…on the other hand

 

On the one hand we have simple, easy to use, single-handed dials.  On the other hand, some dials are not so straightforward.

 

“Fan” clock, anonymous, possibly made in either Netherlands or England, c.1780 (British Museum No. 1958,1006.2174).  Note the enthusiastically-painted windswept trees and windswept bridge

“Fan” clock, anonymous, possibly made in either Netherlands or England, c.1780 (British Museum No. 1958,1006.2174). The clock is showing 10:29 – in one minute’s time the minute hand will fly-back to 30 on the left-hand side.  Note the enthusiastically-painted windswept trees and windswept bridge

This clock has “fly-back” or “retrograde” hands.  The hands proceed clockwise as usual but when they reach the end of the scale, each will spring back and then immediately resume its count.  This design allows the dial to have twice the diameter of a circular dial within a given height.  This, together with the semi-circular form, makes these clocks suitable for positioning over doorways and they were sometimes integrated within wall panelling.  Fly-back hands are still a popular feature in watches, although these are usually (but not always) the preserve of high-end watches.

 

Month-going sidereal and solar regulator by John De Lafons, Royal Exchange, London, c.1780 (British Museum No. 1998,0704.1)

Month-going sidereal and solar regulator by John De Lafons, Royal Exchange, London, c.1780 (British Museum No. 1998,0704.1)

Close-up of the dial, with hours top, minutes centre and seconds bottom.  The clock is showing 19:48:10 mean sidereal time on the main dial plate and 7:46:53 mean solar time on the discs

Close-up of the dial, with hours top, minutes centre and seconds bottom. The clock is showing 19:48:10 mean sidereal time on the main dial plate and 7:46:53 mean solar time on the discs

The dial of this regulator (that I have mentioned before for its escapement) has the typical features which facilitate the use of these clocks in observatories and laboratories.  All hands are positioned on their own centres for disambiguation.  The hours are relegated to a small subsidiary dial and only the minutes have a large hand, for accurate observations.

Less typically, however, this regulator indicates two different systems of time using only one set of hands.  It shows mean sidereal time against the numerals on the main dial plate and mean solar time against the small  discs.  A mean sidereal day is 23 hours 56 minutes and 4.1 seconds, i.e. it is about 4 minutes shorter than the 24 hour mean solar day.  The discs rotate slightly faster than the hands so that after each day they have accrued the 4 minutes difference.  This system is called a differential dial.

Having a clock show both sidereal and solar time might have been a convenience, but a respectable observatory might well be expected to have had separate regulators for providing the required times and so it is possible that this design may have been conceived rather for the use of the (wealthy) amateur.

“Bras en-l’air” watch with verge escapement, Anonymous, Swiss, c.1790 (British Museum No. 1958,1201.1797)

“Bras en-l’air” watch with verge escapement, Anonymous, Swiss, c.1790 (British Museum No. 1958,1201.1797)

This watch has not just hands, but arms and fingers too! It is a type of watch known as “bras en-l’air”, which translates as “hands in the air” (and certainly not “bras in the air” – which might apply another aspect of horology altogether).  When the pendant is pushed the soldier raises his arms, holding pistols to point at minutes (left) and hours (right).

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