after many generations of watching the night sky it was clear to the ancient
observers that there was an underlying order and structure to the cosmos
in addition to the cycle of night and day and the phases of the Moon when
measured against the background of stars they found that the objects they were
watching had a repeatable and predictable motion and a good example of
this is the motion of the moon if an observation is made when the moon is
adjacent to a prominent star then when it returns to that star an interval of
time can be calculated this is known as its side aerial period or in the case of
the moon we simply call it the side aerial month now the ancients were well
aware that this was merely an average representation of the time interval they
might not have had our modern understanding of exactly why it was so
but they certainly knew that the moon sped up and then slowed down during its
trip back to the same reference point and that even that small oscillation was
itself continuously moving throughout each cycle it's perhaps the most
remarkable aspect of the device that such subtle observations were
mechanically represented within its keyring and it began with the
calculation of the mean lunar side aerial month as a further resonance of
the metonic cycle it so happens that 254 sidereal months also almost exactly
equals 19 solar years so much like the metonic gearing the side aerial period
of the Moon can be given a convenient mathematical expression now of course
many aspects of this machine were pushing the engineering limits of the
day at this part of the train in particular takes it closer to the edge
than the rest and it's those engineering limitations that I'd like to go through
now to begin with the gearing rapidly steps
up in this part of the train demanding relatively fast rotations from
assemblies with considerable inertia and this puts a heavy load on some very
small components for the most part the high loads are dealt with in the same
manner as the rest of the mechanism square arbors fitting into square holes
well peened in the position but the teeth are at their greatest risk of
damage in this part of the train and there's no better example of the
material limitations experienced in the mechanism
much like the EM assembly the D assembly must be capable of removal from the main
plate additionally the maker elected to run at
flush against the underside of the assembly platform and in so doing
created the requirement that the D assembly be fastened with a recessed
retaining pin
now you'll have noticed that the wheels and pinions in the mechanism are quite
thin in most cases between one and two millimeters thick and they obviously
need to stay well meshed to function correctly in modern clock making the
meshing is ensured by setting a limiting end shake with the pivot shoulders and
to some extent by using relatively long pinions that can tolerate a small amount
of longitudinal shift between the components but the sleek design of this
machine rules out the option of using longer pinions and the concept of
shouldered pivots had apparently not yet been invented generally the issue was
solved by allowing a small clearance on the inside of the arbors a bit like a
simple axle it provides the required limits to the movement at the cost of a
higher friction penalty a good example is the way that the EM assembly was
fabricated but in this higher load section of the
train the maker discovered that method was unsuitable and was forced to come up
with a more reliable solution it shares some of the features of a modern bridge
but it might be more correctly described as a sort of thrust plate that captures
the small pinion while still permitting its free rotation whatever you choose to
call it it performs almost exactly the same function as the shoulder of a
modern pivot albeit with more friction locking the longitudinal travel of the
assembly and ensuring a very limited and precise end shape but as effective as
this idea is it still leaves one part of the problem unsolved a natural
limitation of the larger wheels due in part to the simple square seating is the
tendency for them to very slightly tilt out of the horizontal plane for the low
load assemblies this isn't really an issue but for the high loads in this
part of the train and elsewhere in the mechanism like the main solar drive
wheel this dramatically affects their efficiency and at worst can quickly lead
to them becoming damaged so it was clear that they required stabilisation
and for that the maker came up with the idea of these small stabilizing strips
known as curves carefully set to the correct height they acted as a guiding
surface that limited the extent to which the wheels could twist out of alignment
now as it happens there's an interesting combination of methods used to fasten
these curves to the main plate some are fastened as you've seen here using pins
but others were soft soldered into position and that's a process that I
figured was worth looking at in more detail so be sure to check out that
video if you haven't already done so the stud for the SI assembly is the final
component to be fabricated for this part of the train with a small notch file to
provide clearance with a d' assembly hub and a pin used to maintain its
orientation with the main plate
and I should also mention that there are two final curbs to install underneath
the main solar drive wheel that if installed now would partly obscure the
internal view of the mechanism so I've decided to hold off fixing those until a
later stage of the build in any event has unorthodox as the solution appears
the curbs turned out to be quite effective the friction penalty has
certainly increased but it is quite acceptable and importantly the objective
has been achieved because the assemblies are now constrained into their operating
planes as required to the extent that the tiny wheel known as e 2 which is now
rotating at the mean side aerial period of the moon is now a reliable input to
the next part of the mechanism the epicyclic been and slight gearing thanks
for watching I'll see you later
you