London dial
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A London dial in the broadest sense can mean any sundial that is set for 51°30′ N, but more specifically refers to a engraved brass horizontal sundial with a distinctive design. London dials were originally engraved by scientific instrument makers. The trade was heavily protected by the system of craft guilds.


A horizontal dial

A gnomon or style is set to point at the celestial north pole, the shadow of the sun is thrown onto the dial plate and will appear at the same position each day of the year, and this position can be calculated using trigonometry, or drawn using geometric construction. In the world of sundials some of the technical terms use an old form of language, so the angle to which the style is set is called the style height. The style height is identical to the geographical latitude, and in
London London is the capital and List of urban areas in the United Kingdom, largest city of England and the United Kingdom, with a population of just under 9 million. It stands on the River Thames in south-east England at the head of a estuary dow ...
this was 51 degrees 30 minutes or 51.50 degrees, which roughly corresponds with
Westminster Bridge Westminster Bridge is a road-and-foot-traffic bridge over the River Thames in London, linking Westminster on the west side and Lambeth on the east side. The bridge is painted predominantly green, the same colour as the leather seats in the ...
. The gnomon has thickness, and thus two shadow throwing edges (the styles) one for the morning and one for the afternoon, there is a gap left on the dialplate the width of the gnomon. A sundial displays the
local apparent time Solar time is a calculation of the passage of time based on the position of the Sun in the sky. The fundamental unit of solar time is the day, based on the synodic rotation period. Two types of solar time are apparent solar time (sundial tim ...
, and watches that use mean time or average time will always be at a slight variance. This difference of time can be calculated and displayed on the dial – again using old language it is known as the
equation of time In mathematics, an equation is a formula that expresses the equality of two expressions, by connecting them with the equals sign . The word ''equation'' and its cognates in other languages may have subtly different meanings; for example, in F ...
. A London dials show hour lines, some show half-hour markers, the quarter hours and some divisions representing half-quarters (7 and a half minutes). The equation of time is shown as a three rings, showing the words ''WATCH SLOWER, WATCH FASTER'', the months and dates and the minutes of time of variance.


Geometrical construction

There are many methods to do this: the one published by Leybourn in 1652 is still popular. Though the one by Dom Francois Bedos de Celles in 1790 is more widely known.


Calculations

;Style height ::SH=\phi ;Hour lines (mark from 4am to 8pm) :: \tan H_H = \sin \phi \tan(15^ \times t) ::where H_H is the angle from noon line, and ::\phi is the geographical latitude.


Definitions

;Centre of delineation: the point where the styles and all the hour lines meet. On a dials with a thick gnomon, there will be two centres of delineation, separated by a noon gap. ;Gnomon: the vertical triangular component. The gnomon is precisely angled for the latitude, in London that is nominally 51° 30' N. The London gnomon has thickness so the style that casts the shadow for the morning hours is the western edge of the gnomon while the eastern edge of the gnomon is the style that cast the shadow for the afternoon hours. ;Chapter ring: this is the ring of written numbers. On the chapter ring may be half-hour indicators and quarter-hour indicators.


Timeline

;1500s :Written evidence of dials (Reign Henry VII) ;1542 : Nicholas Oursian- this is the earliest example in a museum. It is dated with inwardly facing
roman numeral Roman numerals are a numeral system that originated in ancient Rome and remained the usual way of writing numbers throughout Europe well into the Late Middle Ages. Numbers are written with combinations of letters from the Latin alphabet, ea ...
s. The ''origin ''or'' centre of delineation'' is the centre of the dialplale. It has simple decoration, a large Tudor Rose beneath the style. ;1578 :A dial (maker unknown) for
Sir Philip Sidney ''Sir'' is a formal honorific address in English for men, derived from Sire in the High Middle Ages. Both are derived from the old French "Sieur" (Lord), brought to England by the French-speaking Normans, and which now exist in French only as p ...
(1554-1586). Decoration included a ropework border, and scrolls at the end of the ''chapter ring''. ;1580s :Simple punched dials- for use by the less wealthy, though the ''gnomon'' started to have a complex shape. ;1590 : Isaack Symmes, a 'gouldsmith'(sic) was producing dials. The ''centre of delineation'' moved to the southern edge of the dial, the division now included hours, half-hours and quarter hours. The lettering could be gushing, reflecting the developments in calligraphy and silverwork. Some Symmes dials included a
volvelle A volvelle or wheel chart is a type of slide chart, a paper construction with rotating parts. It is considered an early example of a paper analog computer. Volvelles have been produced to accommodate organization and calculation in many divers ...
. ;Jacobean ;1588-1653 :
Elias Allen Elias Allen (c.1588 in Tonbridge – March 1653 in London)H. K. Higton, 'Allen, Elias (c.1588–1653)', Oxford Dictionary of National Biography, Oxford University Press, 2004; online edn, Jan 200accessed 6 Feb 2011/ref> was an English maker of su ...
, the mathematical instrument maker. These were accurate with austere decoration. ;1665 : Henry Sutton (1649-1665), a renowned engraver of scientific instruments, was a victim of the
plague Plague or The Plague may refer to: Agriculture, fauna, and medicine *Plague (disease), a disease caused by ''Yersinia pestis'' * An epidemic of infectious disease (medical or agricultural) * A pandemic caused by such a disease * A swarm of pe ...
. ;1660 :Regime change The restored aristocracy restored their houses and decorated their gardens. Decorative sundials were in demand. ;1680s :
Henry Wynne Sir Henry Arthur Wynne, PC (14 June 1867 – 21 August 1943) was an Irish solicitor. He was Chief Crown Solicitor for Ireland from 1916 to 1922. He was knighted in 1919 and sworn of the Privy Council of Ireland in 1922, one of the last three to ...
;1680 : Flamsteed's Equation of Time Tables were available. Diallists experimented on how the information should be included on the dial.
Thomas Tompion Thomas Tompion, FRS (1639–1713) was an English clockmaker, watchmaker and mechanician who is still regarded to this day as the "Father of English Clockmaking". Tompion's work includes some of the most historic and important clocks and watc ...
used calendar tables, Wynne preferred linear strip tables. The tables were then drawn as concentric arcs around the ''compass rose''. The early arcs were labelled as ''Equation of Natural Days''- later they included the text ''Watch Fast''. This ''chapter ring'' was separated from the compass rose by a decorative ring of oak-leaves. ;1700 :Wynne retired and his workshop was passed to Thomas Wright- Thomas Tuttell and Richard Glynne ;1700s :
John Rowley John Vincent D'Alessio Rowley (12 December 1907 – 30 November 1996) was a South African born English cricketer and colonial governor. He was born in Graaff-Reinet, Cape Province, and went to school at St. Andrew's College, Grahamstown. He ...
made the hour numbers face outwards, this was adopted by all London diallists and by 1750 in all of the provinces. ;1700s :The change of orientation of the ''Equation of Natural Days'' came in two stages, firstly a simple reversal that led to the months running backward, then secondly the months were reversed to run left to right, ie counterclockwise. ;1750 :Mathematical instrument makers moved to a
factory system The factory system is a method of manufacturing using machinery and division of labor. Because of the high capital cost of machinery and factory buildings, factories are typically privately owned by wealthy individuals or corporations who emplo ...
, with workshops of
journeymen A journeyman, journeywoman, or journeyperson is a worker, skilled in a given building trade or craft, who has successfully completed an official apprenticeship qualification. Journeymen are considered competent and authorized to work in that fie ...
just doing a small part of the dial. Thomas Heath had a large workshop. Dials were no longer signed by one man. The name was that of the manufactory or even the retailer. Decorations came from a pattern book, these would be scrolls on the Equation of Time ring, coats of arm and cartouches for signatures. ;1800 :Simplification of design. The oakleaf decoration was replaced by a zigzag design. Troughton & Simms was one of the last of the mathematical instrument makers who made dials. ;1850s :The need for accurate dials decreased as time was transmitted electrically. Francis Barker and Son issued a catalogue of decorative designs. ;1880s :Machine (pantograph) engraving started. The ''Watch Faster/Slower ring'' was removed being replaced by a ''simple table'' then after 1920 a graph. ;1900s :Emergence of fake dials, with mottoes and
Father Time Father Time is a personification of time. In recent centuries he is usually depicted as an elderly bearded man, sometimes with wings, dressed in a robe and carrying a scythe and an hourglass or other timekeeping device. As an image, "Father Ti ...
figures suns and other symbols. These had a false date and spurious makers names: these were sold by companies such as Pearson Page Ltd or Peerless Brass to meet customer demand.


Geometrical markup technique

A horizontal dial, takes the equiangular hour lines of a equatorial dial and projects them onto a plane oblique to the style, the markup simulates this transformation. The Dom Francois Bedos de Celles method (1760) otherwise known as the Waugh method (1973) *Take a large sheet of paper. *Starting at the bottom, draw a line across, and a vertical one up the centre. Where they cross is important call it O. *Choose the size of the dial, and draw a line across. Where it crosses the centre line is important call it F *You know your latitude. Draw a line upwards from O at this angle, this is a construction line. *Using a square, (drop a line) draw a line from F through the construction line so they cross at right angles. Call that point E, it is important. To be precise it is the line FE that is important as it is length \sin\phi . *Using compasses, or dividers the length FE is copied upwards in the centre line from F. The new point is called G and yes it is important- the construction lines and FE can now be erased. *From G a series of lines, 15° apart are drawn, long enough so they cross the line through F. These mark the hour points 9, 10, 11, 12, 1, 2, 3 if you take just 3 and represent the points \tan h \sin\phi . *The centre of the dial is at the bottom, point O. The line drawn from each of these hour point to O will be the hour line on the finished dial. *If the paper is large enough, the method above works from 7 until 12, and 12 until 5 and the values before and after 6 are calculated through symmetry. However, there is another way of marking up 7 and 8, and 4 and 5. Call the point where 3 crosses the line R, and a drop a line at right-angles to the base line. Call that point W. Use a construction line to join W and F. Waugh calls the crossing points with the hours lines K, L, M. *Using compasses or dividers, add two more points to this line N and P, so that the distances MN = ML, and MP = MK. The missing hour lines are drawn from O through N and through P. The construction lines are erased. File:Bedos de Celles method (1790)-(1).svg, Setting out a dial for 52°N. The three initial lines. File:Bedos de Celles method (1790)-(2).svg, Marking the latitude, laying out length \sin\phi , and copying to G on the vertical. File:Bedos de Celles method (1790)-(3).svg, From G casting \tan h \sin\phi on the horizontal. File:Bedos de Celles method (1790)-(4).svg, The actual hour lines 9, 10, 11, 12, 1, 2, 3. File:Bedos de Celles method (1790)-(5).svg, Construction lines removed. File:Bedos de Celles method (1790)-(6).svg, Constructing the 7, 8, 4, 5 lines File:Bedos de Celles method (1790)-(7).svg, Marking the 7, 8, 4, 5 lines File:Bedos de Celles method (1790)-(8).svg, The completed dial plate for 52°N. Bedos de Celles (1790) ''Caveat: These diagrams have not been tested for accuracy. The further north the wider the dial becomes.''


Notable examples

; Beamish museum: There are two London dials, the first is in 'town' by the Solicitors Office. It is dated 1649. The second is in the garden of Pockerley Manor, this is a dial produced by Spencer, Browning & Co, more noted for their production of sextants and
navigational instrument Navigational instruments are instruments used by nautical navigators and pilots as tools of their trade. The purpose of navigation is to ascertain the present position and to determine the speed, direction, etc. to arrive at the port or point o ...
s. ; St Michael's Mount, Cornwall: There is a dial engraved by Troughton and Simms.


References


Bibliography

* * *{{cite book, last1=Waugh, first1=Albert E., title=Sundials : their theory and construction., date=1973, publisher=Dover, location=New York, isbn=0486229475, pag
38–39
url-access=registration, url=https://archive.org/details/sundialstheirthe00albe/page/38 Sundials