hour time and strike-and-alarm spring movements. These are known among collectors as 'baby or miniature steeple' clocks. The case was made of pine veneered with mahogany, rosewood, and other exotic woods. About 1888 the E. N. Welch Manufacturing Co. introduced a form with a single glass door exposing the dial and pendulum. The sharp Gothic case was probably the most popular design ever made during the century of Connecticut clockmaking.
Hand shears or snips are used for cutting thin sheet metal. They may be curved or straight, curved ones for cutting small inside curves and straight ones for making straight or curved cuts. Some shears are designed to allow one handle to be held in a vice, while others have one handle mounted on the bench. Besides the normal cutting jaws these are often provided with a means of shearing wire or rod.
A type of lantern clock in which the chapter ring protrudes a considerable distance beyond the normal lantern-clock frame. Although the sheep's-head clock first appeared c. 1700, it only became popular towards the middle of the 18th century. The larger chapter ring made it easier to tell the time. The distended appearance of this clock, with the exaggerated curves of the chapter ring on either side, perhaps suggested the coiled horns of a ram, though the term 'sheep's head' seems somewhat strange.
A clock designed to sit on a shelf or mantel.
A resinous substance produced as a protective coating by the lac insect of India. It is available as amall orange flakes which are dissolved in methylated spirit to form a varnish or lacquer, and in sticks. It can be melted and used as a cement for holding delicate or awkwardly shaped parts on the face plate or wax chuck or a lathe, or in clockwork for cementing lever pallets and roller jewels in position.
The sidereal day is the time of one complete revolution of the Earth relative to the first point of Aries, or the spring equinox. This is not quite the same time as one revolution relative to the stars, since the first point of Aries moves very slowly around the ecliptic, owing to precession; but the difference is only 0.008 seconds. The length of the sidereal day is 23 hours 56 minutes 4.09 seconds, recorded as 24 'hours' on a sidereal clock.
Post or sidewalk clocks were first commercially produced in the United States c. 1870. The clocks were available in two- or four-dial models from 30 in. to 40 in. in diameter, supported on decorative cast-iron frames, usually measuring 12 to 15 ft. from base to dial centers. The clocks made by E. Howard Clock Co. of Boston had the pendulum movement mouted in a compartment about street level. These were eight-day, weight-operated timepieces, with a locked door at the base opened weekly for winding. The A. S. Hotchkiss of New York models had the movement placed in an adjoining building and connection was made under the sidewalk. Illuminated plate-glass dials were available. Tower and sidewalk clocks were continued to be manufactured during the first quarter of the 20th century. Many sidewalk clocks were later electrified.
A process for forming a slightly matt or frosted finish of chemically deposited silver on brass or copper clock dials, chapter rings, etc. To silver a dial with the engraved numerals already filled with black wax, the surface is first cleaned and given a matt or grained finish. A silvering paste made from silver nitrate, cream of tartar and common salt, mixed with water, is applied to the dial and rubbed on until the surface is whitened with a deposit of silver. The dial is then washed, dried and lacquered to prevent the silvered surface from tarnishing.
Most of the craftmanship and decorative features of the mechanism of a clock are seen only by the repairer and restorer, but the skeleton clock is arranged to reveal as much of the mechanism as possible. While the earliest skeleton clocks are mostly French, it is considered to be mainly an English type, mostly of mid Victorian origin: great numbers were produced at the time of the Great Exhibition of 1851. Most have single striking on a bell at the hour, the striking hammer being lifted by a pin on the minute wheel; more complicated examples have full striking, chiming and other features. Escapements are usually recoil anchor, with chronometer, coup perdu, pinwheel, dead-beat and gravity examples as variations. Ordinary skeleton clocks have movements on a wooden plinth, but those of better quality have a marble plinth; a glass dome keeps the clock free from dust. A clockmaker usually wound these clocks in Victorian times because of the fragility of the glass domes.
Any clock which indicates time without possessing its own internal means of measuring time, depending on an external control, may be called a slave clock. The term is usually applied to the controlled clocks in a system operated by an electric master clock which sends out pulses at either 1-second, 30-second, or 1-minute intervals to advance the hands of the controlled clocks. Carl August Steinheil of Munich was the first to use slave clocks, these being driven by Daniell cells switched by contacts on the master-clock pendulum. Reliable slave clocks were not produced until the beginning of the 20th century, although one installed at the gate of the Old Observatory at Greenwich by Charles Shepherd in 1852 has been operating almost continuously since then. Slave clocks may be operated in series or parallel, by pulses in one direction, or by pulses reversing for each step; generally today slave clocks work from single-direction pulse circuits and are connected in series, large systems having groups of series clocks further connected in parallel and energized by a relay circuit to handle the large current required.
A cam, the spiral profile resembling a snail's shell, commonly used to determine the number of blows struck in rack striking work at the hour or quarter.
A process which imparts a decorative finish to metal clock parts. This finish takes the form of a series of curved lines radiating from a common center. Snailing on steel is produced by a copper mill with a hollow face and a thin projecting rim, charged with abrasive. In operation, the work being snailed is free to rotate between centers while the rotating mill acts upon it. Producing the continuous snailed effect when the mill is charged first with emery and finished with caorse red stuff. For brass or other soft metals, a bone or ivory mill is used.
The term used to describe the uniting of chemically clean surfaces of suitable metals or alloys, which are not themselves melted, by using heat and a fusible alloy or solder. There are two distinct forms of solder - soft and hard. The more fusible one is usually called 'soft' or 'lead' solder; lead has a relatively low melting point. Ordinary soft solder consists of two parts tin and one part lead, but a range of soft solders is available which contains silver with tin, lead, copper, cadmium or zinc added. Depending on purpose, soft solder is suitable for use on copper, nickel, tin, iron, zinc, lead and numerous alloys. Hard or silver soldering, or 'silver alloy brazing' as it is called to distinguish it from soft soldering, is used for most nonferrous metas and alloys as well as for steel and iron. Hall-marked silver is soldered with silver solder of high silver content, and various hardnesses and melting ranges of solders of this type are available. Hall-marked gold is soldered with a gold solder of high quality in various carats, hardnesses and colors. Solders may be purchased in various forms of wire, rod, sheet or powder-paste.
Different fluxes are necessary for different types of work. Soldering fluxes help the metal to flow and prevent oxidation under heat. Although many fluxes are available, soft-solder flux generally consists of resin, or killed sulphuric or hydrochloric acid, or one of a series of patent preparations. Hard-soldering flux is traditionally boracic acid (or borax), made up into a paste with water.
The dates when the sun appears to stand still in the heavens, i.e. when it reaches its greatest declination, north or south. Since these are maximum and minimum values on a smooth curve they cannot be measured precisely; on the Gregorian calendar they occur near 21st June and 22nd December.
The four corners that square off a round clock dial, often featuring painted designs or metal decorations.
In his journal for 27th April 1762, John Wesley gives a description of a speaking clock made by a Mr. Miller of Lurgan, Ireland. The Paris Exposition of 1900 featured a clock 6 ft. high which announced the hours in a human voice by means of a phonograph. The first true speaking clock was the invention of Hernhaard Hiller of Berlin. An endless celluloid band carried 48 tracks upon which was recorded, 'It is one o'clock, it is quarter past one o'clock' and so on, for a 12-hour clock; or the half-hours for the 24-hour clock. A gramophone motor turned the band and a gramophone soundbox converted the recording on the track into sound, amplified by a small trumpet. The best-known speaking clocks today are those which give the exact time via the telephone system, of which the earliest was installed in Paris in the early 1930s. The British system commenced on 24th July 1936 with a voice recorded on glass record discs. A Hipp-Toggle maintained pendulum was used for the time control with master-clock control correction, an error of one-tenth of a second being maximum allowed. An electric, domestic speaking clock was introduced by the Japanese in the 1970s.
An early hood-locking device found on some late 17th-century longcase clocks in which the hood was raised vertically on runners for access to the movement. The spoon fitting consisted of a small pivoted metal plate rather like a spoon handle, which locked the hood in position. Access to the spoon was obtained by unlocking the trunk door; the device could then be depressed to unlock the hood. The spoon fitting served a dual purpose. It prevented access to the winding holes in the dial without first opening the trunk door, rendering any chance of overwinding the weights (with the consequent risk of snapping the gut lines) unlikely. It also prevented any unauthorized person from tampering with the clock hands.
A finish or form of decoration given to chronometer plates or clock parts, consisting of equidistant circular spots or rings which sometimes overlap each other. The rings are produced by a rotating bone or ivory tool charged with an abrasive, and the rings can be arranged in many different patterns - straight rows, wavy rows, concentric circles, etc.
A spring may take many forms, and the perfect spring, when deflected within its elastic limit, always returns to the same shape. The most common material for all clock springs is hardened and tempered steel although, particularly in longcase clocks, certain click and return springs are made from hammer-hardened brass.
A clock whose power is provided by springs, rather than falling weights.
The coiled metal ribbon used to drive a clock's mechanism. Originally clocks were all weight-driven, and it is thought that a spiral spring was first used to drive a portable clock early in the 16th century. Mainsprings are usually made from hardened and tempered steel strip, although recently special alloy steel springs have become available, and these offer advantages such as greater resistance to breakage and more power for a given size over standard steel springs.
The spring steel strip from which the pendulum is hung. Kinking of the suspension spring through careless handling is the most common cause of a pendulum rolling as it swings.
A tool for safely winding a clock mainspring and inserting it into its barrel. There are various types, older ones consisting of a frame with a hooked projecting arbor on which the spring is wound by a handle. The handle has a ratchet and reversible click pawl to control winding or unwinding, and the tool is vice-held. A pivoted bar holds the outer spring hook during winding. Some mainspring winders have a range of false-bottomed barrels with a slit in the side wall; the spring is wound into the false barrel and a pusher used to force it into its clock barrel.
The Dutch name for a longcase clock.
A type of Dutch clock which derives its name from the resemblance of its long swinging pendulum to an animal's tail (staart).
A device used until the early 17th century to equalize the force of the mainspring over its period of run. The mechanism usually takes the form of a cam geared to the mainspring arbor, and a roller following the cam profile, impelled by a powerful spring. The cam profile is designed to resist the force of the mainspring over the first half of the clock's period of run, and to assist it over the remaining part of the run.
A support for work of various kinds. A stake is usually, but not invariably, made of steel. There are many designs, from a simple steel block placed on the bench, to complex tools with a wide variety of specialized parts for such purposes as riveting wheels to their collets or setting jewel stones. In the complex form, in which punches, pushers, sinkers or chamfering cutters and numerous anvils or small stump stakes are essential parts, the tool is usually called a staking tool.
A wheel with pointed teeth, used in conjunction with a snail in rack striking work.
The dowel pins commonly used in pairs to ensure the accurate location of a balance cock, etc.
During the late 19th and early 20th centuries a number of French novelty clocks were made incorporating a clock movement and dial in models of steam engines, railway locomotives, warships and other such machinery. In these clocks the engines are periodically released by the clock movement and their subsequent motion is derived from a separate spring. Thus the beam-engine flywheel or the wheels of the railway locomotive begin to turn, or the guns of the warships revolve.
One type was made in the form of James Nasmyth's steam hammer, the hammer of which moves up and down at each oscillation of the clock pendulum. In this case the pendulum is of U shape, swinging inside the hollow legs of the case and controlling, through a system of levers, a conventional brocot escapement. The clock is of patinated black iron and polished brass fittings, and was retailed by a London dealer about 1900.
A clock with a sharply pointed Gothic case, and two or four finials at each side forming spires.
The German name for what is usually known in English as a bracket clock.
A type of Dutch clock deriving its name from the suspended, chair-shaped bracket or seat (stoel) on which the clock is supported.
The mechanism used to limit the number of turns, up or down, of a mainspring. The usual fusee stopwork consists of an arm pivoted to a stud fixed to the clock plate. The arm is moved from the path of the fusee snail nose by a weak spring, until the clock is nearly wound, when the chain or line moves the stop arm into the path of the snail nose, thus preventing further winding.
Maltese Cross stopwork is commonly used for going-barrel clocks. A disc with a projecting finger is fixed to the barrel arbor and advances the Maltese Cross wheel one notch per revolution of the barrel arbor, until the shoulder contacts the raised contour of the Maltese Cross wheel at the end of the wind. This stopwork also limits the unwinding of the mainspring, reducing the poor performance of non-detached escapements, such as the cylinder, when operating with insufficient driving force.
( Pic 1 )
( Pic 2 )
It is thought that originally clocks indicated the passing of time by striking a bell once each hour, a dial probably being added to allow the clock keeper to see when the next hour was to be struck. The earliest form of automatic striking work is the count-wheel mechanism, also termed 'locking plate' or 'count plate'. Count-wheel striking work has the disadvantage of sounding the hours in succession; thus, if the mechanism is tripped accidentally, the hands and striking get out of synchronism.
See ( Pic 1 ).
The rack and snail striking works was developed towards the end of the 17th century. It has the advantage that the relationship between the hands and striking cannot readily by lost.
In quarter striking work, many different layouts are used, but the action may be understood by careful examination.
See ( Pic 2 ).
Early Dutch clocks sometimes use a striking system in which the hour is struck on a large low-pitched bell and at the half-hour the succeeding hour is struck on a small high-pitched bell. The Dutch striking system is found occasionally in clocks from other countries.
Striking, Grande Sonnerie
In clocks with quarter striking the first, second and third quarters are indicated by striking from one to three strokes on a bell differing in tone from the hour bell. In clocks with four-quarter striking, each quarter, including the hour, is struck on bells differing in tone from the hour bell. In clocks with full grande sonnerie striking, the quarter and the preceding hour are struck at each quarter in passing.
Striking, Hour and Half-hour
Sometimes referred to as 'French striking'. Besides striking the hour, the clock strikes once at the half-hour, usually on the hour bell.
Joseph Knibb designed clocks of long-running duration, and to reduce the power requirements of the striking train, he devised a system of striking which utilized one bell for the Roman numeral I and another of differing pitch for the Roman V. Thus the maximum number of blows necessary to indicate any hour is four. Knibb used the true Roman IV instead of the conventional IIII, saving a further two hammer blows. He thus used only 30 blows instead of the usual 78 in twelve hours.
The striking system known by the German term Sürrerwerk ('whizzing work') has been recorded in Italian and German clocks during the 18th and 19th centuries, and variants of the device are known from the 17th century. The system is based on the use of pins of graduated length mounted on opposite sides of a single pinwheel. These pins lift the two hammers to strike the hours and quarters, six on one side operating the hour hammer, the arbor of which is axially changed in position by a stepped-face cam to engage each of the pins progressively as the hours go by. The three pins on the other side of the pinwheel operate in a similar way to strike the quarters progressively by one, two and three strokes, the pinwheel making one complete revolution each quarter of an hour. In a full day the system thus strikes 228 blows in the following way: hours, 1+2+3+4+5+6x4 = 84 (a six-hour system of striking); quarters, 1+2+3x24 = 144. Grande sonnerie striking by this system is known; it strikes 864 strokes per day.
A clock striking the quarters on two differently pitched bells or gongs, the sound for the first quarter being similar to the name, with each successive quarter indicated by an additional ting-tang. Sometimes, before the hour is struck in the usual way, four ting-tang hammer blows indicate the final quarter.
The great majority of turret clocks use count-wheel control in one form or another. Count wheels appear with notches on either the inner or outer side of the ring. Many of the older clocks had internally cut teeth, and were supported by four arms offset from the disc to allow the driving pinion space to turn.
Another type of tooth has pins set axially in the periphery of the wheel, rather than notches. The pins are set either into a wheel driven directly by a pinion, or into a wheel having ratchet-shaped teeth collected tooth by tooth by a rotating gathering pallet.
Rack striking appears on a few later clocks. In clocks made by the E. Howard Clock Co. of Boston, Massachusetts, the rack slides linearly against the snail rather than rotating as is more common.
Many turret clocks of the wooden door-frame variety have a long lever on the second arbor which has become known as the flail, the end of which is turned at right angles and locks against a similar lip on the locking lever, which in turn normally rests in a notch in the count wheel. The hour wheel carries an arm which lifts the locking lever but prevents the flail rotating by engaging with the lip. When the hour wheel rotates far enough the hour lever disengages the train, which runs until the next notch in the count wheel allows the locking lever to drop and lock the flail.
Another type of control, frequently found on wooden-framed clocks, is the percussive type. The locking takes place against a protruding stud on the second wheel, and a second locking lever falls into a notch in the count wheel. A small arm on the hour wheel raises a pivoted weight and lets it go on the hour. A thin rod or wire communicates the impact of the weight falling to the locking levers and knocks them out of engagement. Before the levers fall again under the action of gravity the train starts to run, and continues to do so until the next notch in the count wheel causes it to relock. Percussive letting-off work is occasionally found on iron clocks.
Pins for lifting the hammers were in use until the mid 19th century. Normally six or eight in number, they were on either the great wheel or second wheel, depending on the number of wheels in the train. Lord Grimthorpe introduced a specially shaped cam for the purpose which gave a rolling rather than rubbing action, with less wear. As many as 60 were cast integrally with the great wheel.
A small portable or carriage clock, designed to stand either in a leaning position supported by a pivoted strut at the rear in the manner of a photograph frame, or vertically supported by a swivel foot mounted on the lower edge. The idea for such a clock was first conceived by Thomas Cole, an eminent Victorian clockmaker, c. 1845. The clocks were made to be as thin as possible when folded and were frequently fitted with standard 30-hour Swiss watch movements. Although many minor variations of shape occur they were generally of rectangular, oval or diamond form. A manually adjustable calendar was sometimes a feature of the dial. These clocks can be regarded as a type of calotte, a travelling clock which has become very popular in the 20th century.
The German verb stutzen means to cut down in size, and as applied to a clock it implies one not made to stand independently, like a longcase clock, but needing a pillar, piece of furniture or mantelpiece to support it.
This usually consists of a horizontal dial with a small number of concentric hour scales, each for use in a single latitude, with a gnomon whose inclination can be varied. Dials of this type were incorporated in some of the fine astronomical compendia made by Christopher Schissler of Augsburg between 1555 and 1565, and were popularized by Michael Butterfield.
It is possible to measure time from the sun's altitude when the date is known. Portable altitude dials with a date setting scale take a great variety of forms, including the chalice, crescent, pillar, quadrant, ring and universal ring dials.
A form of adjustable sundial invented by Michael Butterfield, an English instrument maker who is known to have worked in Paris between 1678 and 1680.
The Butterfield sundial is a small, portable horizontal sundial with a baseplate, usually octagonal but sometimes oval, of brass or silver, with engraved hour lines and a small inset compass. Four or more concentric sets of hour lines are employed, each for a definite latitude, and the folding gnomon has a pivoted style edge, which slides over the main gnomon, so that the angle of inclination of the edge can be varied. The fixed portion of the gnomon carries a scale of latitudes; the pivoted portion is engraved with a bird, the tip of whose bill indicates the inclination on the latitude scale.
A form of scaphe dial in which the dial is in the form of a chalice or cup, usually of metal. The hour lines are engraved on the inside of the cup, and the time is shown by the shadow of the tip of a pin-gnomon which usually rises vertically inside the chalice.
Chalice dials may be either direction or altitude dials. If a direction dial it has a built-in magnetic compass, used to set it in position; if without a compass, the interior walls of the chalice are divided into twelve equal compartments identified by the names of the months or the corresponding zodiacal signs. In use the dial of the chalice sundial is set so that the shadow of the gnomon lies within the month of use. The earliest known chalice dial is dated 1554 and signed by Bartholomew, Abbot of Aldersbach, near Passau, Bavaria.
The Chinese have used sundials since at least the 4th century BC. Equatorial dials have been found, dating to the Han dynasty, measuring the day into 100 parts, or K'o, each equal to 14 minutes 24 seconds of the modern day. Portable sundials were referred to in a manuscript of the late Chou dynasty. Additional information is provided by a 14th-century Chinese manuscript, which indicates that scaphe sundials existed in the 13th century. After the arrival of Europeans in the 16th century, horizontal dials were introduced.
Post-16th-century sundials are of two main types, referred to in Joseph Needham's "Science and Civilization in China" as Type A and Type B. Type A is the Chinese descendant of the European horizontal folding sundial with a compass, string gnomon, and unequal hour gradients for measuring equal-hour intervals at one or more specific latitudes. This type often has a moon dial on the reverse side of the lid. Type B is the direct descendant in Chinese lineage from earlier Chou and Sung models. It is a portable folding sundial with a compass, adjustable inclining dial, and a scale for adjusting the degree of incline of the dial according to the season. The scale has twelve different notches for inclining the dial according to the chhi (fortnight) of the season, going from winter solstice to summer solstice. Each notch is used twice during the year, once in the cycle of rising inclination and once in the cycle of descending inclination. In this manner the dial maintains only moderate accuracy in measuring equal fixed hours according to the season. In summer the dial can also be used for travelling if the degree of inclination for each chhi is known. The dial can then be used as an equatorial sundial, according to the latitude of the locality, but not in winter since the angle of the sun is below the inclined dial and there is no provision for indicating winter hours on the reverse.
The compasses on both types of sundial have the cardinal points and the characters of the Twelve Terrestrial Branches and the Ten Celestial Stems as well as seasonal references engraved in the chart of degrees of azimuth around the compass dial. They refer to the sexagenary cycle and were used for purposes of divination.
A modified form of universal-ring dial in which the ring is divided into two equal semicircular portions placed back to back. The division is made at the '12' marks, so that the semicircles face due east and west respectively when correctly set. The pinhole is replaced by the tip of a curved gnomon, which slides on a date scale.
A form of scaphe dial, namely a dial with a hollowed-out surface. The term can be applied to a chalice dial, but it more often refers to dials with hour lines engraved in a shallow circular depression in a horizontal metal or ivory plate; in this type of cup sundial the gnomon is usually a short vertical pin. The cup dial often indicates Italian or Babylonian hours; these two indications may occasionally appear on diptych dial as accessories to the main horizontal or vertical dial showing equal hours.
A convenient form of pocket compass dial. It consists of a pair of rectangular metal or ivory plates, hinged along a shorter edge. When closed it is flat, but in use it is opened, with the upper plate vertical and the lower remaining horizontal. In this position the string gnomon, which passes through a hole in each plate, becomes taut. This casts a shadow on a dial marked on the horizontal plate, in which a compass is also set. The same gnomon often serves a vertical south dial on the under-surface of the upper plate.
Diptych dials often have other 'furniture', the most common being small horizontal, vertical, or cup dials showing Italian and Babylonian hours, others being pin-gnomon dials showing unequal hours and the length of the daylight period. In some versions, a nocturnal or a volvelle for interconverting lunar and solar times may also be included.
Direction sundials, if portable, are oriented by a magnetic compass. They measure time by the hour angle of the sun relative to the meridian.
Sundial, Equinoctial, or Equatorial
A sundial with its hour scales on a circle parallel to the Earth's equator. Since the great circle in which this plane meets the interior of a celestial sphere was termed by medieval astronomers the 'equinoctial', this term is now generally applied to dials of the type.
A typical universal equinoctial sundial consists of a base plate embodying a magnetic compass, to the north edge of which is hinged a circular hour ring. When this is lifted it engages a latitude arc and can thus be set for latitude. A pivoted diametral bar, to the center of which a pin-gnomon is attached at right angles, traverses the hour ring. The shadow of the gnomon gives the time on the hour ring. The pin-gnomon is turned upwards during the summer half-year and downwards during the winter; no shadow can be obtained at the equinoxes when the sun is in the plane of the ring itself.
The earliest-known equinoctial dials date from the 15th century.
The commonest form of sundial, in which the shadow of a gnomon whose edge is parallel to the earth's axis falls on a horizontal surface. Usually this carries only the hour lines, but occasionally a ring showing the equation of time is added. Portable horizontal sundials take many forms; in the diptych form the gnomon is a tautened string; in the small accessory dials showing Italian and Babylonian hours it is a short vertical pin.
Like most astronomical and horological conventions in Japan before the advent of the Europeans, sundials were probably imported during the Asuka period (6th century AD) from T'ang China. The sundial most useful to the Japanese temporal system was used, and came to be more Japanese than Chinese. It is of an archaic design called a 'scaphe', or more accurately, a 'hemispherium', reputed to have been invented by a Chaldean priest-astronomer called Berosus in the 3rd century BC. The Japanese scaphe was a concave hemisphere with six hour divisions inscribed on the surface and a gnomon on a perpendicular axis. It measured the daylight hours equally for the given solar declination. This system was perfectly suited to the Japanese system of varying the hour duration according to the season.
Many pocket sundials are in the shape of a small 18th-century watch. In one side is the scaphe, and in the other the compass. Often around the circumference of one or both the Twelve Terrestial Branches are engraved, and sometimes the Ten Celestial Stems as well. These refer to the sexagenary calendar and to the other cultural associations related to time and the celestial cycles. The characters were placed so that sunset and sunrise were in the east-west axis, and midnight and noon were in the north-south axis, respectively. The numbers corresponding to the hours were engraved either on the rim around half the circumference, or on the central axis (east-west) of the scaphe.
The pillar dial is an altitude dial, sometimes called a 'column' or 'cylinder' dial; the old name was 'chilindre'. It consists of a vertical pillar, usually of constant cross-section, mounted on a base or suspended from a shackle. The surface of the pillar is divided by vertical lines, usually six, into compartments, each for use in a given pair of months, which are indicated at the top or foot of each compartment. The cylinder is surmounted by a cap inside which is pivoted a gnomon in the form of a blade; for storage this fits in a hole in the pillar. To use the dial, the cap is removed, the gnomon swung out horizontally, and the cap with gnomon replaced on the pillar. The dial is then held with the gnomon pointed towards the sun and the pillar turned until the shadow falls straight down the side of the pillar. The time is then read from the shadow of the gnomon's tip on the hour lines, which twist helically round the pillar.
Pillar dials showing unequal hours are shown in manuscript drawings before 1350; the earliest surviving example, showing equal hours, is dated 1455.
Providing that the style, or shadow-casting edge, of a sundial is parallel to the Earth's axis, hour lines can be drawn on any surface on which the sun shines. This allows the designer of dials to make them in a great variety of shapes, with plane or concave faces. One of the simplest forms is the cube, with dials on the top and four vertical sides. Another is the octagon with vertical sides, with or without corresponding sloping sides above or below the vertical ones. The whole may be capped by a horizontal dial, or by a decorative finial.
One form of the polyhedral dial is the crucifix, a cross hinged to a base with the time shown by the shadow of an edge of the cross on a neighboring face.
An altitude sundial in which the time is shown by light rays passing through a small hole in a ring, giving a spot of light on a set of hour lines on the inner surface of the ring. Such dials are sometimes called 'poke' dials: 'He drew a dial from his poke...and says, very wisely, "It is ten o'clock"' (As You Like It).
Since the sun's altitude at a given hour depends on the date, being greater in summer than in winter, a fixed pinhole with a set of hour lines parallel to the axis of the ring would be correct only for one date each half-year. There are two ways of providing for the whole range of dates. The first is to make the hour lines slope and to divide them into three parallel rings, one for winter, one for summer, and one for spring and autumn. The second is to make the pinhole in a small piece of a ring sliding within the main ring adjustable according to the season. This gives an approximately correct set of readings on hour lines parallel to the axis, though with some seasonal errors.
Sundial, Universal Ring
A ring dial is for use in a fixed latitude, but the universal ring dial can be used over a wide range of latitudes. It consists of three rings and a strip with slider. A flat vertical brass ring, with grooved outer edge, fits within a ring of circular cross-section, and slides within it. The outer ring is suspended from a shackle or mounted on a foot. One side of the inner ring carries a scale of degrees, 0-90, for adjustment to latitude using an index on the outer ring. Pivoted within the inner ring is a further ring which can be swung out at right angles, carrying the hour scale. A diametral bar traverses the interior of this last ring; the bar has a slot in which a small piece with a pinhole slides. The slider moves along a scale of dates, or corresponding zodiacal signs. In use, the second ring is adjusted for latitude and the pinhole set to the date. The instrument is then rotated until the spot of light passing through the pinhole falls on the hour ring, where the time is read.
The invention of the universal ring dial is attributed to William Oughtred (1575-1652). Dials incorporating these principles were manufactured by the London instrument maker Elias Allen in 1652.
In this type of dial the shadow of a gnomon whose edge is parallel to the Earth's axis falls on a vertical surface; if this is due south, the hour lines will be symmetrically placed. Vertical sundials are normally seen on the walls of buildings, but they also appear as the upper members of diptych portable dials and on the sides of polyhedral dials.
A plate attached to the quarter-snail and free to move forward as the star wheel is advanced. It prevents the wrong quarter being struck by a repeating clock, close to the hour.
The introduction of the dead-beat escapement enabled the seconds hand of a longcase clock to advance without the slight recoil which is evident with the anchor escapement. This permitted a much longer second hand, known as a sweep or center second, which had a 'tail' or counterbalance and extended in length as far as the minute band near the outer edge of the chapter ring. The minute divisions then represented seconds as well, the sweep second completing its full revolution in one minute. Sweep center seconds were occasionally used for clocks fitted with chronometer-type detent escapements, and they are now common on synchronous electric clocks, on which they move in a steady progression.
Sweep Second Hand
A hand positioned in the center of the clock that sweeps, in a circular motion, around the dial; also called center seconds hand.
A term first applied to Abraham-Louis Breguet's clock, which had a receptacle into which a pocket watch designed for the purpose could be placed in the evening. The clock would then wind and reset the watch to time, besides regulating it. However, the term now means a clock controlled by a central master clock. Alexander Bain was the first to produce this kind of sympathetic clock, using a pendulum with two solenoids, one for driving the pendulum, the other swinging over a separate magnet, the electrical current thus generated being passed via connecting wires to the solenoid of a second clock, the pendulum of which then swung in sympathetic vibrations. Bain demonstrated two such clocks in 1846, the master clock in Edinburgh, the sympathetic clock in Glasgow, 46 miles away. Frederick James Ritchie of Edinburgh adopted Bain's system with some improvements, the sympathetic pendulums driving the dials of the sympathetic clocks. But none of the sympathetic clocks devised proved successful over a long period. The best of such systems was invented by Professor Charles Féry of France and used in the Paris Observatory for many years in the modified form due to the firm of L. Hatot of Paris.
Frank Hope-Jones, in collaboration with George B. Bowell, devised the synchronome remontoire in 1895 after a visit to an exhibition to see an installation of a system of electric clocks devised by Van der Plancke of La Precision Cie., Brussels. The remontoire was first used to rewind the train of a pendulum clock; later, it was arranged to act directly on the pendulum to give an impulse through a roller and pallet. The roller falling off the pallet caused two electrical contacts to close, and an electro-magnet replaced a gravity arm in preparation for the next impulse. A count wheel was used to measure out periods of half-minutes from one impulse to the next, the closing of the contacts also transmitting pulses of current to slave clocks in a system. The synchronome clock became a standard pattern which varied little over a long period of manufacture. It is probably the best known of its type for use as a master clock, and it was sold in greater numbers than any other thanks to the publicity measures of Hope-Jones. In 1921 it was used by William Hamilton Shortt as the slave clock for his 'free-pendulum clock', installed in Edinburgh Observatory. An accuracy of less than one-tenth of a second a day error was achieved, ousting the Riefler regulator clocks from observatories all over the world.
Synchronous clocks were developed in the United States by Henry E. Warren, who obtained his first patent in 1918. There was no general application for this type of clock until alternating-current power supplies were available to large numbers of consumers and the frequency could be maintained at a fixed and accurately maintained value. A rotor is kept synchronously in step with the generator at the electricity generating station, and gearing is used to drive the clock hands; often worm drives are employed to achieve great reduction with the minimum number of wheels, immersed in oil baths to achieve silent running, as the early rotors ran at high speeds. Later models used a greater number of poles in the field magnet to reduce the rotor speed and, together with the use of plastic gears, minimized the noise levels without the need for lubrication. Strictly speaking these are not clocks but time indicators, for there is no means of measuring time within the synchronous clock mechanism.
The number of revolutions per minute of the driving motor of an electric clock driven from the public electricity supply is directly related to the frequency of the supply. A motor controlled in this way is said to be 'synchronized', and the addition of a simple train of gears to drive the hands makes a cheap, reliable, time indicator possible. A synchronous clock of this type is not a timekeeper, but merely a slave controlled by the frequency of the electricity supply, which is regulated within close limits. In periods of heavy electricity demand, load shedding causes synchronous clocks to lose, but this loss is regained automatically during the following night.
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