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Hand for the determination
of true solar time - shows true solar time on the face of the
clock. True solar time differs from mean time that has been adopted
by humanity for measurement by mechanical (and later electronic) timepieces.
Over the year, the difference between true solar time and mean solar
time ranges between -14 minutes 22 seconds and +16 minutes 24 seconds,
while the change in the length of consecutive calendar days can reach
50 seconds. There are two main factors that contribute to this phenomenon:
the elliptical, rather than circular, orbit of the Earth around the
Sun, and the tilt of the Earth's axis of rotation. While moving around
its elliptical orbit the Earth, little by little, changes its position
relative to the Sun. The distance between them increases and decreases
which influences the length of the true day (midnight to midnight).
In the second half of the eighteenth century sundials were still extensively
used and they could by their nature only show true solar time. These
fairly simple devices for determining time could be found everywhere
in city streets, in parks and squares, even on more elaborate milestones.
The spread of pendulum clocks that displayed mean time was an irritation
to many less sophisticated people and so when he produced his mechanical
masterpiece Johann Georg Strasser included a mechanism for determining
true solar time.
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Mean solar time
- like true solar time is made up of hours, minutes and seconds that
come together to make the day or the year. The length of each mean
day is the same throughout the year and is the product of calculation.
The figure was reached by dividing the total length of the equinoctial
year in true solar days by the number of days in a year. (The equinoctial
year is the time taken for the Earth to complete a full orbit of the
Sun, from spring equinox to spring equinox.) The difference (on 30
June) between the minute hands showing (1) true solar time - 2.14
and (2) mean solar time - 2.04.
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Mechanical organ
- a mechanical device designed to reproduce pieces of music.
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Music barrel -
a cylinder made of wood into which pegs and staples are inserted to
record one or more pieces of music.
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Peg - a small metal
protrusion intended to raise a push-rod of the musical mechanism for
a brief period of time when the barrel is rotated. The brief rise
and drop of the push-rod produces a short note of a particular pitch.
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Staple - a piece
of metal wire shaped in the form of an inverted U intended to keep
the push-rod in a raised position for a longer period of time. The
longer the staple, the longer the time the push-rod is kept raised
and the longer the resultant musical note.
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Push-rod - part
of the musical mechanism that makes it possible to open and close
an air pallet. When a pallet is open, air is directed to the pipes
of the organ which then produce sounds.
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Retaining spring
- part of the musical mechanism that secures the barrel after its
insertion. The spring supports the barrel while it operates within
the mechanical organ.
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The winding mechanism
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Weight-case - part
of the body of the mechanical organ that takes the form of a narrow
vertical cupboard made to contain the weights of the drive mechanism.
The walls of the case also serve as guides to direct the fall of the
weights.
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Kinematic mechanism
- a method of picturing or explaining the principles by which mechanical
devices work. A special feature of the winding mechanism of the Musical
Orchestra is the use of a system of moving pulley blocks that permitted
the maker to increase the length of the cable. This method was often
used to increase the working time of devices driven by descending
weights.
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Bevel gear - a metal
or wooden gear with the teeth cut into a conical surface. It can be
used with a second gear of the same kind to change the direction in
which power is transmitted and to reduce friction between the teeth.
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Pulley - a metal
or wooden disc with a central opening and a grooved rim that takes
a line. If the pulley is placed in a block at a height then a rope
or cord passed through it can be used to raise a weight with less
effort. A system of moving blocks makes it possible to increase the
length of the line. Craftsmen often used this to increase the length
of time mechanical devices driven by falling weights could operate.
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Ratchet - a mechanism
consisting of a toothed ratchet wheel and a pivoted bar or pawl which
drops to prevent the wheel returning. The combination of ratchet wheel
and pawl means a rod to which the wheel is attached can rotate in
only one direction. As a rule ratchets are fitted to the winches or
drive mechanisms so that they can be kept wound up until it is time
for them to operate and discharge their potential energy.
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The air system
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Bellows - a chamber
made of wood and leather that operates in a similar way to an air-pump.
When the chamber is compressed air is expelled from it through a nozzle.
When the bellows are expanded, the chamber fills with air through
a valve. Thus the bellows collect air from outside and direct it onwards
to the pipes of the organ.
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Air valve - a mechanical
device permitting the movement of air through it in one direction,
most often found in air-pumps and systems with a similar principle
of operation. The valve opens when the pump takes in air and closes
when the air is discharged from the pump under pressure. This makes
it possible to move air from one place (usually the atmosphere) to
a reservoir where it is under higher pressure (such as a bicycle tyre).
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Windchest - part
of the air system within the musical mechanism. It is a wooden device
that collects the air produced by several bellows working alternately.
When one of them is discharging air into the windchest, another is
taking air into its chamber, and vice verse. This ensures a continual
supply of air from the working bellows. The windchest is also fitted
with valves that are opened by a system of wooden poles and push-rods
in accordance with the pattern of pegs and staples on the barrel.
When these valves are open, air passes from the windchest to the pipes
of the organ.
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Pallet - an elements
of the windchest, used to permit or stop the flow of air to an organ
pipe. When a push-rod is activated, the wooden rod attached to it
moves the pallet, freeing an opening through which air flows to the
pipe. A metal spring closes the pallet after the push-rod drops.
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Rocker - a metal
bar pivoted in the centre so that it can alternately rise and fall
at one end and the other. In the mechanical organ there is a rocker
in the bellows mechanism that ensures that they open and close alternately.
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Tie-rod - part of
a mechanism intended to convey force vertically or horizontally. It
often acts as the link between turning parts and sliding elements,
such as shutters or valves.
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The organ
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Stop - a group of
organ pipes producing low or high notes within the range or a single
octave.
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Swell - part of
the musical mechanism of a mechanical organ that makes it possible
to vary the properties of the sound produced. It consists of a wooden
box containing small organ pipes with a lid or shutter at the top.
When the lid is opened the volume of the sound from the pipes is greater
than when it is closed.
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